Short T.A. Electric Power Distribution Handbook

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Grounding and Safety 689

disconnected; this may require coordination with other utilities. The NESC

allows using a spark gap as a neutral separation device: the 60-Hz break-

down voltage of the gap must not exceed 3 kV. Other neutral separation

devices are possible although they may not be approved by the NESC (the

NESC allows devices that “perform an equivalent function” as a spark gap).

A saturating reactor has been proposed as an interconnecting device. Under

normal conditions, the reactor is a high impedance (unsaturated), but under

fault conditions, the reactor saturates and becomes a much lower impedance

(but a saturating reactor may allow narrow voltage spikes through). The

reactor has a surge arrester in parallel to conduct lightning-type surges.

Another interconnecting device uses a solid-state switch — when a controller

detects high voltage, it ﬁres thyristors in parallel to short out the neutral

connection. Use neutral isolation devices sparingly. Widespread usage

reduces the effectiveness of the multigrounded neutral since the isolation

devices disconnect the secondary grounds from the primary grounds.

13.5.4 Tree Contacts

Trees cause another possibly dangerous shocking hazard. A tree in contact

with an energized phase conductor may create a touch potential hazard near

the ground. For a tree in contact with one conductor, the resistance of the

tree is high enough to remain a high-impedance connection; it will not draw

enough current to operate a fuse or other protective device, but it may be

enough to create signiﬁcant touch potentials. St. Clair (1999) reported two

electrocutions from tree contacts to 12.47-kV lines, one a tree trimmer and

another at ground level where a man leaned against a tree that had fallen

into conductors.

Daily (1999) analyzed the potentials involved and showed threatening step

voltages. Daily’s measurements of American elms had resistivities from 37

to 55 W-m. A resistivity of 50 W-m translates into 93 W/ft (305 W/m) for a

tree trunk with a diameter of 18 in. (0.46 m). Using a tree resistivity of 50 W-

m and a soil resistivity of 100 W-m, Daily calculated a touch voltage of 140

V along with a body current of 391 mA, deﬁnitely in the fatal deﬁbrillating

range. Lower soil resistivities caused higher body currents (even though the

step potentials were lower).

The body current for a touch potential to a tree involves many factors

including the resistance of the tree, the tree’s resistance to ground in the

earth, the conductor voltage, and a person’s hand and foot contact resistance.

The tree resistance is an important component, and it varies considerably,

especially with the moisture content. Hoffmann et al. (1984) measured a

poplar tree and found the distribution of impedances shown in Figure 13.22,

with impedances of 1.8 kW/ft (6 kW/m) near the base of the tree rising to

25 kW/ft (80 kW/m) near the top of the tree. The total impedance of this tree

was 320 kW, which would draw from 0.2 to 0.7 A from a 12.47-kV line.

Hoffman also found resistances from the base of the tree to ground of about

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690 Electric Power Distribution Handbook

300 W for soil resistivities near 600 W-m. Defandorf (1956) measured resis-

tances to ground in the range of 10 to 40 kW for a tulip tree at typical

distribution conductor heights, which would draw only 0.2 to 0.7 mA from

a 12.47-kV line (7.2 kV line to ground). Hoffman et al. tested a tree contact

to a 20-kV line, which drew 80 mA initially but rose to 600 mA with a

potential at 6.6 ft (2 m) reaching 4 kV. Niagara Mohawk (Finch, 2001) tested

several trees at 7.62 kV from line to ground — the highest was just under

0.5 A (see Table 13.7). Larger tree limbs have lower resistances, so the most

FIGURE 13.22

Resistance measurements on a live poplar tree. (From Hoffmann, E., Rheinbaben, H. V., and

Stosser, B., “Trees in Electrical Contact with High Voltage Lines,” CIGRE Session, 22-03, Sep-

tember 1984. With permission.)

TABLE 13.7

Current Drawn by Several Trees in Contact

with 7.62 kV from Line to Ground

Location Specimen

Maximum Current,

Test 1 Black Gum 92

Test 2 Black Gum 69

Test 3 Black Cherry 110

Test 4 Black Cherry 100

Test 5 White Ash 37

Test 6 Aspen 484

Test 7 Red Maple 125.5

Source: Finch, K., “Understanding Tree Outages,”

EEI Vegetation Managers Meeting, Palm Springs,

CA, May 1, 2001.

10.8 m

0.18 m

1.8 m

f 25 cm

f 20 cm

f 4cm

f 2.5 cm

f 0.3 cm

1kW

12 kW

48 kW

60 kW

150 kW

50 kW

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Grounding and Safety 691

dangerous scenario is a primary conductor solidly contacting a large branch

or the main trunk. Shoes and the insulation that they provide often limit

such currents below the let-go threshold.

Williams (1999) found tree touch voltages for a tree contacting a 12-kV

circuit at 7 ft varied between 90 and 130 V (open circuit). But he found that

the current through a shorting jumper at 7 ft (2.1 m) tied to a ground rod 2

ft (0.6 m) from the tree drew less than a milliamp of current (imperceptible

to some people).

