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3 Lightning—a major source of ground potential rise

3 Lightning—a major source of ground potential rise

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TM

APSC FILED Time: 6/10/2015 9:14:01

AM:1692

Recvd

6/10/2015 9:10:59 AM: Docket 14-069-C-Doc. 109

IEEE Std

-2011

IEEE Guide for the Protection of Communication Installations from Lightning Effects



paths for dissipation into the earth and thus lower the resulting GPR to the adjacent equipment building

grounding system.

Fault current blocking through automatic ac disconnect isolation requires protected equipment to have

power back-up systems such as battery, uninterruptible power supply, or battery/generator equipment for

uninterrupted operation during ac isolation. AC service is automatically reconnected after the threat has

subsided below an acceptable threshold.

AC isolation periods for LGPR will nominally be several minutes but are automatically extended if the

LGPR threat condition persists. Isolation periods for power line fault conditions will be several seconds,

but also extend if power line conditions do not return to acceptable levels. AC isolation will respond to

power line sags, swells, and short term transients exceeding threshold levels.

Effective ac disconnect isolation requires preemptive detection of impending LGPR events. Detection of

LGPR from approaching storms and of rapidly changing local surface electric fields provides the

indications of imminent threat.

There is no means to predict power-related fault conditions. AC isolation will minimize the stress caused

by extended poor ac power quality. AC power is reconnected to the protected equipment after it stabilizes

within selected thresholds. As a result, the site is not exposed to power recovery transients following a

power service failure.



7. Locating (siting) towers

Design engineers attempting to keep transmission losses low, along with the real estate considerations,

usually dictate that the equipment building be as close to the antenna tower as possible. This practice goes

against the design of a reliable and robust equipment system to lightning.

The recommended minimum distance between the equipment buildings associated with nearby antenna

towers is (9 m [30 feet]) in order to minimize the effects of the electromagnetic field associated with

lightning and to reduce the risk of damage to equipment circuits. In general, electromagnetic field strength

drops off as the square of the distance. If real estate considerations prevent the building from being more

than 9 m (30 feet) from its antenna tower, then a Faraday cage (wire mesh) around the interior of the

building should be established. Without a Faraday cage, equipment damage cannot be prevented no matter

how well the equipment is grounded or isolated from remote earth.

The recommended minimum distance between the equipment buildings and the towers also contributes to

keep the LGPR at the tower base from saturating the building grounding system, before a majority of it can

be dissipated.

The two grounding electrode systems (for the tower and for the equipment building) must be bonded

together at one single point. However, a bond of 9 m (30 feet) or more will significantly reduce the

resulting GPR at the equipment building due to the impedance of this lengthy bond. This is one of those

rare exceptions in which a lengthy bond is an advantage in supporting a robust grounding system to

lightning.



8. Grounding (earthing) considerations

Take the following grounding considerations into account to reduce the risk of damages from lightning. An

example of a grounding system is depicted in Figure 7.



11

Copyright © 2011 IEEE. All rights reserved.



TM

APSC FILED Time: 6/10/2015 9:14:01

AM:1692

Recvd

6/10/2015 9:10:59 AM: Docket 14-069-C-Doc. 109

IEEE Std

-2011

IEEE Guide for the Protection of Communication Installations from Lightning Effects



NOTE—Reprinted with permission from Duckworth et al.[B10]



Figure 7 —Example of a grounding system



8.1 Grounding impedance

Use the following steps to design a grounding electrode system.

1)



Conduct a four (4) probe soil resistivity test, per IEEE Std 81 [B19], at each proposed tower and

equipment building location to obtain data for an engineering study to design a grounding system

that will meet specified grounding objectives. See items 2) and 3).



2)



Hardening against lightning GPR damage requires specially designed tower radial counterpoise

grounding system with a grounding impedance not exceeding two (2) ohms.

NOTE 1— If the objective is not economically achievable, provide the lowest possible ground impedance

value, using radial counterpoises, to minimize the grounding impedance (and thus GPR) as much as possible.



3)



Hardening against lightning GPR damage requires an associated tower equipment building

grounding system with a grounding impedance not exceeding two (2) ohms.

