Volume 8 – Mechanical and Electrical Services - Malaysia Geoportal

Chapter 7 ELECTRICAL SERVICES
As for electrical requirement TNB requires the consumer to provide lighting points and switch socket
outlets to be terminated at a DFB or splitter unit. If L.V. supply is not available in TNB substation, it
has to be provided by the consumer by taking supply from the main switchboard.
7.3.4 Main Switch Room
Main switch room (also subswitch rooms, if any) should be big enough to permit easy installation
and maintenance. Usually not less than 0.7m clearance should be allowed between the wall and the
rear of the switchboard. Front clearance of the switchboard should be such that there is still
sufficient passage space with the switch gears full withdrawn. The minimum length of the switch
room should be equal to the length of the switchboard plus 1.2m.
Cable trench in the switch room is usually about 0.75m wide so that while the trench is not
unnecessarily congested with cables, the switchboard can still sit with 150mm to each side of the
trench.
Main switch room should have external door for easy access from the exterior of the building.
7.3.5 Main Switch Board
Main switchboard is normally of the self-contained, floor mounted, flush fronted, metalclad cubicle
type suitable for front and rear access. It shall be designed to with stand fault condition of not less
than 31 MVA at 415V for 1 second as defined in B.S.5486.
Air circuit breaker is normally used as incoming switchgear. It shall have minimum breaking capacity
of 31 MVA at 415V with a short time rating of 1 second. Other components are protection relays,
ammeters, voltmeters, power factor meters, or micro-processed based digital power meters, current
transformer, etc. The MSB must be capable of withstanding certain fault and load conditions and
should be positioned at a suitable location with respect to accessibility, ease of operation and length
of cable run with adequate ventilation ensured. It shall be fabricated by Switch Board manufacturer
approved by Suruhanjaya Tenaga.
Sometimes two transformers are supplied by TNB. This may happen when:-
a) A single transformer is insufficient to cater for the total load of the installation, or
b) A more secured supply system for the installation is required.
Whatever the cause maybe, the main switchboard must be designed to receive the two incoming
feeders from the separate transformer. In this case it is normal practice to incorporate a 4 pole
coupler between the two sections of main busbars fed by the two transformers to provide further
flexibility in the supply system. The coupler must at least be mechanically interlocked with the other
two incoming A.C.B.’s is opened in order to satisfy TNB’s requirement.
7.3.5.1 Circuit Protection
Generally the main incoming breaker should be provided with both O/C & E/F (either IDMT or
instantaneous type) protection. Outgoing breakers may be provided with O/C or O/C & EF
(instantaneous type) depending on the circuit design.
As the majority of electrical faults in government buildings are attributed to earth fault, it is advisable
to provide the main incoming breaker with IDMT E/F protection to achieve a better discrimination
between the outgoing circuits and the main incoming breaker thus reducing the occurance of the
total power failure to the building.
7-10 March 2009