Encyclopedia of Grounding (CA09040E)

THEORY OF PERSONAL PROTECTIVE GROUNDING

This “equipotential zone” limits voltage across the body toa suitably lowvalue toprovide the required measure of safety. Again referring to Equation 2 (I = V / R), by estimating the body resistance and keeping the voltage below the safe level selected by the employer, the desired measure of safety can be achieved. The reduction in body voltage is achievedby limiting themaximumvoltage that can bedevelopedacross theparallel circuit composed of both the body and the jumper. Information on thepersonal protective jumper is aknownquantity. The jumper also will carry the largest amount of current compared to the body and can be used to develop the neededparallel voltage level. Again, it is the responsibility of each employer to specify a level of acceptablebody voltage. At present, there are no standards that specify a value to be used. The key to a successful equipotential zone protection method is to place the worker in a parallel path with a conductor of sufficiently low resistance such that the rise in voltage is held at or below the selected level. The maximum jumper voltage is shown by Equation 2 (V = I X R). Shunting the fault current around the body, through the low resistance path, is the first key. Remember that some current will flow in every possiblepath, but it divides in inverseproportion to the path’s resistance. The use of a low resis tance jumper is the major factor. The second key factor is to have the line protection equipment provide fast fault removal. The use of the system neutral provides a low resistance path for the return of a fault current if it occurs. This does two things: It maximizes the fault current and tends to lower the voltage at the worksite. The maximum fault current ensures the fastest clearing possible of the fault by the sys tem’s protective equipment, such as circuit break er, reclosers, fuses, etc. The reduction in voltage occurs because the neutral conductor resistance is of a similar magnitude as the source conductor. The source and neutral conductors form a series circuit of two resistances, and a division of voltage results. Figure 7-1 illustrates this. The voltage at the worksite is reduced to that represented by the neutral resistance as a fraction of the total series

Isolate/Insulate

The onlymethod of providing absolute protection to a worker is to completely eliminate any current path through the body. There are two ways of doing this. The first is to isolate the worker so that contact with an energized part cannot be made. While effective, this also eliminates the ability to work, so this often is not a viable method. The second method uses suitably rated insulation to eliminate the body as a current path. This is the principle used when doing energized distri bution voltage maintenance using rubber gloves. The gloves provide the insulation to eliminate the body as a current path. An alternate means is to completely cover all energized components with an insulating device to prevent any worker con tact. While insulating products are available, they cannot be used in many of the maintenance tasks encountered by a lineman working aloft or by the groundman insupport. Present insulatingproducts are limited to distribution voltage applications. Apractical andmoreuniversalmethod is toprovide a means of keeping the body extremities at the same or nearly the same voltage. If the difference in voltage across the body can be eliminated, the flow of current is eliminated, remembering Equa tion 2 (I = V / R). Without a difference in voltage there isnocurrent flow. This is a theoretical solution that cannot be fully achieved in practice. If current flows through anything with resistance, a voltage drop will be developed. However, the principle of maintaining a sufficiently low level of voltage across the body is the basis for the development of an “equipotential zone.” The term is slightly misleading as the contact points are not at equal voltage values as the name implies. The points are heldtoapreselectedvoltagedifferencebyknowing the available current andcalculating themaximum resistance of the connecting grounding cables. Equipotential Protection

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ENCYCLOPEDIA OF GROUNDING