Encyclopedia of Grounding (CA09040E)

circuit resistance (see Section 5 for a discussion of series resistances).

The connection to overhead static or shield wires are of questionable benefit for use as a low resis tancepath for the returnof fault current andshould be evaluated before use. Many are not continuous to the power source, therefore, cannot be con sidered a full current return path. Most are steel conductor, which has a much higher resistance than a conductor designed to efficiently carry current. The higher resistance may become hot enough to fuse, depending upon the current level, resulting in its loss as a return path for protection if used alone. They may be used as a secondary current return path in addition to a primary return path as a means of increasing the margin of safety by pro viding multiple paths to and through the Earth. If the static or shield wire is included as part of the “work area” it should be electrically connected to the personal protective grounds at the worksite to extend the equipotential work zone. The use of the Earth alone represents a usable cur rent returnpath for personal protectivegrounding. It has higher resistance than a conductor designed to carry current. This will lower the fault current because its resistance is greater than the con ductive neutral, but possibly not to such a level that the system protective equipment would fail to recognize the fault. However, the resistance of the Earth varies widely. In areas of dry, sandy soil conditions the resistance may approach several hundred ohms. In a moist soil it may be in the low to mid teens. At the Hubbell Power Systems research laboratory, Centralia, MO, the Earth re sistance approaches 18 Ohms. If the neutral is broken or fuses during a fault and it was the only return path to the source, worker protection could be lost. A current return path through the Earth could be used as a back-up path for the system neutral. The use of multiple jumpers and return paths is encouraged. Because this presentation is about accidents and avoiding accidents, a “belt and suspenders” approach may be prudent.

I

R 1

I J

I M

V 1

V SOURCE

V J

V M

N V

R N

Voltage Division Using the Neutral Fig. 7-1

V 1 is about equal to V N if they are about the same length and conductor size. The voltage of the conductor andneutral connections at theworksite will be about equal voltage because of the small voltagedropof the jumper, whichwewill discount.

V 1

= V N = V SOURCE x [R N / (R 1 + R N )]

but R 1 = R N so V 1

= V SOURCE x [R N / (R N + R N )] or = V SOURCE x [R N / 2R N ] or

V 1 V 1

= V SOURCE / 2

and

I J

= I SOURCE

V J

= V SOURCE - (V 1 + V N ) [a very small difference]

or V J

= V M

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7-3