13

Earthing and Screening

13.1 Purpose of Earthing

There are three main reasons why it is necessary to earth, or to ground, electrical equipment:-

• To prevent electric shock to human operators, maintenance personnel and persons in the vicinity
  of electrical equipment.


• To minimise damage to equipment when excessive current passes between the conductors and the
  casing or frame during an internal fault condition.


• To provide a point of zero reference potential in the power system for the conductors.

13.1.1 Electric shock

Electric shock occurs when two factors exist:-

• Two points in an electrical circuit that have unequal potentials are in contact with the human body.


• The difference in these two potentials exceeds a lower threshold value.

At the threshold limit slight perception of pain or ‘tingling’ near to the points of contact
will occur. A continuous alternating current at a power system frequency, e.g. 50 or 60 Hz, of
approximately 1 mA will cause this slight reaction. Increasing the current causes a greater intensity
of reaction. At approximately 12 mA the muscles become very difficult to control, i.e. almost unable
to ‘let go’ of the contact. Between approximately 20 mA and 50 mA the current tends to cause
difficulty in breathing, but not to an irreversible extent. A continuous current above 50 mA and up
to 100 mA will tend to cause ventricular fibrillation and may lead to heart failure and death.

Some of the early detailed work on this subject was published in 1936 by Ferris, King, Spence
and Williams. Much work has been published by Dalziel and his co-authors from about 1941 and
1972, see Reference 1, Chapter 20 ‘Bibliography’ for details. In this reference [B26] and [B29]
showed that the current threshold withstand versus time duration characteristic has an ‘I-squared-t’
form, as follows:-
√
tn=

Kh

Ih

( 13. 1 )

Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry. Alan L. Sheldrake
2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6