144 HANDBOOK OF ELECTRICAL ENGINEERING
7.2 Construction
The switchgear (SWGR) and motor control centres (MCC) considered in this section are those
found in the onshore and offshore oil industry for supplying power to processes and utilities. Extra
high voltage (EHV) transmission and distribution equipment used by electricity authorities is not
considered herein. Hence most of the equipment used onshore and offshore is limited to an upper
service voltage of between 11 kV and 15 kV. Occasionally voltages in the range of 30 kV to 40 kV
are used when the incoming line or generating capacity exceeds approximately 120 MW. Voltages
as high as 69 kV are used for long submarine cable systems.
The SWGR and MCC equipment are invariably housed in a building or enclosed module,
or at least effectively protected against bad weather and aggressive environmental conditions. The
construction is therefore of the metal clad type, in which all the live parts are housed in a mild-steel
sheet metal enclosure. The enclosure is sub-divided so that personnel may work safely on some
compartments without danger or the risk of electric shock.
Various degrees of personnel and ingress protection are commonly available. The degree of
protection is defined in various international standards e.g. NEMA and NEC in USA, IEC in UK and
Europe. For use inside buildings where manual operation and interference is infrequent and where
the atmosphere is cool, dry and clean an enclosure of the IEC60529 type IP40, 41 or 42 or NEMA
type 1 or 2 is usually adequate. If equipment is to be located in a poor atmosphere e.g. dust laden,
damp, hot and where hose-pipes may be used to wash down adjacent plant, then a more demanding
enclosure type is required e.g. IP54 or NEMA type 4, but this would normally only apply to low
voltage equipment.
The main electrical components are:-
- Main busbars.
- Earthing busbar.
- Incoming and busbar section circuit breakers.
- Outgoing switching devices, contactors or circuit breakers.
- Fuses for MCC outgoing circuits.
- Safety interlocking devices.
- Electrical protective relays and devices for all power circuits.
- Control and indication devices.
- Communication or network interfacing system.
- Main connections and terminal compartments.
7.2.1 Main busbars
The main busbars should be made of high-grade copper. Aluminium is not recommended because
it suffers from mechanical problems associated with the soft nature of the metal, which makes the
physical jointing and connection of auxiliary devices difficult. For voltages up to 600 V it is often
required to use four busbars, one being for the neutral. This is because unbalanced loads need to be
supplied as a 4-wire system. In this case a 4-wire feeder from the source is necessary, e.g. a HV/LV
transformer, LV generator. Care needs to be taken when specifying the number of horizontal and