1

IntroductionThis application note addresses the grounding requirements of modern Telecommunications Systems andFacilities.

Proper ground connections are required for all parts of a Telecommunications System to ensure thecorrect range of operation for ground-return circuits and prevent signal degradation caused by distortion,severe crosstalk, and noise interference. Proper ground connections also prevent dangerous voltagepotentials that can injure personnel and damage equipment. The point where a telecommunicationsfacility or system makes metallic contact to earth is commonly referred to as station ground or site ground,but is sometimes referred to as man-made ground, buried ground, or main ground. This contact point iswhere the equipment ground, the cable shielded ground, and the signal ground are tied together.

This application note is divided into five sections:

• Section 1 Equipment and Signal Grounds

• Section 2 Interior Signal Ground Network

• Section 3 AC Protective Network (Equipment Fault Protective Network)

• Section 4 Tower, Waveguide Bridge, and Waveguide Grounding

• Section 5 Grounding Practices for Shielding Buildings and Rooms

Section 1Equipment and Signal Grounds

Equipment ground, sometimes referred to as the protective grounding system, is required to connect all ofthe following equipment:

• Chassis

• Equipment cabinets and racks

• All ferrous shields and covers

• All conduits

• Raceways

• Cellular floor shields and transformer cases

• All local signaling power supplies

Application Note - Rev. A August 1997P/N 650-0002-01 ECO No. 798

Installation Standards for Grounding Requirements

by Paul Linser

2

Approx4”

Approx8”

Not

Les

s T

han

8’-0

”

Copperweld 5/8”Steel Ground Rod

Approx4’-0”

Magnesium Sulphateand Water

Removable Coverwith Holes

RT260901

Figure 1 - Chemical Treatment of the Earth Around a Given Electrode

The signal grounding system—composed of insulated conductors—connects the ground side of all loopsignal supplies and all the nonferrous shields of signal and signal control cables.

Normally, facility grounding systems are installed during the original construction of a facility. Either a wiregrid or a metal plate—usually copper—is imbedded in the foundation when the concrete is poured. Thiswire grid or metal plate is then connected to an underground metal ring (that surrounds the facility) withvertical metallic poles extending up from the foundation. The actual ground wires are then connected tothe vertical metallic poles.

For a facility without an installed grounding system, earth grounds may be made by driving metal rods intothe ground to form an electrical connection to the earth. The metal rod must be a good electrical conduc-tor, capable of resisting corrosion, and have sufficient area in contact with the soil so that the groundresistance is within the rated limits of 5 ohms. If ground resistance is greater than 5 ohms:

1. Drive the rod deeper into the earth.

2. If the grounding resistance is still greater than 5 ohms, install additional rods spaced at least 6 feetapart, and connect them in parallel.

3. Treat the soil with magnesium sulfate.

Caution: The use of salt is not recommended because of its corrosive effect.

4. Place a 4-foot-long, 8-inch diameter tile pipe in the ground approximately 4 inches from the groundelectrode. Fill the pipe, within one 1 foot of ground level, with a magnesium and water solution—seeFigure 1. Initially 40 to 90 pounds of chemical will be required to retain an effective ground treatmentfor two or three years.

Shielded signal wiring is connected to ground using the braided shield. After properly butting the signalcable and exposing the braid, the installer forms a pigtail or connection-to-ground.

3

Ω ACMinimum 25’

Ω AB

Min

imum

25’ Ω

BC

Minim

um 25’

GroundUnder Test

RT263901

Measuring Ground ResistanceGround resistance measurement requires two reference rods in addition to the grounding rod or platebeing tested. Measure ground resistance by using the triangular method or the fall of potential method.

The triangular method measures the series resistance of each pair of grounding rods. Reference rods areinstalled at least 25 feet from the grounding rod under test, and 25 feet from each other—see Figure 2.

Use a Megger to obtain the value of resistance between ground rods A & B, A & C, and B & C—seeFigure 3.

Results from the triangular method are not relevant if the reference grounds have a resistance of morethan ten 10 times that of the ground under test. Compute ground resistance with the following equation:

(resistance AB) + (resistance AC) – (resistance BC)

resistance A =2

Where: A is equal to resistance of ground under test; B is equal to resistance of reference ground, and Cis equal to resistance of reference ground.

Figure 2 - Triangular Method of Measuring Resistance of Earth Electrode

4

ReferenceGround

B

GroundUnder Test

A

Direct ReadingScale in Ohms

C

P

G

Turn Crankat 100 RPM.

