grounding

KNOWLEDGE MANAGEMENT

GROUNDING & EARTHING

M S Suresh

What do they mean?

Earthing Grounding Bonding

Earthing

Earthing is provision of specific return path for fault currents for protective device to operate

Earthing is also connection of electronic circuits to 0V reference line

Grounding

Grounding is connecting electrical circuits and metal parts to planet earth

Bonding

Bonding is connecting adjacent conducting materials together reliably to ensure interconnected objects are at the same potential

Earthing & Grounding

In normal use Earthing and Grounding are mixed up and generally referred for connecting to the planet earth.

Earthing is commonly used in UK and Grounding in USA

In Asia both terms are used. Most commonly used is Earthing.

Earthing Nomenclatures

Clean Earth Dirty Earth Plant Earth Structural Earth Power Earth Neutral Earth Panel Earth Restricted Earth Surge Earth Protective Multiple

Earth(PME)

Screen Earth System Earth Computer Earth Instrument Earth Secure Earth DC Earth Intrinsic Safe Earth Chassis Earth

Ground/Earth Symbols

Ground Point

0V reference(chassis) not connected to ground

Why should we ground?

Grounding is done To prevent potential electric shock hazard. To provide a dedicated reliable low impedance return

path for fault currents so that the fault can be detected and source of power is removed quickly

To minimize the effect of lightning strikes on the installation

To prevent build up of electrostatic discharges To minimize the effect of electrical interference and

provide a signal reference for instrumentation system To provide fault path necessary to ensure the safety of

explosion proof apparatus.

Causes for electrical shock hazard Potential difference between metal parts

and earth causes electric shock when touched.

Potential difference occurs due to: Body of electrical equipments coming in

contact with live wires due to internal fault Static charge developed due to intermittent

contact between non conducting materials

What is the effect of electric shock? Electric shock is the result of current

through the body which interferes with nervous control and functioning of heart and lungs.

The severity of electric shock depends on the amount of current that passes through the body. 1mA Electric shock and pain is felt

5mA Maximum current thru body that is harmless

10-20mA Cannot break away from contact with live parts

50mA Starts interfering with heart functioning

100-300mA

Fatal

How electric shock is prevented?Grounding the metal parts eliminates the potential difference to earth

What is the effect of grounding?

Electric charge in the live parts flows to earth. Earth being large mass the electric charge is

dispersed over large mass and gets neutralized.

Lightning & its effect

Electric charge is developed in the cloud due to moist air currents. When the charge voltage is very high the atmospheric resistance breaks down and lighting occurs.

The potential of the surface where lighting strikes raises very high momentarily until the charge is dispersed in the earth.

Very high potential results in large current flow of the order of 30kA which causes severe damage to the place where lighting strikes

Lightning and Its effect

Protection against lightning

Large Potential difference due to ground resistance

Bond to minimize potential difference

Install lightning arrestor at highest points and ground with low resistance conductor to avoid the lightning current from passing through sensitive equipment

Protecting against lightning

• Lightning on the conductors can be protected by using surge protection devices

Electrical interference The inter-winding capacitance causes stray current to flow through the instrument amplifier and cable capacitance to main supply neutral causing interference problem.Grounding 0V reference line (link X-Y) provides bypass path for interference current and eliminates the path through instrument amplifier

How is grounding done?

Old MethodIn this method, a pit is dug and a large wire mesh is placed. It is covered with charcoal first and then sand. A pipe is inserted to water the pit periodically. Charcoal retains moisture and keeps the ground around mesh wet. Wet ground is good conductor compared to dry ground. Sand fill is to indicate the location of ground pit when some one digs the place accidentally. Sand also helps to rain ground water quickly to the wire mesh.

Ground resistance is low and absorbs electrons quickly due to large surface of wire mesh in contact with the ground.

Expensive to build and requires large area

How is grounding done?

Present Method

In this method a conductive rod is driven in to the ground and surrounding is watered.

