Handbook of Electrical Engineering

EARTHING AND SCREENING 353

is usually taken as 110 volts. If a resistance of 1000 ohms is used as a standard value then from
equation (13.1) the threshold voltageEshockwill be:-

Eshock= 1000 ×

0. 116

√

ts

volts ( 13. 2 )

Wheretsis the time duration of the shock in seconds. This voltage can be withstood by 99.5%
of human bodies weighing 50 kg.

13.1.2 Damage to equipment

Occasionally an electrical fault will occur inside a piece of equipment such as a switchboard or motor
that causes a conductor to touch the casing or frame. In most power systems this type of fault would
cause a much larger than normal current to flow in the conductors. This current would flow through
the casing or frame and in so doing would usually cause serious damage to the conductors, their
insulation and casing metalwork due to sparking or arcing. The damage will usually increase with
time and can only be minimised by a careful design of the electrical protective relaying schemes that
detect the fault current, see Chapters 11 and 12.

13.1.3 Zero reference potential

Most power systems comprise several different three-phase voltage levels, e.g. 11,000 V, 6600 V and
440 V. They are isolated from one another by the use of transformers. Each isolated sub-section is
invariably ‘earthed’ or ‘grounded’ at one or more points. (The term ‘earthed’ will be used hereinafter.)
The purpose of this is to ensure that the voltage difference between any conductor and its casing
cannot rise above a predetermined amount. The voltage difference can increase due to several causes.

Static charge builds up across the insulation and causes the conductor potential to rise. This is
more of a problem with high voltage equipment because the dielectric properties of the insulation are
more pure. The insulation resistance is extremely high and does not discharge the accumulated charge.

If a fault occurs between the primary and the secondary windings of a transformer, the lower
voltage winding may experience a high voltage being impressed upon it.

If a three-phase sub-system is unearthed and a line-to-casing fault occurs, then the two ‘healthy’
lines will have their voltage-to-casing raised by a factor of

√

3. Normally the insulation of machines
   and cables can withstand this increase for a long period of time without harm. It is good practice to
   specify that the insulation systems of transformers, motors, generators and cables should be able to
   withstand an overvoltage of this type continuously.

13.2 Site Locations

The environment in which the power system is located will have an impact on how the methods of
earthing equipment are applied. The environments can be broadly grouped as:-

• Steel structures.


• Land-based plants.


• Concrete and brick-built structures.