Page 368 - Handbook of Electrical Engineering

356 HANDBOOK OF ELECTRICAL ENGINEERING

The local processing equipment will be similar to that described in sub-section 13.2.1, and
therefore the earthing practices will be similar, for example the use of copper interconnected busbars
and bonding conductors. However, if the plant is mounted on concrete foundations then extra earthing
rods will usually be needed at each foundation site. All reinforcing steelwork in concrete should be
earthed to busbars or through their own rods.

13.2.3 Concrete and Brick-Built Structures

Concrete buildings such as offices and storehouses contain reinforcing steel rods and bars inside the
concrete columns, walls, floors and ceilings. Steel beams are used to carry the structural loads of
the building, and these beams are either encased in concrete or exposed. Brick-built structures also
use steel beams in a similar manner. In all cases the unseen steel should be bonded to the earthing
system. This is carried out at the footings and foundations, either by using local earth rods or an
interconnecting cable to a nearby point in the earth system.
All electrical equipment and appliances inside the building must be earthed, including metal
luminairs, socket outlets, MCB and MCCB panels, cooking appliances and the like. Earthing is
achieved by routing separate earthing conductors to each appliance, from a central earth point that
is usually at the main power intake at the building. References 9 and 10 give full details of earthing
practices for buildings.
It has become standard practice in recent years to use sensitive earth leakage current detectors
in circuit breakers to further protect against electric shock. The current sensitivity can be chosen from
a range of standard current values, e.g. 30, 100, 500 and 1000 mA. The 30 mA sensitivity is used
at individual consumer sub-circuits, e.g. feeders to domestic and small power socket outlets, feeders
to luminairs. The higher sensitivities are used in the upstream circuit breakers so that protection
discrimination is achieved.

13.3 DESIGN OF EARTHING SYSTEMS
This section covers the design of high voltage and low voltage earthing systems and highlights some
difficulties that can be experienced in practical installations. The concepts and practical requirements
of References 9 and 10 will be discussed.

13.3.1 High Voltage Systems

A plant requiring more than about 1000 kW of power will normally receive a supply at a high voltage,
drilling rigs are often an exception because they tend to use captive diesel engine driven generators.
The primary source of high voltage power will be generators or supply authority transformers. The
supply authority voltage could range from typically 3000 V to 132,000 V depending upon the total
and future power demands of the plant, and to some extent the distance from the ‘point of connection’
to the central grid or power station. The supply of power would be transmitted through overhead lines
or cables. The authority would take care of the earthing requirements for the supply, in a manner
similar to that described later.
High voltage supplies within a plant are invariably arranged as three-phase star-connected
systems. The star point of the transformer secondary winding or the generator stator winding is