at310_system grounding and gf protection for dcs

System Grounding and Ground-fault Protection for Data Centers

October 2011/AT310

byFrank Waterer, Fellow Engineer, Square D Engineering Services Bill Brown, PE, Principal Engineer, Square D Engineering Services

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Revision #2 10/11

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Summary

Abstract ...........................................................................................................p 3

Background .....................................................................................................p 4

Ground-fault Protection for Solidly-grounded Wye Systems ..............................p 17

Principal Purposes of a Bonding and Grounding System ..................................p 21

Common Electrical System Configurations Employed

in Data Center Facilities ....................................................................................p 26

Commonly Issues Found with Grounding Systems

in Data Center Facilities ....................................................................................p 31

Grounding System Inspections and Testing ......................................................p 34

References ......................................................................................................p 36

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System grounding and ground-fault protection for data centers – What you need to know


Abstract

In order for data centers and related facilities to properly operate as designed and intended, it is extremely

important that care be given to the effective installation and maintenance of bonding and grounding systems.

This paper presents the fundamentals of power system grounding, fundamental definitions and requirements

of the National Electrical Code® (NEC®) with respect to grounding, the need for low voltage ground fault

protection and how it is implemented, and special grounding requirements imposed by the data center

operating environment. This paper also provides a summary of the applicable NEC articles, a review of

clauses from specific IEEE® Standards, and listing of other references related to bonding and grounding in

data center facilities. This paper also outlines and explains the principal purposes for an effectively bonded

and grounded system. Also addressed are the requirement of four-wire systems versus three-wire systems,

arrangements that require the installation of modified differential ground fault (MDGF) protection system, and

the effects of system grounding upon surge protection and power quality.

This paper reviews common installation inadequacies and construction issues, and will provide a list of

common conditions that cause bonding and grounding systems within data center facilities to become

inadequate or deteriorated over time. Testing and maintenance requirements of grounding systems are

also discussed.

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Background

System grounding typesThe system grounding arrangement is determined by the grounding of the power source. For commercial and

industrial systems, the types of power sources generally fall into four broad categories:

A.) Utility Service – The system grounding is usually determined by the secondary winding configuration of

the upstream utility substation transformer.

B.) Generator – The system grounding is determined by the stator winding configuration.

C.) Transformer – The system grounding on the system fed by the transformer is determined by the

transformer secondary winding configuration.

D.) Static Power Converter – For devices such as rectifiers and inverters, the system grounding is

determined by the grounding of the output stage of the converter.

Categories B.) – D.) usually fall under the NEC definition for a “separately-derived system.” The recognition

of a separately-derived system is important when applying NEC requirements to system grounding, as

discussed below.

All of the power sources mentioned above except D.) are magnetically-operated devices with windings. To

understand the system voltage relationships with respect to system grounding, it must be recognized that

there are two common ways of connecting device windings: wye and delta. These two arrangements, with

their system voltage relationships, are shown in Fig. 1. As can be seen from the figure, in the wye-connected

arrangement there are four terminals, with the phase-to-neutral voltage for each phase set by the winding

voltage and the resulting phase-to-phase voltage set by the vector relationships between the phase-to-neutral

voltages. The delta configuration has only three terminals, with the phase-to-phase voltage set by the winding

voltages and no neutral terminal.

Neither of these arrangements is inherently associated with any particular system grounding arrangement,

although some arrangements more commonly use one arrangement vs. the other for reasons that will be

explained further on the next page.

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Figure 1: Wye and delta winding configurations and system voltage relationships

Solidly-grounded systems

The solidly-grounded system is the most common system arrangement, and one of the most versatile.

The most commonly-used configuration is the solidly-grounded wye, because it will support single-phase

phase-to-neutral loads.

The solidly-grounded wye system arrangement can be shown by considering the neutral terminal from the wye

system arrangement in Fig. 1 to be grounded. This is shown in Fig. 2:

Figure 2: Solidly-grounded wye system arrangement and voltage relationships

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Several points regarding Fig. 2 can be noted.

First, the system voltage with respect to ground is fixed by the phase-to-neutral winding voltage. Because

parts of the power system, such as equipment frames, are intentionally grounded, and the majority of the rest

of the environment essentially is at ground potential also, this has big implications for the system. It means that

the line-to-ground insulation level of equipment need only be as large as the phase-to-neutral voltage, which

is 57.7% of the phase-to-phase voltage. It also means that the system is less susceptible to phase-to-ground

voltage transients.

Second, the system is suitable for supplying line-to-neutral loads if the neutral conductor is used. If the

neutral is used, the system is commonly referred to as a solidly-grounded wye, four-wire system. The operation

of a single-phase load connected between one-phase and neutral will be the same on any phase since the

phase voltage magnitudes are equal. If the neutral conductor is not used, the neutral terminal still exists but it

is simply not brought out. Such a system is referred to as a solidly-grounded wye, three-wire system. From a

purely technical standpoint such designations are, strictly, inaccurate; in practice a four-wire system is actually

a five-wire system and three-wire system is actually a four-wire system, with the fifth / fourth conductor being

the equipment grounding conductor as described below.

This system arrangement is very common, both at the utilization level as 480 Y/277 V and 208 Y/120 V, and

also at much higher voltage levels on most utility distribution systems.

While the solidly-grounded wye system is by far the most common solidly-grounded system, the wye

arrangement is not the only arrangement that can be configured as a solidly grounded system. The delta

system can also be grounded, as shown in Fig. 3. Compared with the solidly-grounded wye system of Fig. 2

this system grounding arrangement has a number of disadvantages. The phase-to-ground voltages among all

three phases are not equal. And, without proper identification of the phases there is the risk of shock since one

conductor, the B-phase, is grounded and could be mis-identified. This arrangement is no longer in common

use, but is included for completeness.

Figure 3: Corner-grounded delta system and voltage relationships

The delta arrangement can be configured in another manner, however, that does have merits as a solidly-

grounded system. This arrangement is shown in Fig. 4. While the arrangement of Fig. 4 may not appear at

first glance to have merit, it can be seen that this system is suitable both for three-phase and single-phase

loads, so long as the single-phase and three-phase load cables are kept separate from each other. Note that

the phase A voltage to ground is 173% of the phase B and C voltages to ground. This arrangement requires

the BC winding to have a center tap. Although not commonly used for data centers this arrangement is shown

for completeness.

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Figure 4: Center-tap-grounded delta system arrangement and voltage relationships

A common characteristic of all three solidly-grounded systems shown here, and of solidly-grounded systems in

general, is that a short-circuit to ground will cause a large amount of short-circuit current to flow. This condition

is known as a ground fault and is illustrated in Fig. 5 for a solidly-grounded wye system. As can be seen from

Fig. 5, the voltage on the faulted phase is depressed, and a large current flows in the faulted phase since the

phase and fault impedance are small. The voltage and current on the other two phases are not affected.