RT263902

Transformer

V

A

Test Leads

Variable Res.

Ground MatBeing Test

P Sets Various Points in a Straight LineBetween G1 and the Ground Mat Under Test.

Distances 5 to 10 Times the Maximum Diagonal Dimensionof Ground Mat, But Never Less Than 100 Feet.

Test Leads

G1Fixed

PMovable

To A-C Power Supply

RT263903

The fall of potential method measures a known current that passes between the current return electrodeand the ground being tested.

Conduct the fall of potential method as follows:

1. Place the potential measuring electrode midway between the electrodes—see Figure 4.

2. Place an ammeter across the known source.

3. Place a voltmeter between the potential measuring electrode and the ground being tested.

4. Record values and compute resistance from the equation R = E/I.

Figure 3 - Use of Megger Ground Tester

Figure 4 - Fall-of-Potential Method - Field Setup

5

Grounding Connecting CablesTo connect grounds, use the following standard insulated cables:

• Signal Grounds, 6 to 12 AWG

• Equipment Grounds, 8 AWG

• Protective Grounds, 4 AWG

• Power Grounds, 4 to 8 AWG (depending on application)

See Table A for recommended Ground Reference Wire Sizes for DC Power Supplies and Battery Facili-ties.

roylppuSrewoPdaoLlluFtuptuOytilicaFyrettaB

)CDspmA(gnitaR

eziSdnaepyTeriWdednemmoceRreppoCerasrotcudnoCllA

)lauqeroGWA(

52-0 *wolleY,detalusni/dednarts/dilos21.oN

001-52 *wolleY,detalusni/dednarts/dilos6.oN

002-001 *wolleY,detalusni/dednarts/dilos4.oN

004-002 *wolleY,detalusni/dednarts2.oN

008-004 *wolleY,detalusni/dednarts0.oN

008 *wolleY,detalusni/dednarts0/2.oN

* edortcelednuorghtraeroiretxeehtotgnidliubehtstixeeriwecnereferCDehtfI.dettimreperasecilpsreporP.GWA4.oNebdluohseziseriwmuminimeht,krowten

109872TR

Table A - Ground Reference Wire Size for DC Power Supplies and Battery Facilities

6

Section 2Interior Signal Ground Network

IntroductionThe Signal Reference Ground Network is designed to:

• Control static charges

• Establish a common reference for signals between sources and loads for proper operation

• Minimize interference

The Signal Reference Ground Network can be configured to serve a facility:

• Operating in the low frequency or low data rate range

• Operating primarily at the higher frequencies or high data rates

• Where both low and high frequency and data rate equipment operates

CompositionThe Signal Reference Ground Network is a composite of various parts. The configuration of these net-works depends on the following:

• The frequencies and types of signals involved

• The functions being performed by the equipment

• The signal amplitude between the communications wires or in the cables

Modern telecommunications systems seldom include a single ground network capable of satisfying allgrounding requirements with optimum effectiveness, while preventing coupling or conduction of undesir-able currents and voltages into the network. To minimize interference from the many potential sources,separate networks are required at complex sites. These networks consist of the following:

• VF (Voice Frequency) signal and shield ground

• Switching DC signal and shield ground

• Power supply (station and switching DC) reference ground conductors

Keep VF signal and shield ground conductors separate, so that the VF shield ground wire can be directlyconnected to a known low noise point—such as the exterior earth electrode network. Likewise, keep theswitching DC signal and shield ground conductors separate to prevent the electrical noise of the switchingDC shields from being conducted to lower-level VF signal equipment. High-level DC switching shields areespecially noisy and should be bussed to the earth ground electrode system separately.

Some facilities—such as a telephone exchange—might consist of a single interconnected network whereall components form one complex ring/grid network. Such a network spreads electrical noise over manyparallel paths, reducing its overall effect on the communication facility.

Keep electrically noisy equipment such as telephone switching, switching DC, and DC power separatefrom susceptible, low-level VF equipment. Separate ground networks are required with a common inter-connection only within the earth ground electrode network.

7

Separation of Signal and AC-Protective Ground ConductorsThe Signal Reference Ground Network must be separate from the AC power distribution system to theextent possible with current communications equipment.

Never ground the AC Neutral to the signal reference ground. The AC-protective connection at the equip-ment cabinet is a design limitation: expectations are that future equipment design will provide separateprotective and signal ground points. DC-powered equipment requires no AC-protective grounding.

