scope:

PURPOSE AND SCOPE

The purpose of this Guide is:

1. To review grounding practices with especial reference to safety.

2. To establish, as a basis for design, the safe limits for potential differences which can exist in a station, under fault conditions, between points which can be contacted by the human body.

3. To provide a step by step guide for the design of practical grounding systems based on these limits.

4. To recommend test methods for obtaining data for the design of ground systems, and for verifying the adequacy of ground systems as constructed.

5. To develop mathematical methods as an aid in the understanding and solution of typical ground gradient problems.

6. To make available in English translation, some of the more pertinent and valuable foreign language articles.

7. To provide a comprehensive bibliography, with abstracts, of existing literature related to the subject, for the benefit of those seeking additional information not included herein.

This Guide, as a whole, is primarily concerned with alternating-current outdoor substation installations. A few sections are applicable to indoor portions of such stations or to stations which are wholly indoors.

Obviously, the same ground gradient problems which exist in a substation yard will not be present within a building, provided the floor either furnishes an effective insulation from earth potentials, or else is effectively equivalent to a conducting plate or close mesh grid, which is always at station ground potential; and provided also that the conducting parts of the building structure and fixtures are kept at station ground potential. However, even in the case of a wholly indoor substation, it may be necessary to consider some of the possible hazards mentioned herein from "perimeter gradients" (at building entrances), and from transferred potentials. This applies primarily, of course, to large capacity utility substations where the station ground system may rise to a high potential during a fault.

This Guide does not attempt to cover the many special grounding problems peculiar to direct current substations, which are discussed elsewhere in the literature. 66.

Finally, this Guide makes no attempt to develop a quantitative analysis of the effects of lightning surges. To cover this fully would require another report of nearly equal size. The references, however, contain much information on this. At this point it will merely be stated that the greater impedance offered to steep wave front surges will increase the drop in ground leads to the grid system, and decrease the effectiveness of the more distant parts of the grid. Offsetting this in large degree, is the fact that the body can apparently tolerate far greater current magnitude in the case of surges than in the case of 60 Hz.

As a practical matter, it is felt that ground systems designed as recommended herein will give a high degree of protection against steep wave front surges entering the station and passing to earth through its grounding system. Obviously, a man can be killed by a direct stroke inside the station, just as he can outside.