The fact that a solidly-grounded system will support a large ground fault current is an important characteristic

of this type of system grounding and does affect the system design. Statistically, 90–95% of all system short-

circuits are ground faults so this is an important topic. The practices used for ground-fault protection are

discussed below.

Figure 5: Solidly-grounded wye system with a ground-fault on phase A

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The occurrence of a ground fault on a solidly-grounded system necessitates the removal of the fault as quickly

as possible. This is the major disadvantage of the solidly-grounded system as compared to other types of

system grounding.

A solidly-grounded system is very effective at reducing the possibility of line-to-ground voltage transients.

However, to do this the system must be effectively grounded. One measure of the effectiveness of the system

grounding is the ratio of the available ground-fault current to the available three-phase fault current. For

effectively-grounded systems this ratio is usually at least 60% [2].

In general, the solidly-grounded system is the most popular, is required where single-phase phase-to-neutral

loads must be supplied, and has the most stable phase-to-ground voltage characteristics. However, the

large ground fault currents this type of system can support, and the equipment that this necessitates, are a

disadvantage and can be hindrance to system reliability.

Ungrounded systems

This system grounding arrangement is at the other end of the spectrum from solidly-grounded systems.

An ungrounded system is a system where there is no intentional connection of the system to ground.

The term “ungrounded system” is actually a misnomer, since every system is grounded through its inherent

charging capacitance to ground. To illustrate this point and its effect on the system voltages to ground, the

delta winding configuration introduced in Fig. 1 is re-drawn in Fig. 6 to show these system capacitances.

If all of the system voltages in Fig. 6 are multiplied by √3 and all of the phase angles are shifted by 30° (both are

reasonable operations since the voltage magnitudes and phase angles for the phase-to-phase voltage were

arbitrarily chosen), the results are the same voltage relationships as shown in Fig. 6 for the solidly-grounded

wye system. The differences between the ungrounded delta system and the solidly-grounded wye system,

then, are that there is no intentional connection to ground, and that there is no phase-to-neutral driving voltage

on the ungrounded delta system. This becomes important when the effects of a ground fault are considered.

The lack of a grounded system neutral also makes this type of system unsuitable for single-phase phase-to-

neutral loads.

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Figure 6: Ungrounded delta system arrangement and voltage relationships

In Fig. 7, the effects of a single phase-to-ground fault are shown. The equations in Fig. 7 are not immediately

practical for use, however if the fault impedance is assumed to be zero and the system capacitive charging

impedance is assumed to be much larger than the phase impedances, these equations reduce into a workable

form. Fig. 8 shows the resulting equations, and shows the current and voltage phase relationships.

As can be seen from Fig. 8, the net result of a ground fault on one phase of an ungrounded delta system

is a change in the system phase-to-ground voltages. The phase-to-ground voltage on the faulted phase is

zero, and the phase-to-ground voltage on the unfaulted phases are 173% of their nominal values. This has

implications for power equipment – the phase-to-ground voltage rating for equipment on an ungrounded

system must be at least equal the phase-to-phase voltage rating. This also has implications for the methods

used for ground detection, which is explained further in this whitepaper.

The ground currents with one phase is faulted to ground are essentially negligible. Because of this fact, from

an operational standpoint ungrounded systems have the advantage of being able to remain in service if one

phase is faulted to ground. However, suitable ground detection must be provided to alarm this condition (and

is required in most cases by the NEC [1] as described below). In some older facilities, it has been reported that

this type of system has remained in place for 40 years or more with one phase grounded! This condition is not

dangerous in and of itself (other than due to the increased phase-to-ground voltage on the unfaulted phases),

however if a ground fault occurs on one of the ungrounded phases the result is a phase-to-phase fault with its

characteristic large fault current magnitude.

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Another important consideration for an ungrounded system is its susceptibility to large transient overvoltages.

These can result from a resonant or near-resonant condition during ground faults, or from arcing [2]. A resonant

ground fault condition occurs when the inductive reactance of the ground-fault path approximately equals the

system capacitive reactance to ground. Arcing introduces the phenomenon of current-chopping, which can

cause excessive overvoltages due to the system capacitance to ground.

Figure 7: Ungrounded delta system with a ground-fault on phase A

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Figure 8: Ungrounded delta system: simplified ground-fault voltage and current relationships

The ground detection mentioned above can be accomplished through the use of voltage transformers

connected in wye-broken delta, as illustrated in Fig. 9:

Figure 9: A ground detection method for ungrounded systems (see commentary for explanation of variant used in low-voltage systems)

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In Fig. 9, three ground detection lights “LTA”, “LTB” and “LTC” are connected so that they indicate the A, B and

C phase-to-ground voltages, respectively. A master ground detection light “LTM” indicates a ground fault on

any phase. With no ground fault on the system, “LTA”, “LTB” and “LTC” will glow dimly. If a ground fault occurs

on one phase, the light for that phase will be extinguished and “LTM” will glow brightly along with the lights for

the other two phases. Control relays may be substituted for the lights if necessary. Resistor “R” is connected

across the broken-delta voltage transformer secondaries to minimize the possibility of ferroresonance. Most

ground detection schemes for medium-voltage ungrounded systems use this system or a variant thereof. For

low-voltage systems, a wye-wye connected transformer (with both primary and secondary neutrals grounded)

is commonly used in lieu of the wye-broken delta transformer shown in

Fig. 9; this is feasible due to the much smaller likelihood of ferroresonance on low-voltage systems. In such

an arrangement the master fault light “LTM” is typically not present.

Note that the ground detection per Fig. 9 indicates on which phase the ground fault occurs, but not where

in the system the ground fault occurs. This, along with the disadvantages of ungrounded systems due to

susceptibility to voltage transients, was the main impetus for the development of other system grounding

arrangements.

Modern data center power systems are rarely ungrounded due to the advent of high-resistance grounded

systems as discussed below.

High-resistance-grounded systems

One ground arrangement that has gained in popularity in recent years is the high-resistance grounding

arrangement. For low-voltage systems, this arrangement typically consists of a wye winding arrangement with

the neutral connected to ground through a resistor. The resistor is sized to allow 1–10 A to flow continuously if

a ground fault occurs. This arrangement is illustrated in Fig. 10:

Figure 10: High-resistance-grounded system with no ground fault present

The resistor is sized to be less than or equal to one-third of the magnitude of the system charging reactance to

ground. If the resistor is thus sized, the high-resistance grounded system is usually not susceptible to the large

transient overvoltages that an ungrounded system can experience. The ground resistor is usually provided with

taps to allow field adjustment of the resistance during commissioning.

