Page 394 - Handbook of Electrical Engineering

382 HANDBOOK OF ELECTRICAL ENGINEERING

a) Switchboards and motor control centres

It is the normal practice to provide a copper busbar at the base of the switchboard or motor
control centre for earthing all the high power circuits, e.g. cable armouring, motor earthing cables,
and low power circuits that are not sensitive to noise pick-up. This busbar is insulated from
the frame, and at one or both ends there is an isolating link with bolts that bonds the busbar
to the steel frame. The steel frame is bonded to the local earthing system, e.g. steel decking in
a marine installation, earthing conductor or rod in a land-based installation. The isolating link
can be opened for checking the earth-loop impedance or for making measurements of the noise
voltages. It is often the practice to install one or two external earthing busbars in the locality
of the switchgear. For example in a switchroom a busbar would be located near to each of the
two opposite walls, and in reasonable proximity to the switchgear. Equipment such as switchgear,
neutral earthing resistors, transformers, have their internal earth busbars or star points connected
by single cables of large cross-sectional area to the external earthing busbars described above.
These external earthing busbars are often mounted on insulators or bushings and fitted with bolted
isolating links that are again used for testing purposes.
A typical offshore platform will have several modules or large equipment rooms and so
all the external earthing busbars will be interconnected by single-core insulated cables of large
cross-sectional area. The interconnections are preferably made in the form of a ring circuit so that
continuity is highly assured. A similar ring circuit approach can be used for land-based plants
where the items of equipment are located near to each other, otherwise a radial interconnection
system or one with local grids and rods would be more economical.

b) Earthing within cubicles and panels
Instrumentation cubicles, SCADA cubicles, control panels, computer equipment and the like
require to be earthed in a particular manner so as to avoid or minimise the pick up of noise.
Some of the internal circuits may be very sensitive to noise pick-up from earth sources, e.g. input
amplifiers, signal conditioning units. These circuits may have their own special noise elimination
devices, as described in References 20 and 22, but it is better to assume that they have not for the
purposes of designing a good earthing system in the first place. It is common practice therefore
to provide two separate internal earthing busbars, one for general earthing and the other for the
special circuits. These will be isolated and insulated from each other.

The general earthing busbar would be used for earthing the framework, chassis metalwork
and cable armouring. The special earthing busbar, often called the ‘clean earth’ busbar, would be
used for signal core screens, earth reference points of input circuits, and earth reference points
of output circuits. Both the ‘general’ and the ‘clean’ earthing busbars would be mounted near
the cable gland plate on insulated bushes. The level of insulation need not be high because in
practical testing the potential to earth with the links removed would only be a few volts. (It is more
governed by the expected level of cleanliness in the area at ground level, which may contaminate
the bushings and cause a leakage current to pass and upset the measurements taken.) If the plant
is not prone to earth pick-up noise then the general busbar could be bonded to the same local
earthing boss as the main frame or cubicle. However, where earth pick-up is a problem then the
clean earth busbar would be interconnected by a large section cable to the copper ring system.
The general and clean busbars serve as ‘single-point’ earths, thereby eliminating pick up between
distributed earthing points due to conducted noise.