NOTE 2— If the objective is not economically achievable, provide the lowest possible ground impedance

value, using radial counterpoises, to minimize the grounding impedance (and thus GPR) as much as possible.



4)



The total overall site ground impedance (tower and building) should not exceed one (1) ohm.

NOTE 3— This may require significant real estate space if the site soil resistivity is greater than 500 meterohms at the anticipated grounding electrode depth.

NOTE 4— If the objective is not economically achievable, provide the lowest possible ground impedance

value, using radial counterpoises, to minimize the grounding impedance (and thus GPR) as much as possible.



As tested 5 ohms

5)



Measure the final total site grounding impedance, per IEEE Std 81 [B19] using the three (3) probe

method, at the single point ground (SPG) bar, to verify that the site grounding system meets the

specified objectives prior to the electrical connection of the power multi grounded neutral (MGN)

to site system ground.



12

Copyright © 2011 IEEE. All rights reserved.



TM

APSC FILED Time: 6/10/2015 9:14:01

AM:1692

Recvd

6/10/2015 9:10:59 AM: Docket 14-069-C-Doc. 109

IEEE Std

-2011

IEEE Guide for the Protection of Communication Installations from Lightning Effects



8.2 Grounding requirements

Consider the following items when designing and constructing the grounding system.

1)



All conductors for the grounding system are to be 2 AWG solid bare tinned copper (SBTC).



2)



Use low impedance conductive cement placed around all grounding conductor radial

counterpoises at locations where the soil resistivity is greater than 500 meter-ohms at the

grounding electrode depth.

Follow the items below for the installation procedure:

a)



The trench for the radial counterpoise is to be opened to a depth of a minimum of 457 mm

(18 inches) to a maximum of 610 mm (24 inches) or below the frost line.



b)



Place the conductor centered in the trench.



c)



Then use a 50 mm (2 inches) covering of dry, low impedance conductive cement on top of

the radial conductor. (Moisture from the earth will harden the low impedance conductive

cement within one week).



d)



Then, backfill the trench with removed earth, this will then cover the low impedance

conductive cement and radial wire and will level the earth thereby closing the trench.



NOTE 1— Low impedance conductive cement will not corrode, or crack, and is extremely low in resistivity.

Other materials might change resistivity depending on moisture content.



3)



All ground rods for the grounding system are to be stainless steel, copper, or galvanized steel and

a minimum of 2.4 m (8 feet) in length and 15.87 mm (5/8 inch) in diameter.



4)



All bonds to the grounding system in contact with the earth are to be done by exothermic welding

or irreversible compression connectors listed for the purpose.



5)



Provide an external ring ground, which should include ground rods, for the tower and the

equipment building. The ring ground is to be composed of 2 AWG SBTC conductors placed

below the frost line. Also provide a minimum of 4 radial counterpoises each 7.6 m (25 feet) in

length (see Figure 7). This combination of ring grounds and radial counterpoises provides

capacitive coupling of the lightning high frequency current to earth.

NOTE 2— The scheme described above needs a minimum of 30 m (100 feet) for the total combined length

of the radial counterpoises for best results.



6)



In corrosive environments, consideration should be given to the use of sacrificial magnesium

anodes against the effects of corrosion (to protect grounding system).



8.3 Radial counterpoises

Place the radial counterpoise conductor in a trench (500–600 mm [18–24 inches] in width) and low

resistivity cement, conductive cement, bentonite, or similar material, around the conductor.

The recommended minimum length of each radial counterpoise conductor is 7.6 m (25 feet). If the desired

resistance to earth is not achieved at this length then use longer radial counterpoise conductors in order to

obtain the desired resistance objective. Bond the radial counterpoise conductor to the tower base and to the



13

Copyright © 2011 IEEE. All rights reserved.



TM

APSC FILED Time: 6/10/2015 9:14:01

AM:1692

Recvd

6/10/2015 9:10:59 AM: Docket 14-069-C-Doc. 109

IEEE Std

-2011

IEEE Guide for the Protection of Communication Installations from Lightning Effects



ground ring conductor using an exothermic weld or irreversible compression connectors listed for the

purpose (i.e., below or above ground use).