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If no ground fault current is present, the phasor diagram for the system is the same as for a solidly-grounded

wye system, as shown in Fig. 10. However, if a ground fault occurs on one phase the system model is as

shown in Fig. 11. As can be seen from Fig. 11, the ground fault current is limited by the grounding resistor. If

the approximation is made that —ZA and

—ZF are very small compared to the ground resistor resistance value R,

which is a good approximation if the fault is a bolted ground fault, then the ground fault current is approximately

equal to the phase-to-neutral voltage of the faulted phase divided by R. The faulted phase voltage to ground in

that case would be zero and the unfaulted phase voltages to ground would be 173% of their values without a

ground fault present. This is the same phenomenon exhibited by the ungrounded system arrangement, except

that the ground fault current is larger and approximately in-phase with the phase-to-neutral voltage on the

faulted phase. The limitation of the ground fault current to such a low level, along with the absence of a solidly-

grounded system neutral, has the effect of making this system ground arrangement unsuitable for single-phase

line-to-neutral loads.

Figure 11: High-resistance-grounded system with a ground fault on one phase

The ground fault current is not large enough to force its removal by taking the system off-line. Therefore, the

high-resistance grounded system has the same operational advantage in this respect as the ungrounded

system. However, in addition to the improved voltage transient response as previously explained, the high-

resistance grounded system has the advantage of allowing the location of a ground fault to be tracked.

A typical ground detection system for a high-resistance grounded system is illustrated in Fig. 12. The ground

resistor is shown with a tap between two resistor sections R1 and R2. When a ground fault occurs, relay 59

(the ANSI standard for an overvoltage relay, as discussed later in this guide) detects the increased voltage

across the resistor. It sends a signal to the control circuitry to initiate a ground fault alarm by energizing the

“alarm” indicator. When the operator turns the pulse control selector to the “ON” position, the control circuit

causes pulsing contact P to close and re-open approximately once per second. When P closes, R2 is shorted

and the “pulse” indicator is energized. R1 and R2 are sized so that approximately 5-7 times the resistor

continuous ground fault current flows when R2 is shorted.

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The result is a pulsing ground fault current that can be detected using a clamp-on ammeter (an analog

ammeter is most convenient). By tracing the circuit with the ammeter, the ground fault location can be

determined. Once the ground fault has been removed from the system pressing the “alarm reset” button will

de-energize the “alarm” indicator.

Figure 12: Pulsing ground detection system

This type of system is known as a pulsing ground detection system and is very effective in locating ground

faults, but is generally more expensive than the ungrounded system ground fault indicator in Fig. 9.

Three-wire vs. four-wire systems

Regardless of the system grounding arrangement, questions frequently arise in data center power system

design as to whether a three-wire or four-wire system should be employed. The concept of three-wire vs. four-

wire systems was discussed in section “Solidly-grounded Systems” on page 5. In general, a three-phase, four-

wire wye system is distinguished by the use of the neutral conductor. The reason for using a neutral conductor is

to be able to supply single-phase line-to-neutral loads. An example of such a load is 277 V lighting.

Ultimately, if single-phase line-to-neutral loads are not required to be supplied, a three-wire system is sufficient.

Such a system is less expensive due to the removal of the need for neutral busses in power equipment,

running neutral conductors, etc. As discussed in section “Solidly-grounded Systems” on page 5, what is

commonly referred to as a “three-wire system” is actually a four-wire system if the equipment grounding

conductor, as described below, is counted.

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Grounding and bonding definitions and descriptionsThe following definitions are taken from The National Electrical Code [1], section 100 unless otherwise noted.

Where some text for a definition is given in quotation marks (“ ”), the portion of the definition in quotation marks

is from the NEC definition itself, along with additional explanatory text not in quotation marks.

•Bonding Jumper: “A reliable conductor to ensure the required electrical conductivity between metal parts

required to be electrically connected.” Every bonding jumper must be adequately sized to effectively carry all

phase-to-ground fault current likely to be imposed on it.

•Equipment Bonding Jumper: The connection between two or more portions of the equipment

grounding conductor.

•Main Bonding Jumper: The connection between the grounded circuit conductor and the equipment

grounding conductor at the service.

•System Bonding Jumper: “The connection between the grounded circuit conductor and the equipment

grounding conductor at a separately derived system.” The primary function or purpose of the system bonding

jumper is to provide for an applicable reference to earth for the system voltage at the origins of the specific

and separately derived system. (This definition was added to the NEC for the first time in the 2005 Edition.)

•Grounded: Connected to earth or to some conducting body that serves in place of the earth.

•Effectively grounded: Intentionally connected to earth through a ground connection or connections of

sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltage

that may result in undue hazards to connected equipment of persons.

•Solidly Grounded: Connected to ground (earth) without inserting any resistor or impedance device.

•Grounded Conductor: “A system or circuit conductor that is intentionally grounded.” The grounded conductor

carries current during normal operations of the power distribution system. Normal operations imply the absence

of a faulted condition or surge event. The grounded conductor is also referred to as the neutral conductor.

•Grounding Conductor: “A conductor used to connect equipment or the grounded circuit of a wiring system

to a grounding electrode or electrodes.” A grounding conductor is intended to only carry current during an

“abnormal” operation as during a faulted condition or surge event.

•Equipment Grounding Conductor: The conductor used to connect the non-current carrying metal parts

of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode

conductor, or both at the service equipment or at the source of a separately derived system.

•Grounding Electrode: “A device that establishes an electrical connection to the earth.” This definition was

added to the NEC for the first time in the 2005 Edition.

•Grounding Electrode Conductor: The conductor used to connect the grounding electrode(s) to the

equipment grounding conductor, to the grounded conductor, or to both, at the service, at the building or

structure where supplied by a feeder(s) or branch circuit(s), or a source of a separately derived system.

•Ground-fault Protection of Equipment: A system intended to provide protection of equipment from

damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded

conductors of the faulted circuit. This protection is provided at current levels less than those required to

protect conductors from damage through the operation of a supply circuit overcurrent device.

•Ground Fault Current Path: NEC 250.2 defines a ground fault return path as, “an electrically conductive

path from the point of a ground fault on a wiring system through normally non-current-carrying conductors,

equipment, or the earth to the electrical supply source.”

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•Effective Ground Fault Current Path: NEC 250.2 defines an effective ground fault return path as, “an

intentionally constructed, permanent, low-impedance electrically conductive path designed and intended

to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the

electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault

detectors on high-impedance grounded systems.”

In addition, NEC 250.4(A)(5) mandates that the “electrical equipment and wiring and other electrically

conductive material likely to become energized shall be installed in a manner that creates a permanent, low-

impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance

grounded systems.” Also an effective ground fault current path “shall be capable of safely carrying the

maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground

fault may occur to the electrical supply source.” “The earth shall not be considered as an effective ground-fault

current path.”

The following figure shows, in graphical form, several elements of system grounding as defined above:

Figure 13: Elements of system grounding as defined by [1]

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Ground-fault Protection for Solidly-grounded Wye SystemsBecause the ground fault is the most common type of system fault, and because low-voltage systems are

necessarily the largest portion of most industrial and commercial facilities, low-voltage, ground-fault protection

has become a specialized area of development for system protection. These systems are specially designed to

provide sensitive protection, even for four-wire systems with imbalanced loads.