Minimizing Electrical NoiseSignal Reference Ground Networks should be designed and installed so that interference coupling can becontrolled. However, not all system and circuit noise problems will be solved by correctly designed andinstalled ground systems, whether they are low- or high-frequency systems. Measures such as reducingthe levels of the interference sources through relocation, insulation, shielding, filtering, or alternate designand installation techniques are more effective in limiting electrical noise to a non-interfering level. Commu-nications equipment can also be designed to have high-noise immunity using the following methods,methods which are currently being applied to new equipment designs:

• Balanced inputs and outputs

• Twisted, shielded pairs

• Higher-level signal operation

8

Single-Point Signal Grounding NetworksSee Figure 5 for a configuration of a Single-Point Signal Reference Grounding Network.

Single-Point Signal Reference Grounding Networks are useful inside equipment and for low frequencyand DC switching grounding applications. For this plan, low frequencies include those below 30 kHz.Single-point grounding consists of a main ground point at a central location to provide a single point ofinterconnection among all grounded components.

RT263904

Sig Gnd

Sig Gnd

Sig Gnd

Sig. GndEqpt Wires

Note

AC Prot.Wires

Sig. GndBR Wires

Sig. GndBox

To OtherEquipmentLocations

Sig. GndTrunk Cable

Ground Rod(3 PLs)

Continuationof Earth GndConductor

Earth GroundConductor Network

To ACPwr House

Main AC PwrEntry PNL

AC PwrDist PNL

C-E Eqpt.AC Pwr PNL

Lightning RodDownlead

(2 PLs-TYP)

Lightning Rod(3 PLs-TYP)

C-E Eqpt.

Figure 5 - Example of Single-Point Signal Grounding Network - Profile View

Note: This method requires separate signal and AC protective ground points within the equipment.

9

RT263905

#8 AWG. STR.INS. Downleads

#6 AWG. STR.INS. Copper Wire

Interior Ring GND#2 AWG. STR. INS

Copper Wire

WG ToTower

GND Rod(Every 20’-25’)

#2 AWG. STR. INSCopper Wire

AC Prot.Wires

2’-4’Earth Ground

Electrode Network

Lightning RodDownlead

(4 PLs TYP)

Lightning Rod(4 PLs TYP)

AC Prot. toAC Pwr House

Main ACPower

Entry Panel

Multiple-Point Signal Grounding NetworksSee Figure 6 for an example of a Multiple-Point Signal Grounding Reference Network—a high frequencysignal reference subsystem. This multiple-point grounding network utilizes many conductive paths fromthe earth electrode network to the various electronic facilities and equipment within the site. Thus,numerous parallel paths exist between any two points in the facility’s ground network.

Figure 6 - Example of Multiple-Point Signal Grounding Reference Network - Oblique View

The multiple-point configuration is very effective in minimizing potential differences between RF and DCequipment. It is also useful in minimizing the effects of RFI (Radio Frequency Interference). This networkgenerally employs two concentric rings. An exterior earth electrode ring network surrounding buildings,towers, antennas, etc., is used in conjunction with an inner ring inside each building that surroundscommunications equipment in the building interior. Multiple bonds between exterior and interior rings—plus connections to antenna towers and metallic objects both in the ground and in the building—form anequal potential plane. By design, most communications facilities are of the multiple-point variety. Numer-ous metallic contacts between equipment cabinets, racks, ducts, cable trays, conduits, and their supportsfrom the building structural elements, plus the AC power grounding interconnections, comprise themultiple paths to ground.

Building structural metal, cable tray, ladders, and ducts can be bonded in an approved manner to aug-ment the deliberate copper wire network—especially in new construction, where the necessary bonding ofstructural metal can be assured. In older facilities, the communications electronics equipment and cablesupports can still be used to fill in the deliberately installed ground wire networks.

10

VF PatchVF Line Cond. & Test

VF Video & MW Eqpt.TTY ShieldGnd Wire CCFB

TTY Sig & ShieldGnd Box*

TTY Eqpt.

VF Sig & ShieldGnd Box*

* Optional

VF Sig Gnd Grid

WGs to Tower

TTY SigGnd Grid

CDF

Mux Mux Mux Mux MWRadio

VF ShieldGnd Wire

Gnd Rod

Earth GndElectrodeNetwork

RT263911

Composite Single and Multiple-Point Grounding NetworksIn many facilities, both high- and low-frequency equipment operates in the same general area. In suchfacilities, a signal reference network is required which consists of a hybrid of both configurations. Ifengineered and installed meticulously, an isolated single-point Signal Ground Network can be providedfor low-frequency or electrically noisy equipment, and a high-frequency ring/grid can be provided for VF,video, and microwave equipment. In a hybrid system, the earth electrode network serves as the tie pointbetween the single- and multiple-point ground networks. See Figure 7 for an illustration of a compositeSignal Grounding Network.

