Short T.A. Electric Power Distribution Handbook

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638 Electric Power Distribution Handbook

Ground the shield wire at each pole. Lightning has such fast rise times

that if a pole is not grounded, and a lightning strike hits the shield wire it

will ﬂash over before the grounds at adjacent poles can provide any help in

relieving the voltage stress.

At all poles, obtain a ground that is 20 W or less. Good grounding is vital!

Exposed sections of circuit such as at the top of a hill or ridge should have

the most attention. Getting adequate grounds may require:

• More than one ground rod. Make sure to keep them spaced further

than one ground rod apart.

• Deeper ground rods

• Chemical soil treatments

• Counterpoise wires (buried lengths of wire)

Figure 12.26 shows estimates of performance versus grounding for several

insulation levels based on the approach of IEEE Std. 1410-1997. In order to

ensure that lightning hits the shield wire and not the phase conductors,

maintain a shielding angle of 45∞ or less (as deﬁned in Figure 12.27).

12.7.4 Line Protection Arresters

Arresters are normally used to protect equipment. Some utilities are using

them to protect lines against faults, interruptions, and voltage sags. To do

this, arresters are mounted on poles and attached to each phase. For protec-

FIGURE 12.26

Performance of a shield wire depending on grounding and insulation level.

300 kV CFO200 kV100 kV

10.0 100.020. 200.50. 500.

100

Pole footing resistance, ohms

Percentage of direct strikes

that cause flashovers

1791_book.fm Page 638 Monday, August 4, 2003 3:20 PM

Lightning Protection 639

tion against direct strikes, arresters must be spaced at every pole (or possibly

every other pole on structures with high insulation levels) (McDermott et

al., 1994). This is a lot of arresters, and the cost prohibits widespread usage.

The cost can only be justiﬁed for certain sections of line that affect important

customers.

Arresters have been used at wider spacings such as every four to six poles

by utilities in the southeast for several years. This grew out of some work

done in the 1960s by a task force of eight utilities and the General Electric

Company (1969a, 1969b). Anecdotal reports suggest improvement, but there

is little hard evidence. Recent ﬁeld monitoring and modeling suggest that

this should not be effective for direct strikes. One of the reasons that this

may provide some improvement is that arresters at wider spacings improves

protection against induced voltages. Nevertheless, arresters applied at a

given spacing is not recommended as the ﬁrst option. Fixing insulation

problems or selectively applying arresters at poles with poor insulation are

better options for reducing induced-voltage ﬂashovers.

For direct strike protection, arresters are needed on all poles and on all

phases. The amount of protection quickly drops if wider spacings are used.

Lead length is not as much of an issue as it is with equipment protection,

but it is always good practice to keep lead lengths short. The arrester rating

would normally be the same as the existing arresters.

Grounding is normally not an overriding concern if arresters are used on

all phases. If grounds are poor, one effect is that surges tend to get pushed

out away from the strike location (since there is no good path to ground).

One of the implications is if just a section of line is protected (such as an

exposed ridge crossing), then grounding at the ends is important. Good

grounds at the end provide a path to drain off the surge.

One concern with arresters is that they may have a relatively high failure

rate. Direct strikes can cause failures of nearby arresters. Something in the

range of 5 to 30% of direct lightning strikes may cause an arrester failure.

This is still an undecided (and controversial) subject within the industry. It

FIGURE 12.27

Shield wire shielding angle.

Shield

wire

Phase

wire

Shielding

angle

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640 Electric Power Distribution Handbook

is recommended that the largest block size available be used (heavy-duty or

intermediate-class blocks) to reduce the probability of failure.

Field trials on Long Island, NY, of arresters at various spacings did not

show particularly promising results for distribution line protection arresters

(Short and Ammon, 1999). LILCO added line arresters to three circuits. One

had spacings of 10 to 12 spans between arresters (1300 ft, 400 m), one had

spacings of ﬁve to six spans (600 ft, 200 m), and one had arresters at every

pole (130 ft, 40 m). Arresters were added on all three phases. We also mon-

itored two other circuits for comparison. None of the three circuits with line

arresters had dramatically better lightning-caused fault rates than the two

circuits without arresters. Statistically, we cannot infer much more than this

since the data is limited (always a problem with lightning studies). One of

the most signiﬁcant results was that the circuit with arresters on every pole

had several lightning-caused interruptions, and theoretically it should have

had none. Missing arresters is the most likely reason for most of the light-

ning-caused faults. One positive result was that the arrester failure rate was

low on the circuit with arresters on every pole.

Automated camera systems captured a few direct ﬂashes to the line during

the LILCO study. Figure 12.28 shows a direct stroke almost right at a pole

protected by arresters (arresters were every ﬁve to six spans on this circuit).