The NEC [1] requires at least one level of ground-fault protection for most solidly-grounded electrical systems

1000 A or more and above 150 volts to ground but not exceeding 600 V phase-to-phase. For this reason, the

ground-fault systems described herein are prevalent in systems meeting these criteria.

Ground-fault protection for low-voltage radial systems is straightforward. For electronic trip units the tripping

logic is typically built into the circuit breaker, and only the neutral CT or sensor must be connected to complete

the ground fault protection system. Such an arrangement is illustrated in Fig. 14:

Figure 14: Low-Voltage Ground-Fault Protection for 4-wire radial system

In Fig. 14 the neutral sensor may be an air-core CT or a conventional iron-core CT. Note that the ground fault

current is diverted around the neutral sensor when it is placed on the load-side of the main or system bonding

jumper. Under normal unbalanced-load conditions the neutral sensor will detect the neutral current and prevent

the circuit breaker from tripping. Note that if the system is a three-wire system without a system neutral the

neutral CT is omitted.

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If the circuit breaker is not equipped with an electronic trip system, an external ground fault relay may be used

with a zero-sequence sensor to trip the circuit breaker. The circuit breaker must be equipped with a shunt trip

attachment in this case. Fig. 15 shows an example of this arrangement. In Fig. 15 the external ground fault

relay is noted as “GS”. In low-voltage systems this is the typical notation rather than “51G,” although “51G”

could be used also. Note that in a three-wire system the neutral is omitted, and the zero-sequence sensor

surrounds the phase conductors only.

Figure 15: Low-voltage ground-fault protection for four-wire radial system with zero-sequence sensor and relay

Because four-pole circuit breakers are not in common use in the USA, the issue of multiple ground current

return paths has a large effect upon ground-fault protection in four-wire systems. To illustrate this point,

consider a secondary-selective system as shown in Fig. 16:

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Figure 16: Secondary-selective system with radial ground-fault protection applied

A ground fault on one bus has two return paths: Through its source-transformer main/system bonding jumper

or the other source-transformer main/system bonding jumper neutral. How much ground fault current flows

in each path is dependent upon the ground or zero-sequence impedances of the system, which is difficult

to evaluate. Therefore, Fig. 16 assumes that a factor of (A x the total ground-fault current) flows through the

source transformer main/system bonding jumper neutral and (B x the total ground-fault current) flows through

the other transformer main/system bonding jumper, where A + B = 1. As can be seen from Fig. 16, the ground-

fault protection for the faulted bus can be de-sensitized or, worse, the wrong circuit breaker(s) may trip.

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The solution is the modified-differential ground fault system. A typical concept example of such a system is

shown in figure Fig. 17:

Figure 17: Modified-differential ground-fault protection for secondary-selective system

In Fig. 17 the breaker internal sensors are shown, but the trip units are omitted for clarity. The ground-

fault function for CB-M1 is noted as GM1, for CB-M2 is noted as GM2, and for CB-T is noted as GT. In

this arrangement, regardless of the ground current dividing factors A and B the correct circuit breakers will

sense the ground fault and trip. Note that this system works regardless of whether CB-T is normally-open or

normally-closed. Non-electronic circuit breakers could also be used, but external CTs and ground relays would

have to be utilized.

For unusual system arrangements or arrangements with more then two sources, the system of figure Fig. 17

can be expanded. These are usually custom-engineered solutions.

Another possible option, in lieu of the modified differential ground fault system, is the use of four-pole circuit

breakers. These switch the neutral as well as the phase conductors, separating the neutrals of multi-source

circuits. This method will not work if sources are paralleled. Four-pole circuit breakers are not common in the

USA for this reason, as well the increased physical equipment sizes they necessitate.

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Principal Purposes of a Bonding and Grounding SystemThe principal purposes for an “effectively bonded grounding system via a low impedance path to earth” are

intended to provide for the following:

Provide for an applicable reference to earth to stabilize the system voltage of a power distribution system during normal operationThe system voltage is determined by how the secondary winding of any power class or distribution class

transformer is actually configured as well as how the windings are referenced to ground or earth. The primary

function or purpose of the system bonding jumper is to provide for an applicable reference to earth for

the system voltage at the origins of the specific and separately derived system to stabilize the voltage.

(i.e., 600 Y/347 V, 480 Y/277 V, or 208 Y/120 V, three-phase, four-wire, solidly grounded, WYE systems)

The system bonding jumper is employed as a direct connection between the Xo terminal of a supplying

transformer, generator, or UPS output terminals and earth. The system bonding jumper is usually connected

within the same enclosure as the power supply terminals and the jumper is not normally sized to carry large

magnitudes of phase-to-ground fault current.

Create a relatively controlled path for ground-fault current to flowThe exact point in time where a phase-to-ground fault might occur can not be determined. However,

depending on the exact point of the phase-to-ground fault within a specific power distribution system, multiple

return paths are likely to occur between the point where the fault conductor makes contact with a conductive

surface and the Xo terminal of the supplying transformer or local standby generator. Consequently, it is

desirable and preferred that the majority of the ground fault current flow primarily in the specific equipment

bonding jumpers and equipment ground conductors directly associated with the faulted circuit. If the

impedance in the equipment bonding jumpers and equipment ground conductors associated with the faulted

circuit is too high, then significant magnitudes of phase-to ground fault current will likely take various other

parallel paths in order to return to the source winding of the power supply. These various other uncontrolled

and unexpected return paths can subject facility personnel to dangerous touch potential differences that can

cause death, injury or permanent damage to internal organs. In addition, other unaffected equipment could be

negatively affected or damaged by potential rises and unintended flow of current.

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Create an effective and very low impedance path for ground-fault current to flow in order for overcurrent protective devices and any ground-fault protection systems to operate effectivelyDuring the time of the phase-to-ground faulted condition the subjected equipment bonding jumpers and the

equipment grounding conductors are intended to function as a very low impedance path between the point of

the fault and the ground bus within the service equipment or the stand by generator equipment. Consequently,

these affect equipment bonding jumpers and the equipment grounding conductors constitute 50% of the

total power circuit during the period in which phase-to-ground fault current is flowing. If the impedance in the

ground fault return path is not low enough, then the overcurrent protective devices employed in the circuit such

as fuses and circuit breakers will be ineffective to prevent substantial equipment damage. In extreme cases the

resulting flow of phase-to-ground fault current might actually even be lower than the rating of the fuses and

thermal-magnetic circuit breakers installed to protect the affected circuit.