Figure 7 - Example of Composite Signal Grounding Network - Top View

Note: (1) The AC protective wires are omitted for clarity. (2) The TTY equipment must be insulated fromthe VF, video, and microwave equipment except for the common earth ground connection.

11

Separation of the Various Ground ConductorsRegardless of which signal reference configuration is used, certain equipment operating at high levels willrequire special attention to minimize electrical noise. Separate busses for VF and switching DC signal andshield grounding are advisable. All switching DC equipment generates a significant amount of electricalnoise during normal operation. A separate switching DC signal and shield ground bus grounded directly tothe earth electrode network will be effective in keeping much of the electrical noise off the VF signalreference ground network. In addition, cable shields from switching DC equipment should be taken to theearth ground electrode separately.

In a facility containing VF and switching DC equipment in the same general area, the equipment shouldbe physically separated into two groups:

• VF and any associated video and/or microwave equipment in one group

• Switching DC equipment in another group

Placing the equipment in two groups will facilitate separate equipment grounding.

Install separate insulated VF and switching DC ground networks: a grid for the VF and associated equip-ment, and a separate bus for the switching DC and shields. The switching DC bus can be connected tothe earth electrode at more than one point if required. Avoid direct metal contacts through cable trays,ducts, or metal flooring (false floor).

Certain low-level circuits may also require special attention. For example, a separate VF cable shieldground bus can provide lower overall electrical noise on shields if the point at which the bus is connected(such as the earth electrode subsystem directly) has a lower overall electrical noise level than the generalSignal Ground Network.

12

DC Power EquipmentDC power equipment—such as rectifier-chargers, DC-AC inverters, and DC-AC converters—produce alot of electrical noise. This equipment requires only a protective ground conductor to the main AC circuitbreaker, an intermediate panel, or the panel that powers the equipment, in order of preference.

Caution: Never connect DC power equipment to signal ground conductors.

For best results, the DC power equipment should be physically and electrically separate (insulated) fromthe communications equipment. Whenever possible, the DC power equipment should be located in aseparate building, a basement, or a separate room in the communications facility. Note that DC powerequipment is acoustically, as well as electrically, noisy.

DC power equipment is grounded for safety only by the AC protective wire. The effect of electrical noiseconducted by the AC protective wire can be minimized by connecting it to one of the following (in order ofpreference):

• The AC protective bus bar in the main AC entry panel

• An intermediate power distribution panel away from the communications equipment

• The AC panel supplying the DC power equipment

Where the DC power and communications are in separate areas, the AC power panel supplying the DCpower equipment should be used.

The voltage return load bus bar (plus a bar for a negative facility) must be insulated from the equipmentrack or cabinet. Connect the ground reference wire to the common distribution point closest to the loadsor at one central load which is not likely to be removed. At the other end, connect the ground referencewire directly to the exterior earth ground electrode network.

The DC ground reference conductor should have a green/yellow insulating covering or be color codedwith green/yellow plastic tape at key points to distinguish it from other ground conductors. Do not connectany other conductors to the DC ground reference wire. Base the size of the DC ground conductor on thecapacity of the power supply—see Table B.

Table B - DC Ground Conductor Size

roylppuSrewoPdaoLlluFgnitaRtuptuOtnalPyrettaB

)CDspmA(

-eziSdnaepyTeriWdednemmoceRreppocerasrotcudnoclla(lauqeroGWA

)wolley/neergdekramdna

52-0 *detalusni/dednartsrodilos21GWA

001-52 *detalusni/dednartsrodilos6GWA

002-101 detalusni/dednarts4GWA

004-102 detalusni/dednarts2GWA

008-104 detalusni/dednarts0GWA

008revO detalusni/dednarts0/2GWA

* edortcelednuorghtraeroiretxeehtotgnidliubehtstixeeriwecnereferCDehtfI.dettimreperasecilpsreporP.)eziseriwmuminim(eriw4GWAesu,krowten

209872TR

13

Additional filtering is generally required when the noise voltages (ripple, impulse, and wideband) acrossthe supply distribution points of a station VF power supply or battery plant exceed 100 mV p-p. Varioustypes of filters—capacitive, inductive, or LC—are used depending on the noise identified. For electricalnoise of mainly ripple, capacitive filtering is used; for wideband electrical noise, LC filtering is used. SeeFigure 8, Figure 9A, Figure 9B, and Figure 10 which illustrate filter types commonly used.

