Encyclopedia of Grounding (CA09040E)

contact is expected, in parallel with the worker to provide protection to the worker aloft. Again, the distance from the fully installed set must be considered. In this case, there always will be a full set of protective ground sets present and a low resistance ground set in parallel with the worker, assuring lower current through the worker and rapid removal of the line voltage. Additional distance away from the full set is achieved by adding the jumper to the personal jumper described earlier. Placing the ground sets from the cluster bar to the neutral and from the cluster bar to the phase being worked ensures the worker always will be in parallel with a low resistance ground set. Assume the worksite is now five spans from the installed personal protective ground set on the side away from the source. R L and R N are now (5 X 0.024) = 0.120 Ohms each.

Personal protective jumpers between theworksite and Source A:

= 10 milliampere or 10 volts impressed across the worker I M = I SOURCE X [R J1 / (R J1 + R L + R M + R J2 + R N )] = 10,000 x [(0.001/(0.001 + 0.024 + 1,000 + 0.001+ 0.024)]

I

L R

I

R

M

J1 R

R J2

R

N

Personal protective jumpers opposite from the worksite and SourceB: This is a situation to avoid. I M = I SOURCE X (R L + R J1 + R N ) / (R L + R J1 + R N + R M + R J2 ) = 10,000 X (0.024 + 0.001+ 0.024) / (0.024 + 0.001+ 0.024+ 1,000 + .001) = 490 milliampere Or 490 volts impressed across the worker. Utilities vary on the allowable distance fromthe in stalledset, that is, thenumberof allowedspans. The calculation is based upon their available fault cur rent, selectedmaximumworker voltage, conductor resistance (length and resistance/unit length) and direction tosource. Thismethod requiresbothfield judgments by the maintenance workers and the reviewof the safety and engineering departments of each utility. It will be necessary to adjust both R L and R N in the above example. Modified Worksite Remote from Grounds by Adding a Personal Jumper In some situations, working away from grounds is required to complete the task. As explained ear lier, this can be a hazardous situation. Use of the personal groundmentioned above in theWorksite Remote from Grounds (Limited Distance) section can be modified by adding a protective jumper from the cluster bar to the conductor, as here

I

L 5R

} R EQ

M R

J3 R

R

J1

J2 R

5R

N

R EQ = (R M X R J3 ) / (R M + R J3 )

I EQ = I SOURCE X [R J1 / (R J1 + R L + R EQ + R J2 + R N )] = 41.2 Amp

I MAN = I EQ X [R J3 / (R J3 + R MAN )] = 41.2 microamp

Or a body voltage of 41 millivolts

Now assume the worksite is five spans from the installed personal protective ground sets on the side toward the source. R L and R N remains = 0.120 Ohms each.

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ENCYCLOPEDIA OF GROUNDING