For example, in order for fuses or thermal/magnetic circuit breakers to interrupt a phase-to-ground fault in a

480 Y/277 V, three-phase, four-wire power distribution system, the maximum impedance in the ground fault

return path must be lower than is calculated below for specifically rated device:

Table 1: Overcurrent device size vs. maximum impedance in ground return loop for device operation during a ground fault, 480 Y/277 V system

Fuse Size (A) (a) CB Size (A) (b) Maximum Impedance in Ground Return Loop (Ω)*

5 10 18.48

10 20 9.23

15 30 6.12

20 40 4.62

30 60 3.08

40 80 2.31

50 100 1.75

75 150 1.23

100 200 0.924

125 250 0.739

150 300 0.616

175 350 0.528

200 400 0.462

250 500 0.370

300 600 0.308

400 800 0.231

500 1000 0.093

600 1200 0.154

800 1600 0.116

1000 2000 0.093

n/a 2500 0.074

1500 3000 0.062

2000 4000 0.047

* The maximum impedance shown is equal to 277/(3 x fuse size) or 277/(1.5 x circuit breaker size)

AT310 | 22


From the data in the previous example it can be clearly seen that fuses or thermal/magnetic circuit breakers

cannot possibly provide any effective and reliable circuit protection for phase-to-ground fault conditions if the

impedance in the ground fault return path is too high.

Per NEC® 250-4(A)(5) in order to meet the requirements of an effective ground-fault current path “electrical

equipment and wiring and other electrically conductive material likely to become energized shall be installed in

a manner that creates a permanent, low-impedance circuit facilitating the operation of the overcurrent device

or ground detector for high-impedance grounded systems.” The ground fault current path must be capable of

effectively and safely carrying the maximum ground-fault current likely to be imposed on it from any point in a

specific power distribution system where a ground fault may occur to the return to power supply source. Earth

can not be considered as an effective ground-fault current path. Therefore, randomly inserting individual ground

rods into the soil to connect to remote electrical equipment will not provide an effective return path for phase-

to-ground fault current.

The primary function or purpose of the main bonding jumper (or MBJ) located within the service equipment

is to provide a low impedance return path for the return of phase-to-ground fault current from the ground

bus in the service equipment to the respective power supply source such as service transformers, stand by

generators, or the output terminals of onsite UPS via the neutral conductors. The MBJ must be adequately

sized to effectively carry all phase-to-ground fault current likely to be imposed on it. In addition, the MBJ is

another bonding jumper that is often employed to stabilize the system voltage with respect to ground or earth.

However, the MBJ is only a small portion of the ground fault return path for phase-to-ground fault current to

return to the Xo terminal of the respect power source.

Limit differences of potential, potential rise, or step gradients between equipment and personnel, personnel and earth, equipment and earth, or equipment-to-equipmentIt is extremely important that all conductive surfaces and equipment enclosures associated with any power

distribution system be effective bonded together via a low impedance path. As partial explained in paragraph

“Create a relatively controlled path for ground-fault current to flow” on page 18, without a very low impedance

path for ground fault current to flow in a relatively controlled path potential rises or step potential differences are

likely to occur at other locations within the power distribution system. However, during non-faulted conditions

part of the normal load current will flow through the conductive surfaces, equipment enclosures, and earth if

any current carrying conductor is connected to earth at more than one location. For example, if any grounded

conductor (neutral) were to become connected to any conductive surface or equipment enclosure downstream

of the MBJ, then part of the load current will flow through the conductive surface, equipment enclosure, or the

earth because a parallel path will have been created.

Any current flow in any parallel paths that results in current flowing in any bonding or grounding conductor

is identified as objectionable current flow. NEC 250.6 provides information on arrangements to prevent and

alterations to stop objectionable current over grounding conductors. “The grounding of electrical systems,

circuit conductors, surge arresters, and conductive non–current-carrying materials and equipment shall be

installed and arranged in a manner that will prevent objectionable current over the grounding conductors

or grounding paths.” If the use of multiple grounding connections results in objectionable current, then a

facility must discontinue one or more but not all of such grounding connections, change the locations of

the grounding connections, interrupt the continuity of the conductor or conductive path interconnecting the

grounding connections, or take other suitable remedial and approved action.

AT310 | 23


Limit voltage rise or potential differences imposed on an asset, facility, or structure from lightning strikes, a surge event impinging on the service equipment, and phase-to-ground fault conditions, or the inadvertent commingling or unintentional contact with a different voltage systemWhen lightning strikes an asset, facility or structure the return stroke current will divide up among all parallel

conductive paths between attachment point and earth. The division of current will be inversely proportional

to the path impedance Z, (Z = (R2 + X2)1⁄2, square-root of the square of resistance plus square of inductive

reactance). The resistance term should be very low, assuming effectively bonded metallic conductors.

The inductance and corresponding related inductive reactance presented to the total return current will be

determined by the combination of all the individual inductive paths in parallel. The more parallel paths that exist

in a bonding and grounding system will equate to lower total impedance.

Lightning should be considered an extremely stiff current source. The potential difference between the charged

ions in a cloud formation and earth can exceed 100 kV. A given stroke will contain a certain amount of charge

(coulombs = amps x seconds) that must be equalized during the discharge process. For example, if the return

stroke is 50 kA, then that is the magnitude of current that will flow, whether it flows through an impedance

path that consist of one ohm or 1000 ohms. Subsequently, extremely high potential rises, or differences of

potential, can very rapidly result relative to the path impedance. For example, assume a magnitude of current

stroke to be 10 kA. If the impedance in the bonding and grounding system is 1 Ω, then the potential rise across

the impedance of 1 Ω can equate to potential difference of 10 kV. The higher value of impedance in a bonding

or grounding system equates to a higher potential rise between specific points during a lightning event. The

magnitude of current in a lightning stroke can not be controlled.

Therefore, achieving the lowest possible path impedance serves to minimize the transient voltage developed

across the impedance path through which the current is flowing [e(t) = I(t)R + L di/dt)].

The potential rise throughout an asset, facility or structure can occur without a direct lightning strike to the

asset, facility or structure itself. A direct lightning strike to earth near an asset, facility or structure can result in

current flowing in the soil directly under the asset/facility/structure. Subsequently, any asset, facility or structure

above ground level could effectively become a parallel path above the surface of the soil for lightning current

to flow in numerous parallel paths. For example, if the impedance between one end of a single story structure

to the other end of the same structure through the steel frame were 1 Ω and 10 kA of lightning current were

to flow under the building, then the potential rise across the frame of the steel could equate to a potential

difference of 10 kV. Any increase in the resistance through the steel frame of the structure or an increase in

the magnitude of lightning current flowing across the surface of the soil would equate to a higher potential rise

across the frame of the structure. Installations that have assets and structures spread over a relatively wide

geographic area, as large office building complexes or data center complexes, are more likely to be adversely

affected by direct and indirect lightning strikes.

AT310 | 24


Because of the miles of overhead distribution lines employed for the distribution of electricity, it is quite

common for power distribution equipment within a facility to be adversely affected by surge energy impinging

on the service equipment from atmospheric conditions outside of the facility. Consequently, it is extremely

important that the Xo terminal associated with the secondary winding of the service entrance transformer and

the ground bus within the service equipment be effectively bonded together and solidly grounded to earth via a

low impedance path.