Today’s technology is rapidly moving toward solid state filtering techniques. Filtering is especially impor-tant in large DC-AC inverter use as the inverters generate electrical impulse noise as high as 6 V p-p.This noise must be isolated from the communications load(s).

Figure 8 - Typical 48V DC Capacitor Filter Panel for Installation in an Existing Fuse Distribution Rackto Provide Additional Filtering of the Station 48 V DC Power Supply

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

C10C9C8C7C6C5C4C3C2C1

Part of 48 V DCCopper Bus BarAcross Back ofFuse HoldersFuse

Panel Alarm Bus

1/4 AFuse

1/4 A Type 70Alm Fuse(2 PLs.)

30 ASlo-BloFuse

30 ASlo-BloFuse

#6 Awg, Ins, Stranded Black

To FuseAlm Pnl

#6 Awg, Str,Ins, Wht

#12 Awg,Ins, Wht Part of 48 V DC Return Bus

Ear at Top of 48 V DC FuseDistribution Rack

RT263906

#12 Awg, Ins, Blk

Note: (1) Construct using a suitable panel for rack mounting. (2) Install in fuse distribution rack. (3) C1 toC10, 10,000 - 20,000 µF, 75 WV DC. (4) The capacitor filter is very effective for frequencies belowapproximately 100 kHz.

14

RT263907

C120,000 uF

L115 A

10 mH

75 WV DC

Fuse Alm

Typical-48.8 V DC

Input

+

Typical -48.5 V DC@ 10 A Output

Attenuation:20 dB @ 200 Hz45 dB @ 4 kHz

RT263908

C1 C2

L130 A

5 - 20 mH10,000 - 20,000 uF

75 WV DC

Fuse Alm

Typical-48.8 V DC

Input

+

Typical -48.5 V DC@ 25 A Output

10 Hz to 25 MHzNoise < 100

mV p-p

Figure 9A - Typical LC and PI Filter

Figure 9B - Typical LC and PI Filter

Note: Typical LC communications equipment decentralizing filter for loads up to 10 A DC. LC filters forother voltages and larger loads of 25 A, 50 A, and 100 A are available or can be constructed. The LCfilter should be used where an input capacitor already exists and wideband (high -and low- Fre-quency) noise filtering is required.

Note: Typical 48 V DC P1 communications equipment decentralizing filter for loads up to 25 A DC. P1network filters for other voltages and all types of loads are available or can be constructed. The P1network filter is used where no input capacitor exists or greater effectiveness is required.

15

C1 C2 C3 C4

C1 C2 C3 C4

L13-5 mH

+

-

DC Filter #2 (Noise 2)

DC Filter #1 (Noise 2)

RT263909

(Note 3)

(Note 3)

L13-5 mH

LoadBars

(Note 1)

150 A

Slo-BloFuse

150 A

Slo-BloFuse

5-kVA Inverter(Main)

5-kVA Inverter(Hot-Standby)

Control Rack Inverter Cabinet

Note: (1) This additional filtering is only required if the electrical noise on the common load bars isexcessive as determined by communications equipment operation. Generally, 1000 mV p-p is permis-sible where decentralizing filters are also used for the sensitive equipment. (2) Each inverter inputmust be filtered separately. There may be enough stored energy in filter inductor L1 to blow the fuseof the standby inverter in case of failure (open or short) of the main inverter if the two inputs areconnected to one filter. (3) C1 ,C2 , C3, C4 -- 35,000 µF, 75 WV DC.

Figure 10 - Typical 48 V Filtering of Large Inverters

16

Interior Signal Grounding Conductor Types and ColorsInterior signal ground wires should be copper, solid or stranded, and insulated with green/yellow covering.Yellow should be reserved for shield ground wires, and green for all protective wires. Existing wires neednot be replaced merely for the color of insulation. Instead, use colored tape around the existing wire atkey places to color-code signal, shield, and protective grounding wires. Make ground conductor intercon-nections with bronze, brass, or copper tap connectors or cross lugs. Where movement due to vibration orcontact with AC and DC power equipment or metal pipes is anticipated, insulate connectors with plastictape or covers. Make connections to equipment with terminal lugs and screws, or screws only when astrap is used. Minimize the use of dissimilar metals.

