Principals Of Electrical Grounding

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Principles of Electrical  Grounding   By:      John C. Pfeiffer, P.E.     Pfeiffer Engineering Co., Inc.


Abstract:

This is a discussion of the basic principles behind grounding systems and how grounding is
related to safety and the effective operation of circuit protection devices such as fuses and circuit
, Z5 U" G4 s. O7 q- q) ?8 Rbreakers. The discussion moves quickly from a basic study of grounding to simple examples of a
& m; _8 ?4 s3 G* F& I7 t, S& u. @single building installation and onto facilities with multiple buildings and structures. Finally the
discussion will briefly cover grounding as it applies to lightning protection and the control of
static electricity.

introduction:

Grounding to most engineers, technicians and electricians is a simple subject and little attention
is paid to it other than knowing that something is required. To some people there are grounds and
) g, l. S. b) j8 p- o& U8 @then there are “Clean Grounds”. Thirty years ago when computers were relatively new, there
were many approaches to grounding, particularly for electronics and computers. Some of these
8 B2 G. |( J. o/ o) V  }8 Yapproaches established what was called a “Clean Ground” which was often isolated from the
power grounds.

! |6 Z1 P% o1 P; J1 X! B: XMany of these ideas have been proven to be ineffective and sometimes dangerous to equipment
4 T* U2 T) A* e! b9 p' Dand personnel. As frequencies became higher and higher (computer speeds faster and faster)
# b3 |& F5 z+ l8 ^+ i, ~  Cresearch accelerated into the subject of grounding, shielding, EMI, lightning protection and static
electricity. The research resulted in the basic science behind grounding. This subject is not as
simple as once thought and one needs a clear understanding of the basic principles.

First, the ground or grounding of a circuit is a misnomer. For most purposes the term means
/ G" E" y3 V8 `+ h4 o5 eearthing or connecting the circuit to earth. In actuality, it=s connecting the circuit to a common
" G/ o' ?5 I& ?$ y& {) c/ Hpoint of reference; for most systems that is the earth.
4 `/ W9 f" `4 E- Y
Grounding=s main purpose is to provide a common point of reference between various sources of
, V" |# D; @: g# F( O! jelectrical energy, i.e.
         Different power systems            - Public Utilities, on-site generation, battery systems
  b0 A- @: P3 f$ w' y3 v  I( D         Different voltage systems          - 138,000v, 13,800v, 480v, 120v, etc.
( @2 Z, Z9 P( X; B, A  z$ y( F         Different energy sources           - Electrical Energy, Lightning, Static Electricity, RF
+ g% C& J+ o5 Q5 h) y. ~+ p         Energy

In any discussion of the grounding of electrical systems, particularly as it applies to power
systems, there also needs to be a consideration of how the grounding system relates to
& |, f& O4 d# m: B$ ?overcurrent protection. They go hand in hand.

There are also a number of misconceptions that affect how a ground system works. The
- E# v+ u' W0 _7 Nfollowing facts are generally not known or are misunderstood.
; O8 r! F' Y7 ^8 T0 C# B         The earth is NOT always a good ground.
         What is acceptable at 60 Hz does not always work at high frequencies.
         The interconnection of systems with even large conductors may be ineffective when

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