Electrical services that are solidly grounded are with respect to earth can be effectively operated within the

voltage rating of the conductors and equipment and are relatively free from excessive system over voltages or

destructive transient over voltages associated with ungrounded systems. The potential rises possible at various

points during a phase-to-ground fault condition are limited by an electrical service that is solidly grounded.

In addition, during such abnormal conditions where by a medium voltage distribution line might drop and

inadvertently make contact with the low voltage service conductors, the solidly grounded service will assist in

limiting the potential rise throughout the power distribution system.

AT310 | 25



Common Electrical System Configurations Employed in Data Center Facilities

Solidly grounded systems in a radial serviceIn addition to the definition of a solidly grounded system in NEC 100, Clause 1.4.6 of IEEE Standard 142, “solid

grounding refers to the connection of the neutral of a generator or power transformer directly to the station

ground or to the earth.” Presently solidly grounded systems are the most common type of power distribution

system employed within the North America.

Solidly grounded power distribution systems provide stable system voltage relative to earth and prevent the

excessive system over voltages and voltage instabilities that are commonly associated with ungrounded power

systems. In addition, phase-to-ground faults are easier to detect and sense. Under a proper installation multiple

levels of ground fault protection can be employed through a power distribution system to selectively remove

a fault circuit from the system. Intended selective coordination is the main reason that most all local utilities or

electric service providers distribution electrical power as three-phase, four-wire, solidly grounded WYE system

and supply service entrance utilization transformers with secondary windings configured as three-phase, four-

wire, solidly grounded WYE.

Fig. 18 and Fig. 19 below indicate how the service entrance equipment with it main overcurrent protective

device are commonly configured and terminated in a typical radial service:

SOURCE 1

GROUND BUS

N

52-1

52-1PS

Ø

52-1X2

X1

2

1NS

Figure 18: Solidly-grounded radial service, one-line diagram arrangement

AT310 | 26


SOURCE (LINE)

NEUTRALDISCONNECTLINE

MAINBONDINGJUMPERGROUNDDISCONNECTLINE

ØA ØCØB

ØA ØCØB

52

X2

X1

NS

52

52

TEST

13

14

15

16

17

18

19

22

R

PS

LOAD

CHASSISGRD

N

N

Figure 19: Radial service, three-line (elementary) diagram arrangement

The most common utilization voltage configuration employed in data center facilities in the USA is 480 Y/277 V,

three-phase, four-wire with a solidly grounded WYE. Therefore, a ground fault protection system is often

necessary. ground fault protection system is a designed, coordinated, functional and properly installed system

that provides protection from electrical faults or short circuit conditions that result from any unintentional,

electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally

non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment or earth. NEC

230.95A mandates that solidly grounded WYE electrical services rated 1000 A or more with a system voltage

of more than 150 volts to ground have a ground fault protection system installed. Consequently, it is very

important that an effective ground fault return path remain in existence at all times. In addition, NEC 230.95C

mandates that “the ground-fault protection system shall be performance tested when first installed on site.”

“The test shall be conducted in accordance with instructions that shall be provided with the equipment.

A written record of this test shall be made and shall be available to the authority having jurisdiction.”

Ground-fault protection of downstream feeder and branch circuits and equipment is not required by the

NEC if protection is provided on an upstream feeder overcurrent device or at the main overcurrent device in

the service equipment. However, additional levels of ground-fault protection on feeder and branch circuits is

recommended so that a phase-to-ground fault any where down stream in the power distribution system does

not interrupt power to the majority or the entire electrical system. Because 95% to 98% of all electrical faulted

conditions originate as phase-to-ground fault, it is recommended that multiple levels ground fault protection

become an essential protection function within important electrical distribution equipment whenever possible

regardless of the minimum NEC requirements.

AT310 | 27


Solidly-ground systems in a multiple-ended serviceMany larger data center facilities employ more than one interconnected electrical service or have large standby

generator onsite that are interconnected to the service equipment as an alternative electrical power source. The

installation and interconnection of two or more electrical supplies into one switchboard or switchgear line up of

service equipment create unique bonding and grounding issues along with challenges for an effective ground

fault protection. Within service or distribution equipment that is supplied with multiple sources of electrical

power the neutral bussing and the ground bussing internal to the equipment are effectively connected in

parallel via two or more main bonding jumpers. Consequently, there are neutral-to-ground bonds downstream

of each main bonding jumper. A common service equipment arrangement is where there is a one common

load bus supplied by two possible power supplies. One of the power supplies would naturally be a power class

transformer provided by a local utility or an independent electrical service provider and other power supply

might represent an alternative electrical source as a large stand-by generator permanently installed onsite.

Refer to Fig. 20 below:

SOURCE 1 SOURCE 2

GROUND BUS

COMMON LOAD BUS

N N

52-1

52-1PS

Ø

N Ø

Ø

52-2PS

52-1

X2

X1

16

1717

1652-2 52-2NS

X1

X2

NS

Figure 20: Double-ended service – Main-main arrangement

AT310 | 28


SOURCE 1 SOURCE 2

GROUND BUS

LOAD BUS NO. 1 LOAD BUS NO. 2

N N

52-1

52-1PS

Ø

N Ø

52-1

X2

X2

X2

X1

1

X1

X117

1716

52-2

52-2PS

Ø

17

1616

52-2NS NS

52-T

52-T

NS

52-TPS

N Ø

Figure 21: Double-ended service – Main-tie-main arrangement

Another common supply and bussing configuration is where two power class transformers supply two

separate load bus. A tie circuit breaker is installed between the two separate line ups of service equipment

and distribution equipment as a means to supply either load bus from either source. This bussing arranged is

commonly identified in the industry as “Main-Tie-Main” configured service equipment. See Fig. 21 above.

In order to eliminate or restrict objectionable current flow NEC 250.24(A)(5) mandates that “a grounding

connection shall not be made to any grounded conductor on the load side of the service disconnecting

means.” However, service or distribution equipment supplied with multiple sources of electrical power have

their neutral bussing and the ground bussing effectively connected together in parallel via the main bonding

jumpers internal to the service equipment or upstream from the distribution equipment. Consequently, some

neutral imbalanced currents are subject to circulate about internal to the equipment between the neutral bus

and the ground bus. Because of the presence of multiple main bonding jumpers, as required by NEC 250-

24C, the service equipment construction results in the interior neutral bus and ground bus functioning as

a parallel current paths for the flow of any imbalanced neutral currents or ground fault currents. Depending

on the number of main bonding jumpers installed within a specific line up of service equipment numerous

circulating current paths are inadvertently constructed within the interior of the interconnected equipment. Such

circulating current flow within the interior of multiple-ended service equipment is normal and expected and is

an expression of Kirchhoff’s Current Law. (The sum of the current entering any junction is equal to the sum of

the current leaving the same junction.) As long as there are no extraneous or objectionable “neutral to ground”

bonds inadvertently connected downstream of the service equipment there is no harm to personnel or the

protective functions of the equipment. However, extreme care should be taken to ensure that interconnected

multiple supply systems do not negate proper phase and neutral current sensing by the ground-fault protection

equipment. A careful engineering study must be made to ensure that fault currents do not take parallel paths to

the supply system, thereby bypassing the ground-fault detection device.