If the possibility of corrosion exists because of dissimilar metals, use a compatible washer and suitablecovering between the dissimilar metals. Scrape equipment connections clean of insulation or corrosion toexpose bare metal prior to connection. If exposed to weathering, the joint should be covered with siliconeor another suitable covering after connection. See Table C, Table D, and Table E for metal compatibilityand ground conductor sizes for various exterior and interior grounding applications.

Table C - Strap and Wire Types and Sizes for Grounding Applications

noitacilppAeriWreppoCniegnaR

)GWA(seziSdednemmoceRlacipyT

)GWA(

sdnuorGgniRedistuO 0/4#ot2#,partsleetsdezinavlagdilosmm5.3x03

ropartsreppocdiloshcni61/1xhcni1eriwerabdennitdilos2#

sdaelnwoDdoRgninthgiLrewoT 0/1#ot4# eriwerabdilos2#

gnidnuorGediugevaWediw-hcni-1oteriw6#

diarbreppocnevow

ropartsreppocdiloshcni61/1xhcni1roerab,eriwdednartsesraocfodilos6#

detalusni

dnuorGevitcetorPCA MCM008ot41# neerg,detalusni,dednartsrodiloS

seriWdnuorGlartueNCA MCM008ot41# yargroetihw,detalusni,dednartsrodiloS

dnuorGrotcetorPegatlovrevOseriW

4#ot8#,detalusni,dednartsesraocrodilos6#

neerg

dnuorGedisnIotdnuorGgniRdirG

0/4#ot2#,detalusni,dednartsesraocrodilos2#

wolleyroneerg

)gniR(seriWdnuorGedisnIniaM 0/2#ot4# wolleyroneerg,detalusni,dednarts2#

)dirG(dnuorGniaMmorfsrupS 4#ot6# wolleyroneerg,detalusni,dednarts6#

edisnIniaMmorfsdaelnwoDskcaRrostenibaCotdnuorG

6#ot01# wolleyroneerg,detalusni,dednarts8#

kcaRrotenibaCedisnI 01#ot81#,detalusni,dednartsrodilos21#

wolleyroneerg

ssenraHeriWdnuorGFDC 01#ot41# wolley,detalusni,dilos21#

skcolBlanimreTotseriWdnuorGFDCno

21#ot81# wolley,detalusni,dednartsrodilos81#

ecnerefeRylppuSrewoPCDsrotcennoCdnuorG

0/4#ot21# noitacilppanotnednepeD

309872TR

17

Table D - Metals

slateMfoseireScinavlaG,evitcA,dnEcidonA

evissaP,dnEcidohtaC-gnicifircaS

muisengaM

syollAmuisengaM

cniZ

munimulA

muidaC

norIroleetS

norItsaC

)evitca(leetSsselniatS8:81

)evitca(leetSsselniatS3:8:81

sredloSniTdaeL

daeL

niT

)evitca(lekciN

)evitca(lenicnI

)evitca(CyolletsaH

ssarB

reppoC

eznorB

syollAlekciNreppoC

lenoM

redloSrevliS

)evissap(lekciN

)evissap(lenocnI

)evissap(leetSsselniatS8:81

)evissap(leetSsselniatS3:8:81

)evissap(CyolletsaH

revliS

etihparG

dloG

munitalP

:etoN evissaproevitcasadetacidnislatemehTehtfoerutanehtnognidnepedsnoitisopegnahc

.muidemevisorroc

409872TR

18

Table E - Size of Equipment Grounding Conductors for Grounding Raceway and Equipment

tnerruCcitamotuAfognitaRfodaehAtiucriCnieciveD

toN,.cte,tiudnoC,tnempiuqE)spmA(gnideecxE

)GWA(eziSeriWreppoCdalCreppoCromunimulA

)GWA(eziSeriWmunimulA

51 41 21

02 21 01

03 01 8

04 01 8

06 01 6

001 8 6

002 6 4

004 3 1

006 1 0/2

:etoN .koobdnaHedoCcirtcelElanoitaNehtsielbatsihtrofecruosehT

509872TR

19

Section 3AC Protective Network (Equipment Fault Protective Network)

The AC Protective Network is designed for protecting personnel and equipment in the event of a malfunc-tion (through an electrical short to a metal equipment enclosure or frame) by causing protective devices tooperate promptly. To accomplish this function, ground connections must be adequate for fault currents tentimes or more the normal value for a limited time period, usually less than one second. In general, the ACProtective Network must conform to the requirements established in the National Electrical Code.