Texas A&M researchers tested several scenarios involving tree contacts to

a single-phase portion of a 12.47-kV circuit (Butler et al., 1999). At 7 ft (2.1

m) the step voltages ranged from 200 to 1000 V. Voltage gradients along the

bottom portion of the tree reached as high as 118 V/ft (387 V/m) during the

tests. The current into the tree during the tests was about 1 to 2 A. Although

they did not make any current measurements through a resistor or jumper,

we can verify that these voltages can produce dangerous currents. For an

800-V step potential across 6 ft (1.8 m) of the tree and 2 A of current ﬂow,

this length of tree represents a resistance of about 400 W. For a body in parallel

with the tree resistance, the current through the body is determined by a

current divider. For this scenario with 2 A of current from the utility primary

through the tree, any body contact resistance less than 3600 W causes a

possibly fatal 100 mA through the body. Trees passing this level of current

hissed and crackled, which hopefully warns bystanders to stay away.

The most dangerous conditions are when the ground is wet and the tree

is wet. Low earth resistivity “grounds” the tree better, so it draws more

current from the line. Foot-to-earth contact resistances are also lower with

drenched soil, which draws more current through your body. A wet tree —

due to rain or humid weather — is more dangerous as the tree’s resistance

drops appreciably.

Strategies to reduce the possibility of tree-related shocks include the

following:

• Educate the public on the dangers of cutting trees in the vicinity of

power lines.

•Trim trees away from power lines.

•Train tree-trimming crews and ground support crews properly.

• Use covered wire instead of bare wire.

Wooden poles can also cause dangerous touch potentials if a phase con-

ductor contacts the pole directly. The U.S. National Institute for Occupational

Safety and Health has documented more than one case where an electrical

worker died when helping guide the butt of a pole into its hole at ground

level after the top of the pole contacted a live distribution circuit. Resistance

of the pole plays an important role. Wood poles have a wide resistance,

depending primarily on their internal moisture content and surface moisture

(the resistance is largely independent of the pole species). Poles normally

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692 Electric Power Distribution Handbook

have higher resistance than live trees because they are drier. Stewart (1936)

measured resistances of approximately 500 W/ft to 10 MW/ft (1600 W/m to

30 MW/m) on a variety of treated and untreated poles. The lowest resistances

are for new poles with moisture contents above 30%. Surface moisture, due

to humid weather or rain, decreases a pole’s resistance. Seasoned dry poles

normally have resistances above 2.5 MW/ft (8 MW/m) when dry and 50 kW/

ft (160 kW/m) when wet.

13.6 Protective Grounding

Extensive repair or maintenance or installation work requires working the

circuit deenergized. Remember the line worker’s gospel:

If it is not grounded, it is not dead!

Or, better yet:

If it is not tested and grounded, it is not dead!

During deenergized work on a structure, protective grounds tie the phase

conductors to a ground point. The main purpose of these protective jumpers

is to protect workers in case the circuit is accidentally energized (another

worker could close the disconnecting switch or another energized circuit

could fall into the deenergized circuit). The jumpers also help protect against

voltages from magnetic or capacitive coupling and backfeed through trans-

former or capacitor banks. With a properly grounded protection system, line

workers do not need rubber gloves.

Utilities use two types of grounding systems: bracketed grounds and

work-site grounds (IEEE Std. 1048-1990). Bracketed grounds are attached on

structures on both sides of the work site. Work-site grounds are attached on

the work-site structure. Work-site grounds protect workers (lower possible

body currents) better than bracketed grounds. Work-site grounds provide

the best protection, but keep them far enough away so that they do not

interfere with the work on the structure and so the work does not disturb

the jumpers. With single-point grounds, it is safest to have the grounding

set on the source side of the work site. But, be careful, many lines may be

energized from more than one direction. Bracketed grounds can effectively

protect workers. Puget Sound Power & Light allows bracket grounds up to

250 ft from the worker (King, 1990). If grounds are farther than one span,

rubber gloves or other work practices may be appropriate (based on a thor-

ough review).

Normally, crews tie protective grounds to the multigrounded neutral.

On circuits without a multigrounded neutral, crews tie bonds together at

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Grounding and Safety 693

a pole band, which may be bonded to a driven ground rod or guy anchor

or other grounding point (see Figure 13.23). Crews may tie individual

grounding jumpers from each phase to the neutral (or other bonding point),

or they may jumper the three phases together and tie another jumper from

a phase jumper to the pole ground. Excessive jumper lengths should be

avoided.

Testing the line for voltage before installing jumpers is safer; attempting

to jumper an energized line is extremely dangerous — a jumper that causes

a fault creates an arc and the magnetic forces can swing the jumper. A variety

of testers attached at the end of live-line tools are available.

When the work is done, the protective grounds should not be removed

until everyone is clear of the work site. When removed, the phase end should

be removed ﬁrst with an appropriate hotline tool.