Ideally, the arresters on that pole should divert the surge current to ground

without a ﬂashover. The arresters prevented a ﬂashover on that pole, but

two of the three insulators ﬂashed over one pole span away. An arrester at

the struck pole also failed.

Another ﬁeld study showed more promise for line arresters. Common-

wealth Edison added arresters to several rural, open feeders in Illinois

(McDaniel, 2001). ComEd uses an arrester spacing of 1200 ft (360 m); as a

trial, they tightened the arrester spacing to 600 ft (180 m) on 30 feeders (and

all existing arresters that were not metal oxide were replaced). The 30

feeders with the new spacing were compared to 30 other feeders that were

left with the old standard. Over three lightning seasons of evaluation, the

upgraded circuits showed that circuit interruptions improved 16% (at a 95%

conﬁdence level). Note that most of the interruptions were transformer fuse

operations.

Another way to apply arresters is to use them on the top phase only. The

top phase is turned into a shield wire. When lightning hits the top phase,

the arrester conducts and provides a low-impedance path to the pole ground.

Just as with a shield-wire system, grounding and insulation are critical. A

top-phase arrester application cannot be used on typical three-phase cross-

arm designs because there is no top phase.

In areas where grounding is poor and arrester failure is a concern, arresters

can be used with a shield wire system. The shield wire takes away most of

the energy concerns, and the arresters protect against backﬂashovers. This

provides very good lightning protection (and is very expensive).

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Lightning Protection 641

FIGURE 12.28

Lightning-caused fault on a Long Island Lighting Company 13-kV circuit. (From Short, T. A.

and Ammon, R. H., “Monitoring Results of the Effectiveness of Surge Arrester Spacings on

IEEE. With permission.)

FIGURE 12.29

Impact of grounding.

Phase wire

Neutral wire

5 kA

200 ohms

10 kA

= 5 kA(400 ohms) = 2000 kVV

30 kA

5 kA

10 ohms

5 kA

= 5 kA(10 ohms)

= 50 kV

= 2000 kV - 50 kV

= 1950 kV

flashover

= 0V

= 10 kA(200 ohms) = 2000 kVV

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642 Electric Power Distribution Handbook

12.8 Other Considerations

12.8.1 Role of Grounding

With poor grounding, lightning strikes to distribution lines subject more

equipment to possibly damaging surges due to ground potential rise. Light-

ning current must ﬂow to ground somewhere. If the pole ground near the

strike point has high impedance, more of the surge ﬂows in the line, exposing

more equipment to possible overvoltages. At the point of a direct stroke to

a distribution conductor, the huge voltages ﬂash over the insulation, shorting

the phase to the neutral. From there, the lightning travels to the closest

ground. If there is no proper pole ground nearby, the next most likely path

is down a guy wire. If the path to ground is poor, the phase conductor and

neutral wire all rise up in voltage together. This surge (on both the phase

and neutral) travels down the circuit. When the surge reaches another

ground point, current drains off the neutral wire, which increases the voltage

between the phase and the neutral. The voltage across the insulation grows

until a surge arrester conducts more current to ground or another insulation

ﬂashover occurs. Figure 12.29 shows a drawing describing the ground poten-

tial effect, and Figure 12.1 shows a photograph of a direct strike that caused

another ﬂashover remote from the ﬂash point.

Good grounding helps conﬁne possible lightning damage to the immediate

vicinity of the strike. If the distribution insulation ﬂashes over, the short

circuit acts as an arrester and helps protect other equipment on the circuit

(as long as grounds are good).

Normally, the ground resistance right at a piece of equipment protected

with an arrester does not impact the primary-side protection. The primary

surge arrester is between the phase and ground. For a lightning hit to the

primary, the arrester conducts the current to ground and limits the voltage

across the equipment insulation (even though the potential of all conductors

may rise with poor grounding).

Grounding does play a role for transformers, as they are vulnerable to

surge current entering through the secondary. Poor grounding pushes more

current into the transformer on the secondary side and increases the possi-

bility of failure. Poor grounding also forces more current to ﬂow into the

secondary system to houses connected to the transformer. And, poor ground-

ing may also push more current into telephone and cable television systems.

12.8.2 Burndowns

The lightning arc itself can pass enough charge to burn small primary and

secondary wires although sometimes it is hard to tell whether it is the

lightning or the fault arc that does most of the damage. Normally, it is the

power frequency arc that does the most damage. Chisholm et al. (2001) cites

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Lightning Protection 643

a damage rate on Hydro-One’s primary distribution system of 0.13/100 mi/

year (0.08/100 km/year) for an area with a ground ﬂash density of about 1

ﬂash/km

/year. This is about 1% of the lightning ﬂashes hitting circuits.

The lightning arc causes damage the same way that a fault current arc

(and also an arc welder) does: the heat of the arc melts conductor strands.