The AC Protective Network should generally follow a crows-foot configuration from a central or mainground point which, ideally, should be located at the primary station transformer ground point. It shall beconnected directly to the earth electrode subsystem at the same point as the AC Neutral line from thecommunications equipment. Note that the AC Protective Network is in addition to any Signal GroundNetwork. The Signal Ground Network cannot be relied upon to operate protective devices promptly. Apath all the way back to the AC power source may not exist. Conversely, the single-ended AC protectivenetwork cannot ensure an equal-potential ground for all equipment, such as a ring/grid network can. Bothare generally necessary to fulfill all functional requirements at the site/station.

To protect personnel from exposure to hazardous voltages, all exposed metallic elements of AC-poweredelectrical and electronic equipment are connected by means of the AC protective wire (normally green).This ensures that in the event of inadvertent contact between the hot lead and the chassis frame (orcabinet) through human error, insulation failure, or component failure, a good, direct, known, fault currentpath is available to quickly remove the hazard.

20

Install a separate AC protective conductor with AC power cables if not already provided in the cable itself.When run as a separate conductor, place it in the same conduit or duct with the phase and AC neutralconductors for best results. If this is not possible, it should be installed alongside the AC conduit or ductand connected to the power panel protective bus bar. Never connect the AC protective conductor to thesignal reference ground conductors. A minimum 2-inch separation should be maintained between ACprotective conductors and communications or signal reference conductors if run parallel and not encasedin conduit or ducts.

Pay special attention to situations that might result in flashover or a capacitive discharge between equip-ment, cabinets, or racks caused by operating personnel walking between them. This may occur frompower faults or lightning strikes where metal components that could store a static charge are not ad-equately bonded to a discharge path. Accepted lightning protection practices dictate that, within a metal-roofed or metal clad building, all metal within 6 feet of the metal sides, roof, or down conductor must beconnected to the external metal or down conductor network. Also, all cabinets and racks that are closetogether must be bonded to each other.

By proper segregation of ground routes, careful location of ground nodes, and elimination of unnecessaryground loops, one can avoid the flow of objectionable currents between two grounding points having agiven ground resistance or impedance.

This is the basis of ground segregation networks as discussed above. Provide separate ground paths forcircuits which have different functions and sensitivities, and avoid ground loops (mandatory). A typicalground segregation in a complex system should consider separate grounds for the following:

• Low-level analog returns

• High-level signal returns

• Protective shield grounds

• RF coaxial shield returns

• High-speed logic returns

• Power returns

• Safety

• Lightning

Some of these conductors will probably have one point in common. To avoid circulating currents amongthe different layers of the grounding hierarchy, make sure they do not have more than one common point.

21

Section 4Tower, Waveguide Bridge, And Waveguide Grounding

Tower GroundingConnect towers to the earth ground electrode network underneath at all legs. The tower ground networkshould consist of ground rods not less than 8-feet-long and interconnected by means of #2 AWG bare ortinned, solid or coarse, stranded copper wire or a 30 x 3.5 mm galvanized steel strap. For very tall, squaretowers with wide bases, install 10-foot rods and an additional cross from the diagonal legs—see Figure11. Radial wires extending from each leg can be added for extremely tall towers in areas of severelightning activity. Make wire leads or strap connections as short and direct as possible.

RT263910

Note 1

#2 Awg Solid Copper WireBuried 18” Below Grade

Tower LegBase

Note 2

Cross Only ForVery Large Towers

Shortest Direct Routfor Ground Strap toGround Electrode

8 FT. (Minimum)Copper-Clad Steel

Ground Rod

Figure 11 - Tower Grounding

Note: (1) Normally, two separate conductors are used to interconnect the tower ground ring with thebuilding ground ring. One of these should follow the route of the WG support structure. If a secondWG support is used, the second interconnecting ground conductor should follow the same route,since the WG supports require grounding also. (2) Optional radial wire to lower the ground impedance(1 to 4 places) - required only in areas of severe lightning activity.

22

Steel TowerMember

Waveguide

Earth ElectrodeRing

#6 AWGWire

Ground Rods#2 AWG GroundConductor Buried

18” to 24” Below Grade

Earth ElectrodeRing

LightningRod

RT263912

Figure 12 - Grounding of Waveguide and Supporting Structures

Equip all concrete and nonmetal towers with lightning rods. Also equip metal towers with lightning rodswhere antennas, solar panels, or obstruction lights are mounted at, or near, the top. Downleads in goodcondition are required for metal towers with tubular/flange legs and all concrete or other nonmetallicconstruction. For wiring sizes and types, see Table C.