AT310 | 29


NEC 250.6 (B)(6) provides alterations to manage objectionable current flow by taking suitable remedial and

approved actions. The engineering solutions that Square D employs is the engineering and installation of a

Modified Differential Ground Fault (MDGF) protection system to monitor and relay all circulating current flows

during normal operations and phase-to-ground fault conditions. The MDGF protection system insures that

there will not be any inadvertent nuisance tripping of either main or tie circuit breakers during any imbalance

current flows conditions. The MDGF protection system also insures that the appropriate electronic circuit

breakers open to effectively sectionalize the actual faulted bus or cable.

Resistively grounded systemsIn resistively grounded systems a specifically rated resistor is deliberately inserted in series between the Xo

terminal of the service entrance transformer or stand by generator and earth. The employment of resistively

grounded systems are intended to prevent the destructive transient over voltages commonly associated with

many ungrounded system and the high magnitudes of ground fault current associated with some solidly

grounded system. The selected ohmic value of the resistor can result in a low resistance grounded system or

a high resistance grounded system. In high resistance grounded systems the magnitude of phase-to-ground

fault current is usually limited to 10 A or less. In low resistance grounded systems the magnitude of phase-to-

ground fault current is usually limited to range between 50 A and 100 A.

The installation and operations of a power distribution system employing resistively grounded system requires

special engineering expertise and regularly scheduled inspections and maintenance. In addition, operations

and maintenance personnel require specific training to be qualified to maintenance and troubleshoot such

power distribution systems. In addition, in resistively grounded systems no phase-to-neutral loads can be

utilized unless separately-derived systems fed from the main resistively-grounded system are employed (this

is usually accomplished via delta-wye transformers). For these reasons resistively grounded power distribution

systems require careful consideration when used in data center facilities, however with proper system design

such a system can increase the over-all reliability of the data center.

AT310 | 30



Commonly Issues Found with Grounding Systems in Data Center Facilities

Grounded conductor not brought into service equipmentBecause the majority of the 3Ø loads within data center facilities are three-phase, three-wire, there is a

common perception that the grounded conductor or system neutral is not necessary or required. Due to

this, in some cases the grounded conductor is not brought into the service equipment from the supplying

transformer. Such an installation where the grounded conductor is not brought into the service equipment from

the supplying transformer would be a violation of NEC 250.24C. NEC 250.24C requires that the grounded

conductor be brought into service equipment and clearly states that, “where an ac system operating at less

than 1000 volts is grounded at any point, the grounded conductor(s) shall be run to each service disconnecting

means and shall be bonded to each disconnecting means enclosure.”

Without the installation of the neutral conductors between the Xo terminals of the supplying transformer and

the service equipment, there is no effective path for phase-to-ground fault currents to return to the supplying

transformer except through the soil. Employing the earth as a ground fault return path is a violation of NEC

250.4(A)(5) and NEC 250.54.

Structures and utilities not effectively bonded togetherNEC 250.104 requires the bonding of all piping systems and exposed structural steel. This includes metal

water pipes and gas pipes. Bonding the metal water piping system of a building or structure is not the same

as using the metal water piping system as a grounding electrode. Bonding all piping systems and exposed

structural steel to the grounding electrode system places the bonded components at the same voltage level.

The bonding jumper(s) shall be sized in accordance with NEC250.66 and based on the size of the feeder or

branch circuit conductors that supply the building. Of specific note is that the metallic piping associated with

the gas service can not be employed as a grounding electrode for obvious reasons. Also, the effective bonding

of all piping and metal air ducts within a facility will provide an additional level of safety.

EMT conduits with set screw couplings employed as ground fault return pathIn many installations the metallic conduits are employed as the intended ground fault return path. As a cost

saving EMT conduits with set screw couplings is the most common type of metallic conduit installed within my

facilities. The installation and routing of conduits is often overhead and out of reach. Consequently, over time

the set screws often become loose from the effects of vibration, corrosion and rust. Loose fittings will impede

the effectiveness of the intended ground fault return path and return in the violation of NEC 250.4(A)(5).

AT310 | 31


No grounding bushings employedStandard locknuts or sealing locknuts are not acceptable as the sole means for bonding on the line side of

service equipment. NEC 250.92(B)(4) requires other similar devices, be employed such as listed bonding-

type locknuts or bushings. Grounding and bonding bushings for use with rigid or intermediate metal conduit

are provided with means of a clamp with set screws that make positive contact with the conduit for reliably

bonding the bushing and the conduit on which it is threaded to the metal equipment enclosure or box.

Improper or loose connectionsLack of maintenance or improper installation is the usual cause of this situation.

Undersized grounding conductorsPer Clause 4.1.5 of IEEE Standard 142, “one factor that should not be overlooked in designing a grounding

system is the current loading capacity of a connection to earth. The temperature and moisture conditions

immediately surrounding the electrode have a direct effect on the resistivity of this section of the grounding

circuit. Currents passing from the electrode into the earth will have a definite effect on these two conditions.

Therefore, the current loading capacity of a connection must be analyzed from the standpoint of the nature of

the grounding circuit and the types of loading which it can normally be expected to carry.” Per the NEC, “each

bonding jumper shall be sized in accordance with Table 250.66 based on the largest ungrounded conductor of

the separately derived system.”

Lightning abatement system directing lightning currents into the building via connections to building steelAlthough NEC 250.106 allows for the lightning protection system ground terminals to be bonded to the

building structure or the grounding electrode system, allowing lightning current to be intentionally directed into

a facilities building steel will likely results in unintended potential or step gradients between equipment and

personnel, personnel and earth, equipment and earth, or equipment to equipment.

In addition, with few exceptions the Xo terminal of most all 208 Y/120 V, three-phase, four-wire distribution

transformers (sized ≤500 KVA), lighting transformers (sized ≤75 KVA), and PDUs are bonded to the building

steel as allowed per NEC 250.30(A)(6) and NEC 250.104(D). Consequently, when the structure of a data center

facility experiences a direct lightning strike then the high energy from the lightning strike has direct access to the

lower voltage windings of the transformers and all sensitive electronic equipment connected to the load side of

those transformers. Therefore, the connection of the system bonding jumper to the Xo terminal of the transformer

should be effectively connected to the grounding conductor associated with the primary supply circuit.