This method is easy to make and least expensive

Absorption rate slower compared to old method but effective

Grounding requirementsIS Barriers

A fault current of 100A will raise the voltage at X1 to 100V assuming resistance of ground wire(X1-X) is 1 Ω. This will result in potential difference of 100V between body of instrument and internal components in hazardous area, which is not acceptable. It should be restricted to 10V. This means ground wire resistance should be ≤ 0.1Ω. The ground wire resistance can be higher if the fault current is lower. Standards specify ≤ 1Ω

Body is at 0V and internal circuit is at 100V

Body is at 100V and internal circuit is at 0V

Grounding requirements IS Barriers & Isolators

As in the previous example a 100A fault to ground will result in 100V potential difference between screen and body of instrument in hazardous area. The ground wire resistance should be limited to ≤ 0.1Ω to limit the potential difference to 10V.

Grounding requirementsSurge ArrestorsIs grounding in fig-1 correct?The voltage across conductor and body of protected equipment is Vc+Vt. Vc - SPD limiting voltage . Vt - Surge voltage. Vt can be 1000V or more depending on ground wire resistance. Even though SPD limiting voltage is 16V, the potential difference in protected equipment is 1016V or more. This will damage the protect equipment. In fig-3 the potential difference is SPD limiting voltage Vc thus equipment is protected

Fig - 1 Fig - 2 Fig - 3

Why?

Surge Arrestors groundingDigital signals

Where should the SPD earth be connected?

Connect to Panel Earth in MP

This will protect the Power supply but not the module. Why?

Connect to System Earth.

This will protect the module but not power supply. Why?

Surge Arrestors groundingDigital Signals

Insulate PS from panel and ground to system earth

Surge Arrestor grounding Analogue Signals

Grounding requirementsSurge Arrestors & IS barriers

Grounding wire resistance shall be selected to ensure potential difference in the field device is not more than 10V for maximum fault current of the system. It shall not be more than 1Ω as per standard

Grounding requirementsNon hazardous application

Why run separate ground wire to earth mat?

If there is a power fault in the cabinet, fault current flows from C to G. The potential of C raises due to ground cable resistance but does not affect the potential of A and B. Similarly if there is an external fault on field device current flows from A to G and potential of A raises but does not affect B and C.

G

A

B C

Cabinet

CB

Power fault in cabinet will raise the potential of C due to ground wire resistance. Since B is connected to C, potential of B will also raise and affect sensitive circuits.

Cabinet

Common or Separate Earth?Power fault

The power fault current flows from 0v ground mat to neutral ground mat and back to power supply neutral point. Since the ground resistance is generally high, fault current is low and not sufficient to activate protective devices. Potential shock hazard and damage to sensitive equipment.Lightning strikes may create large potential difference between 0V line and equipment. May damage sensitive equipment.

Fault current

0V

Common grounding Errors

Marshalling Cabinet

System Cabinet

Safety Earth

System Earth

Modul

e

TP

Vc

Vt

Equivalent Circuit

SPD limiting voltage Vc is typically 16V and peak inductive voltage Vt across ground wire can be as high as 1000V. Voltage across the module terminal and body is therefore 1016V. This will damage the module.

To protect the module the SPD DIN rail should be insulated and connected to System Cabinet earth bar as shown in dotted line. The voltage across the module is Vc.

Vc+VtVc

Common grounding errors

Marshalling

Cabinets

Power Distribution Cabinet

Ground wire in power cable does not serve any purpose. It may cause circulating current if the ground mats are different.

Use two core cables and save money

Power Cable

MCB MCB

Common grounding errors

Rack Insulated

Safety Earth

System Earth

Insulation between cabinet and base frame serves no purpose as it is shorted by the grounding of non-insulated earth bar in the cabinet to safety earth mat.

Insulation

Common grounding errors

Rack

System Earth

Safety Earth

Safety Earth has no function as it is not grounding any item.This is done under the false notion that system earth does not provide safety.