As a minimum, connect waveguide bridges to the exterior earth ground electrode network at the first andlast support columns. Make all wire leads as short and direct as possible. Connections in the earth shallbe mechanically strong and brazed or welded. Above ground connections can be bolted.

Ground waveguides to the tower and earth ground electrode network at two points: just above the verti-cal-to-horizontal transition near the base of the tower, and at the waveguide entry port. In addition, groundwaveguides connected to antennas at or near the top of the tower shall be grounded to the tower near theantenna—see Figure 12.

Note: Only the waveguide to the topmost antenna need be grounded at the top. All waveguides will begrounded near the bottom of the tower and at the waveguide entry port.

Use solid copper strap or wire equal to #6 AWG to ground waveguides. Coarse-stranded, insulatedcopper wire can be used if solid wire is unavailable. Do not use braid or fine stranded wires. Make surewire leads are as direct as possible, with no up- or right-angle bends.

23

Section 5Grounding Practices for Shielding Buildings and Rooms

Because of either the severity of outside interference threats or the vulnerability of the inner system,electomagnetic shielding is often required at building or room level. Some examples are as follows:

• Buildings where EMI (Electromagnetic Interference)/RFI (Radio Frequency Interference)testing is performed

• Hospitals using NMR (Nuclear Magnetic Resonance) scanners

• Buildings which host sensitive electronic equipment and are near powerful broadcast transmitter, radar, etc.

• Embassies, government facilities, and industrial headquarters handling confidential data,where eavesdropping must be prevented.

Whether the entire building, or only the specific room, is to be shielded, will depend on the application.

Currently, shielding an entire building is rare, but this practice may become more common as the numberof EMI occurrences increases. This solution is viable when the building contains many rooms which needshielding and when target attenuation levels are rather modest—40 dB, up to approximately 100 MHz.The walls can be covered with aluminum foil, conductive textile, metal mesh, or conductive paint.

In addition to all the mounting precautions to avoid seam leakage, two general grounding practices shouldbe followed:

1. Bond cable conduits, pipes, shields, etc. to the building shield at their points of penetration,since the whole building now constitutes a Faraday cage.

2. Connect the building overall shield to the earthing conductor for added safety.

When superior attenuations are required—for example 80, 100, or 120 dB— a real shielded room mustbe installed. To avoid alteration of the Faraday cage performance from installation, the following principlesshould be observed:

1. Connect the shielded room to the building safety ground conductor. This connection must bemade at the exterior of the room, preferably at the power input box or filter. Metal ducts,pipes, etc., must be electrically interrupted with a dielectric spacer at the point where theypenetrate the cage. Then, equip the entry hole with a waveguide or honeycomb barrier topreserve the high attenuation of the room to HF fields.

2. The grounding of the room down to the earthing terminal can be made by a dedicatedconductor, but take precautions to avoid ground loops by making no other ground connectionto the shielded room.

3. Do not use a dedicated earth rod since it would create both a noise problem and a safetyhazard (that is, two earthing resistances in series) in case of a power fault to ground. SeeFigure 13 and Figure 14 for examples of some common mistakes in grounding a shieldedroom and their corrections.

24

A/C Heat Ducting

FiltersDisconnect

Box

ShieldedEnclosure

Excitation &MonitoringEquipmentP

lem

um

Safety (Green) WirePanel Service

Entrance

Earth Impedance

EarthingRod

a. Typical Grounding of Shielded Enclosure RoomShowing Multiple Ground Loops

A/C Heat Ducting

DisconnectBox

ShieldedEnclosure

Safety (Green) Wire

Panel ServiceEntrance

RF Chokes &Shielded Power Line

Isolation Transformers

AsbestosBellows

Break Conduit& Safety Wire

b. Eliminating Ground Loops Including Shielded Enclosure (One Approach)

RT263913

Figure 13 - Bad and Preferred Grounding Practices for Shielded Rooms or ShieldedEnclosures in a Building

25

A/C Heat Ducting

DisconnectBox

ShieldedEnclosure

Safety (Green) Wire

Panel ServiceEntrance

RF Chokes &Shielded Power Line

Isolation Transformers

AsbestosBellows

Break Conduit& Safety Wire

Triple Shielded IsolationTransformer & Ground

RT263914

Figure 14 - Eliminating Ground Loops - Second Approach