Damage is mainly a function of the charge in the ﬂash. Lightning tends to

be more damaging than the equivalent ac fault arc. A negative direct current

is about 50% more damaging than an alternating current passing equal

coulombs. Negative ﬂashes are more destructive than positive ﬂashes; it

takes about three times the charge for a positive stroke to cause the same

damage as a negative stroke. Lightning also tends to stay in one place more

than an arc because it does not have the motoring effect.

12.8.3 Crossarm and Pole Damage and Bonding

Although service experience indicates that lightning rarely damages poles

or crossarms; in high-lightning areas, concern is warranted. Darveniza (1980)

cites a survey by Zastrow published in 1966 that found poles failing from

lightning in the range of 0.008 to 0.023% annually (versus an overall failure

rate of 0.344 to 1.074% with more than half of these due to decay). Other

surveys summarized by Darveniza also found minimal lightning-caused

damage. Lightning overvoltages damage and shatter poles and crossarms

when the wood breaks down internally rather than along the surface. If the

wood is green and wet, internal breakdowns and damage are more likely.

Damage within the ﬁrst year of service is more likely.

If historical records show that wood damage is a problem, bonding the

insulators (grounding the base of each insulator) protects the wood, but this

shorts out the insulation capability provided by the wood. Better are surface

electrodes ﬁtted near the insulator pin, including wire-wraps, bands, or

other metal extensions attached near the insulator in the likely direction of

ﬂashover. This local bonding encourages breakdown near the surface rather

than internally.

Preventative measures for lightning damage to wood also reduce the

likelihood of pole-top ﬁres. Leakage current arcs at metal-to-wood inter-

faces start ﬁres (Darveniza, 1980; Ross, 1947). Local bonding using wire

bands or wraps helps prevent pole damage. Bonding bridges the metal-to-

wood contacts where ﬁres are most likely to start. Local bonding is better

than completely bonding the insulators because the insulation level is not

compromised.

12.8.4 Arc Quenching of Wood

Wood poles and crossarms can prevent faults from forming after lightning

ﬂashes across the wood (Armstrong et al., 1967; Darveniza et al., 1967).

Whether this arc quenching occurs depends mainly on the power frequency

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644 Electric Power Distribution Handbook

voltage gradient along the arc at the time of the ﬂashover. If the power-

frequency voltage happens to be near a zero crossing when the lightning

strikes, the lightning-caused arc is likely to extinguish rather than becoming

a power-frequency fault.

The voltage gradient for an arc inside wood is much higher than an arc

voltage gradient in the air. Darveniza (1980) found voltage gradients in wood

on the order of 1000 V/in. (400 V/cm) through wood compared to about 30

V/in. (12 V/cm) for arcs in air. When dry wood insulation arcs, the path is

often just barely below the surface (about a millimeter); and the arc is sur-

rounded by wood ﬁbers, so the arc-suppression action occurs. Wood ﬁbers

raise the arc voltage primarily by cooling it and absorbing ionized particles

(similar to the arc-absorbing action of fuses). The arc voltage gradient is

lower for wet wood (ﬂashovers are no longer fully contained and tend to

arc on the surface rather than just below the surface). There is considerable

variability in the arc voltage, especially for wet wood. To transition from a

lightning arc to a power-frequency fault, the power-frequency voltage must

be higher than the arc voltage to keep the arc conducting.

Darveniza calculated several probabilities for arc quenching based on the

arc voltage gradients (see Figure 12.30). Arc quenching is less likely on wet

wood and when multiple phases ﬂash over (even if the ﬂashovers are on

the same phase). To achieve signiﬁcant beneﬁt, voltage gradients must be

kept less than 3 kV/ft (10 kV/m) of wood. For a 12.47Y/7.2-kV system, this

is at least 2.4 ft (7.2/3 = 2.4 ft or 0.7 m) of wood in the phase-to-ground

ﬂashover paths and at least 4.1 ft (1.2 m) of wood in the phase-to-phase

FIGURE 12.30

Probability of a power-frequency fault due to a lightning-caused ﬂashover over wet wood

crossarms with different numbers of ﬂashover paths. (Adapted from Darveniza, M., Electrical

Properties of Wood and Line Design, University of Queensland Press, 1980. With permission.)

1f, 1 path

3f, 6 paths

3f, 3 paths

1f, 3 paths

0 5 10 15

0.0

0.2

0.4

0.6

0.8

1.0

Voltage gradient, kV

rms

per foot of wood

Probability of a power arc

following a lightning-caused flashover

1791_book.fm Page 644 Monday, August 4, 2003 3:20 PM

Lightning Protection 645

ﬂashover paths. And, if we really want to count on this effect, we should at

least double these lengths.

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