On three-phase circuits, grounding all three phases is generally preferred

since all three conductors on the pole are grounded. If the circuit is acciden-

tally energized, a three-phase fault has higher fault current and may trip

protective equipment more quickly. A three-phase fault also keeps most of

the fault current out of the ground, which reduces step and touch potentials.

Protective grounding jumpers must withstand the heat generated during

a fault, and they must withstand the mechanical forces. ASTM F-855 (1997)

speciﬁes grounding cables, clamps, and ferrules.

FIGURE 13.23

Work-site protective ground. (From Hubbell/Chance, “Tips of the Trade: Grounding,” 2000.

Bulletin 09-9001. With permission.)

Grounding

Jumper

Neutral

Jumper

Cluster Bar

(Chain Binder)

Below Working

Position

Phase

Conductor

Jumper

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694 Electric Power Distribution Handbook

Maintenance and proper application of grounding jumpers is important.

Always clean conductor surfaces and tighten all terminals. Utilities should

test protective-grounding sets periodically.

Underground work has special protective grounding issues. With insulated

cables, protective grounding at the work site is not always possible. Normally,

the cable is opened and protective grounds are placed at the nearest switching

point, but this may be hundreds of feet (meters) from the work site. In urban

installations, duct banks often have several cables. Faults on adjacent cables

may induce voltages at the work site (see Figure 13.24). The sheath covering

the cable limits capacitive coupling between phases, but inductive coupling

may induce voltages on the cable sheath. Since the cable sheaths are tied

together, a portion of the fault current ﬂows along the cable under repair;

this current induces voltage in the cable phase conductors.

Field tests by Rajotte et. al. (1990) found very small neutral-to-earth volt-

ages in the manhole, less than 20 V on a 25-kV circuit with a 4800-A fault

current. The voltages were low because the circuit tested had low ground

resistivity (50 W-m) and many connections to ground.

On cables, the highest touch voltages occur between the two phases of the

separated cable, and the highest voltages occur when the protective grounds

are far from the work site. The voltages on the phase conductor are induced

by current ﬂow in the cable’s concentric sheath. Since the phase and the

concentric neutral have tight inductive coupling, signiﬁcant voltage devel-

ops on the phase conductor for current ﬂow through the concentric neutral.

Rajotte et al. (1990) measured 169 V on the phase conductor of the cable; the

protective ground on that cable was 1.6 mi (2.5 km) from the work site. The

voltage between the two cables (V

– V

in Figure 13.24) reached 221 V. The

two phase conductors have voltage of opposite polarity, so the voltage

between the two is the sum of the voltages. The best way to reduce the

voltages on the phase conductors is to keep the protective grounds close,

preferably right at the work site.

The voltage induced between the phase conductor and the neutral depends

on the fault current (I) and the distance to the protective ground (d):

FIGURE 13.24

An underground cable under repair with a fault on an adjacent cable. (Adapted from Rajotte,

Y., Bergeron, R., Chalifoux, A., and Gervais, Y., “Touch Voltages on Underground Distribution

IEEE. With permission.)

Working

ground

Work site

Working

ground

Fault

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Grounding and Safety 695

f-N

= k d I

where k is a factor in ohms per unit length that is the coupling factor to the

phase conductor; k depends on the number of cables in the duct bank, their

size, and their position. Rajotte et al. (1990) calculated k = 0.016 W/mi (0.01

W/km) for a conﬁguration with ﬁve 750-kcmil cables and k = 0.1 W/mi (0.06

W/km) for a conﬁguration with two 350-kcmil cables. The voltage between

two cable phase conductors is based on the sum of the length to the protective

grounds on each side since the voltages add.

The Occupational Safety and Health Administration (OSHA) regulates

several aspects of utility operations and equipment related to safety in OSHA

Standard 29 CFR parts 1910 (Occupational Safety and Health Standards) and

1926 (Safety and Health Regulations for Construction). The Lineman’s and

Cableman’s Handbook (Kurtz et al., 1997) also has useful safety information.

Following proper procedures and using common sense are critical. Most

accidents are not from equipment failure (see Figure 13.25); they are from

human failure — carelessness, doing something stupid. The best piece of

safety equipment is a brain — an alert, well-trained, and cautious brain.

References

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FIGURE 13.25

Transmission and distribution fatalities (4 – 46 kV) by cause from 1988 – 1990 from the Inter-

national Brotherhood of Electrical Workers (IBEW) database (n = 66). (Data from [EPRI 1001917,

2002].)

Protective equipment failure (hotstick, etc.)

Mislabeled conductors

Safe working distance not observed

Backfeed

Equipment failure (transformer, etc.)

Clearance error

No ground on line

Not wearing gloves or safety equipment

Human error

010203040

Percent of fatalities

010203040

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696 Electric Power Distribution Handbook

ASTM F855-97e1, Standard Speciﬁcations for Temporary Protective Grounds to Be Used

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