The ideal number of radial counterpoises recommended is ten (see Block [B5]. The maximum effective

length of each radial counterpoises (see Block [B5]) is 24 m (80 feet) each. Longer length radial

counterpoises will offer little dissipation improvement because the lightning strike current will not remain

on the radial counterpoises for much over 24 m (80 feet).

In sites with limited space (i.e., real estate limitations or restrictions), the recommended grounding system

is, at a minimum, 60 m (200 feet) of grounding conductors. This includes a ring ground of 12 m (40 feet)

and four radial counterpoises, each 12 m (40 feet) in length.

Placing a ground rod in rocky soil is not always practical (costly and not efficient with high resistivity

soils). The use of radial counterpoises in close contact with the rocky soil and covered by low impedance

conductive cement will provide a less expensive and more efficient solution. Anchoring the radial

counterpoises at the end to minimize movement is recommended.

In all cases, the use of low impedance conducting cement (or a similar low resistivity material) placed

around the radial counterpoises at the time of installation will help reduce the grounding impedance of the

radial counterpoise. The low impedance conductive cement will harden into concrete both protecting

(mechanically) the grounding system (giving it many years of additional life), and giving the system a

much better (lower) ground resistance.

Capacitive radial grounding will dissipate the high frequency current components in lightning. Adding

ground rods to a radial counterpoise ground wire will not significantly improve the dissipation of the high

frequency components in lightning.

Ground rods are most effective at the origination location (tower ring) of the radial ground and dissipate the

low frequency components of lightning, including dc.

Direct radial counterpoises placed off of the tower ground ring away (opposite direction) from the

equipment building. Direct counterpoises placed off of the equipment building ground ring away from the

tower.

Radial counterpoises from one structure (i.e., building) may extend around the structure if they do not get

too close to another structure (i.e., tower) or to the radial counterpoises from that structure. If they get too

close they could originate voltage differences and increase the GPR.



8.4 Grounding conductor requirements in equipment buildings

Consider the following requirements for the grounding conductors in equipment buildings.





Do not use U-shaped grounding conductor runs.







When conduit is required, place all grounding conductors in nonmetallic conduit only. If metallic

conduit cannot be avoided, bond both ends of the metallic conduit to the grounding conductor.







Minimize the length of all grounding conductors. Run the conductor as straight as possible

avoiding unnecessary bends, loops, and sharp bends. The minimum bend radius for a 2 AWG wire

is 305 mm (12 inches).







Place grounding conductors through nonmetallic sleeves in floors, walls, ceilings, etc.







Keep runs as short as possible.



14

Copyright © 2011 IEEE. All rights reserved.



TM

APSC FILED Time: 6/10/2015 9:14:01

AM:1692

Recvd

6/10/2015 9:10:59 AM: Docket 14-069-C-Doc. 109

IEEE Std

-2011

IEEE Guide for the Protection of Communication Installations from Lightning Effects



8.5 Interior equipment ground ring (IEGR)

1)



Consider placing an interior equipment ground ring (IEGR) to facilitate common bonding of

equipment and to minimize passage of lightning current through equipment (racks).



2)



Place the IEGR as close to the same height as the building entrance panel (hatch plate or

bulkhead), typically 2–3 m (7–8 feet).



3)



Bond the IEGR to the master ground bar (MGB) which is then bonded to the exterior single point

ground (SPG) bar.



4)



Construct the IEGR of 2 AWG stranded or solid bare conductor mounted in an open loop, that is,

open at one point, around the perimeter of the room at the recommended height. See item 2).



5)



Bond items to the IEGR using 6 AWG conductors.



6)



Bond all equipment frames directly to the MGB when the IEGR is not used.



7)



When protection against radio-frequency interference is necessary, bond metal objects such as

door frames, air conditioners, electrical boxes, cabinets, water pipe, etc., to a closed loop ground

ring. This ring is typically referred to as a halo ring. Do not bond a halo ground ring to fire

protection systems or to any electronic equipment whatsoever (see ATIS 0600334 [B3]).



8)



The halo ground ring is only bonded to exterior building ring ground through the MGB.



9)



Bond the MGB directly to the single point ground.



8.6 AC power grounding electrode

Consider the following items when designing the ac power grounding electrode.