Insulation

IMPORTANTGrounding eliminates potential difference between equipment and earth and always provides safety by whatever name it is called

Safety violations

Fan230V AC

L

N

Fused Terminal

Disconnecting Terminal

Both lines are live. It is possible technician may open only one line for attending to problem and will suffer electric shockGround the neutral line

OR

Fan230V AC

L

N

Use Double Pole MCB. Both line and neutral must be isolated at the same time

Grounding - Daisy chain connection

Ground bar in building

A B C D E

G

0.1Ω

0.1Ω

0.1Ω

0.1Ω

1Ω1Ω

A B C D E

100A

Fault in cabinet C will result in fault current flowing from C to G via C-B-A-G and C-D-E-G. Since the resistance is equal the current in two branches will be 50A each as the total resistance to ground is equal. The potential of ground bars in each cabinet ignoring the inductive effect will be:A & E – 50V, B & D – 55V, C – 60V

50A 50A

In this arrangement potential of ground bars in all the cabinets will be affected when there is a ground fault in any cabinet.

Grounding - Star connection

Ground bar in building

A B C D E

G

1Ω1Ω

A B C D E

100A

100A

1Ω1Ω1Ω

Fault current will flow directly from C to G. The potential of ground bar in C will raise to 100V and other cabinets are not affected.

In real life situation since cabinets are connected by mounting frame and metal frames of cabinets, there will be some current flow through other cabinets’ ground bar and there will be some potential raise in all the cabinets. This however will be very small.

Why System Cabinet is insulated?

System

Cabinet

Other Cabin

et

1Ω1Ω

98Ω

100A

1A 99A

1V 99V

100A

100V0V

MΩ

When the system cabinet is not insulated, it is connected to other cabinets via the panel metal parts. If there is fault in the other panel, then part of the fault current will flow through system cabinet ground bar, raising its potential. Insulation electrically isolates the system cabinet from other cabinets and prevents the flow of fault current from other cabinets through its ground bar affecting its potential.

Cable ScreenWhy ground only on one side?

Ground Potential Difference

Circulating Current

Although the different grounds in the plant are bonded, potential difference still exists due to capacitance and inductive current flowing to the grounds. This results in circulating current in the cable screen which induces emf in the conductors.Connect Screen at marshalling end only to ensure all cable screens are at same potential

Junction Box

Marshalling Panel

Grounded or Ungrounded supply UPS

UPS rated 30 KVA and above are three phase type.Insist on grounding the neutral. If the neutral is not grounded the neutral point voltage will not be zero for unbalance loads and will affect individual phase voltages.

R

N

Y

B

Grounded or Ungrounded supplyDI Signals

?

Ground fault in field will result in fault current flow as shown. If the ground resistance between plant earth and instrument earth is high then the fuse will not blow. If the earths are bonded then fault current will be sufficiently high to blow the fuse

Grounded System

Grounded or Ungrounded supplyDI Signals

No change when an earth fault occurs. Earth leak current will flow as shown and ELR will indicate earth fault. To locate the fault, each circuit has to be isolated one by one. This may be difficult in running plant. External fault or interference can affect the signal voltage and prolonged fault condition may damage the module

ELRUngrounded Supply

Grounded or Ungrounded supplyDO Signals

Grounded Supply

?

Ground fault in field will result in fault current flow as shown. If the ground resistance between plant earth and instrument earth is high then the fuse will not blow. If the earths are bonded then fault current will be sufficiently high to blow the fuse

Grounded or Ungrounded supplyDO Signals

Ungrounded Supply ELR

No change in the status. Earth leak current will flow as shown and ELR will indicate fault. External fault or interference may affect the module.A second earth fault will result in short circuit and one of the fuses in the associated circuit will blow. ELR will continue to indicate fault status as the circuit with healthy fuse still has earth fault.

Overall grounding – With SA

Overall grounding – Without SA

References

TP 1121 – A definitive guide to earthing and bonding in hazardous

area (MTL)

TAN 1003 – Earthing guide for surge protection (MTL)

IEC 60364-1 – Electrical Installation of buildings

IEC 60364-4-41 – Protection against electric shock

IEC 60363-4-54 – Earthing arrangements, protective conductors and

protective bond conductors

Further reading

Practical Grounding, Bonding, Shielding and Surge Protection byG. Vijyaraghavan, Mark Brown & Malcom Barnes

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