AT310 | 32


Sheet metal screws employed to secure ground wires or grounding lugsSheet metal screws are often found employed to readily secure ground wires or grounding lugs to the painted

metallic frames of enclosures or terminal compartments. By design sheet metal screws are short and have

wide course threads that do not allow for tight or permanent mechanical connection. Consequently, sheet

metal screw can not satisfy the requirement of NEC 250.4(A)(5) that mandate that “electrical equipment and

wiring and other electrically conductive material likely to become energized shall be installed in a manner that

creates a permanent, low-impedance circuit. “ Consequently, NEC 250.8 requires that, “grounding conductors

and bonding jumpers shall be connected by exothermic welding, listed pressure connectors, listed clamps, or

other listed means. Connection devices or fittings that depend solely on solder shall not be used. Sheet metal

screws shall not be used to connect grounding conductors or connection devices to enclosures.”

Cable trays not effectively bonded and grounded per the requirements of NEC 392.7.Also refer to [6] for detailed information on the manufacturers’ recommendations and requirements for bonding

and grounding cable trays.

No accessibility to an external ground grid system or the grounding electrodeNEC 250.68 requires that, “the connection of a grounding electrode conductor or bonding jumper to a

grounding electrode shall be accessible.” However, in many installation and facilities the connection point(s) to

the grounding electrode is not known and can not be located for assessment or testing.

In addition, NEC 250.50 in the 2008 Edition introduces the importance and requirements for all grounding

electrode described in NEC 250.52 to be effectively bonded together to create a “grounding electrode system”.

Special importance and attention is being focus by many local electrical inspectors (AHJs) when reinforcement

bars as concrete encased electrodes are employed as a grounding electrode or grounding electrode system.

Of specific importance to AHJs are the connections of copper grounding electrode conductors to steel

reinforcement bars encased in concrete. AHJs are wanting to visually inspect the bonds before the concrete is

poured and they are requiring that all copper grounding electrode conductors bonded to steel reinforcement

bars encased in concrete be readily accessibly and available for inspections.

Deterioration of external ground grid systemGrounding systems exposed to soil or to a harsh external environment will deteriorate with time. The rate of

deterioration is a function of the type and concentration of soluble chemicals in the environment, moisture,

temperature and the compactness of the soil.

Lack of documentationDocumentation such drawings or records for the facility’s grounding system design, records of testing of

the ground grid system after initial installation, records of regular inspections and maintenance of grounding

systems should all be maintained.

AT310 | 33



Grounding System Inspections and TestingGrounding system are rarely inspected or maintained. Nowhere within the body of NEC [1] is there any mention

of grounding system inspection or testing. However, if one is diligent enough to actual test the resistivity of the

soil and it is determined that the resistivity is above 25 Ω, then per the NEC all that is required is the insertion

and bonding of a supplemental ground rod to the first rod at a distance of at least 6' away. No initial testing or

subsequent testing is required.

Per Clause 7.1 of [3], “earth resistivity varies not only with the type of soil but also with temperature, moisture,

salt content, and compactness. The values of earth resistivity vary from 0.01 Ω-meter to 1 Ω-meter for sea

water and up to109Ω-meter for sandstone. The resistivity of the earth increases slowly with decreasing

temperatures from 25° C to 0° C. Below 0° C the resistivity increases rapidly. In frozen soil, as in the surface

layer in winter the resistivity may be exceptionally high. Although allow by NEC 250.56 and easily obtained is

many regions of the USA, the resistance value of a single electrode of 25 Ω is by the nature of electrical physics

an excessively high value and can subject a facility to dangerously high potential rise during a lightning strike or

a phase-to-ground fault condition.

Per Clause 4.1.3 of [2], “it is strongly recommended that the resistivity of the earth at the desired location of

the connection be investigated. The resistivity of soils varies with the depth from the surface, the type and

concentration of soluble chemicals in the soil, the moisture content, and soil temperature. The resistivity is that

of the electrolyte in the soil. The presence of surface water does not necessarily indicate low resistivity.”

Clause 4.4.3 of [2] states that, “test should be made periodically after the original installation and test so

that is can be determined whether the resistance is remaining constant or is increasing. If later tests show

that the resistance is increasing to an undesirable value, steps should be taken to reduce the resistance

either by remaking corroded connections, by adding electrodes, by increasing the moisture content, or by

chemical treatment.”

Per Clause 3.1 of [3] the “measurements of ground resistance or impedance and potential gradients on the

surface of the earth due to ground currents are necessary to verify the adequacy of a new grounding system,

detect changes in an existing grounding system, determine hazardous step and touch voltages, and determine

ground potential rise in order to design protection for power and communication circuits.” Earth resistivity

measurements are useful for estimating potential gradients including step and touch voltages.

Clause 8 of [3] prescribes the establish methods for testing and measuring ground impedance. “Connections

to earth in general are complex impedances, having resistive, capacitive, and inductive components, all of

which affect their current-carrying capabilities. The resistance of the connection is of particular interest to

those concerned with power frequencies because it is affected by the resistivity of the earth in the area of the

connection. Ground-impedance measurements are made to determine the actual impedance of the ground

connections, as a check on calculations, to determine the rise in ground potential and its variation throughout

an area that results from ground fault current in a power system and the suitability of a grounding connection

for lightning protection.” Ground-impedance measurements are also made “to obtain data necessary for the

design of protection for buildings, the equipment therein, and any personnel that may be involved. Ground

connections of all power and communication systems should be studied to determine the variation in ground

potential that can be encountered during ground-fault conditions so as to ensure personnel safety, adequacy of

insulation, and continuity of service.”

AT310 | 34


Clause 8.2.1.1 of [3] provides information on the “Two-Point” Test Method.

Clause 8.2.1.2 of [3] provides information on the “Three-Point” Test Method.

Clause 8.2.1.3 of [3] provides information on the “Ratio” Test Method.

Clause 8.2.1.4 of [3] provides information on the “Staged Fault” Test Method.

Clause 8.2.1.5 of [3] provides information on the “Fall-of-Potential” Test Method.

The “Two-Point” and “Three-Point” Test Methods are the most common test methods employed to test

grounding systems.

AT310 | 35



References

[1] NFPA 70®, 2008 National Electrical Code, Quincy, MA: NFPA

[2] EEE Std.142-1991, IEEE Recommended Practice for Grounding of Industrial and Commercial

Power Systems

[3] IEEE Std. 81-1983, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth

Surface Potentials of a Ground System

[4] IEEE Std. 1100-2005, IEEE Recommended Practice for Powering and Grounding Electronic Equipment

[5] IEEE Std. 446, IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial

and Commercial Applications

[6] NEMA Publication VE-2 – 2006, Cable Tray Installation Guidelines

AT310 | 36


Document Number AT310 October 2011 tk

Schneider Electric USA, Inc.

Data Center Solutions1010 Airpark Center DriveNashville, TN 37217615-844-8300sedatacenters.com ©

2011

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at310_system grounding and gf protection for dcs

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