1)



The ac service grounding must meet the requirements of NFPA 70 National Electrical Code®

(NEC®) and any other applicable local code.



2)



A conductor must be placed from where the ac power service ground is derived to the metallic

water pipe system (if present).



3)



Bond the ac power service entrance panel board neutral to the tower building’s MGB.



4)



Bond the ac grounding electrode (ground rod) at the meter to the SPG bar.



8.7 Coaxial cable, waveguide, and building entrance panel (BEP)

Consider the following items when grounding the cables entering the structure.

1)



Ground the coaxial antenna cable shield and waveguide to the tower, at the top and bottom of the

tower, and every 15 to 20 meters (50 to 75 feet) in between.



2)



Place the bottom grounding kit for the cable shield at the bend where the coaxial cable and

waveguide transition from the vertical to horizontal.



3)



Route all antenna cable and waveguide into the equipment building through the BEP. Ground the

cable shields to the BEP.



4)



The preferred coaxial cable entrance into the equipment building and off of the tower is at ground

level for both. This design eliminates the need for large copper straps required to ground the BEP,

and intercepts the tower magnetic field.



5)



The BEP at the equipment building is to be grounded to the building ring ground on outside of

building and to the MGB inside the building.



6)



Place SPDs on the coaxial cable just before the coaxial cable enters the BEP.

15

Copyright © 2011 IEEE. All rights reserved.



TM

APSC FILED Time: 6/10/2015 9:14:01

AM:1692

Recvd

6/10/2015 9:10:59 AM: Docket 14-069-C-Doc. 109

IEEE Std

-2011

IEEE Guide for the Protection of Communication Installations from Lightning Effects



8.8 Communication facility isolation from a lightning induced ground potential rise

Consider the following when addressing the isolation of communication circuits into the facility.

1)



Isolate from remote ground all wire-line (i.e., metallic) communication facilities, i.e., copper pairs

that may enter an equipment building.



2)



Do not use standard communication pair shunting protection, such as GDT or carbon type primary

protectors to protect equipment at lightning GPR locations. They are too slow and will allow the

lightning surge to pass by and damage equipment. Primary solid state hybrid SPD may be used to

protect against residual surges (surges passing through the isolation equipment).



3)



NOTE—In some cases, primary gas tube protectors in combination with secondary solid state



elements may provide adequate protection.

4)



Ground the metallic shields of wire-line communication cables entering equipment buildings to

the single point ground location when wire-line isolation is not used.

NOTE—NFPA 70 requires the grounding of cable shields entering buildings. NFPA 70 also requires primary

protection for wire-line conductors.



5)



When fiber optic cable is used to provide communications services use all-dielectric cable with no

metallic strength members or metallic shield. Place the cable in a PVC conduit (Schedule 80) with

a minimum diameter of 50 mm (2 inches). Locating should be done with electronic (frequencybased) devices or passive reflectors external to the optical fiber cables (see IEEE Std 1590).



8.9 Single point ground

Consider the following when designing a single point ground.

1)



Place a copper ground bar at the single point ground location (in the earth or on the equipment

building).



2)



Provide suitable access (a hand hole) for the bar installed at the single point ground location to

allow for future inspection, maintenance, etc. when placing the bar in the earth instead of on the

equipment building.



3)



Damage to equipment in buildings may be minimized by providing a Faraday cage in the

equipment building’s concrete walls utilizing the rebar within the concrete. A Faraday cage is

established by electrically connecting the rebar cage, ¼-inch rebar at every rebar intersection, with

203 mm (8 inch) rebar spacing on all four building sides including the roof, and bonded to ring

ground.



4)



Use tinned copper bars when the ground bars are placed in exterior locations. Tinning provides

corrosion protection. All connections to ground bars are to be made using two-hole lugs.



8.10 Installation of ground rods and bonding requirements

Consider the following when installing ground rods and determining bonding requirements.

1)



Place all ground rods in undisturbed soil and below the frost line.



2)



Provide suitable access (a hand hole) for the ground rods placed at each corner of the ground rings.

The hand hole will allow future inspection, maintenance, etc., of the ground rods without digging.



3)



Bond external ring grounds to any metal object or structure, including fences, within 2 m (6 feet)

of the ground ring.



16

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