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6. PROTECTIVE CO-ORDINATION
6.1 General
Type of System
The primary purpose of a circuit protection system is to prevent damage to series connected equipment and to
minimise the area and duration of power loss. The first consideration is whether an air circuit breaker or moul-
ded case circuit breaker is most suitable.
The next is the type of system to be used. The three major types are:
Fully Rated, Selective and Cascade Back-Up.
Fully Rated
This system is highly reliable, as all of the breakers are rated for the maximum fault level at the point of their
installation. Discrimination (selective interruption) can be incorporated in some cases. The disadvantage is that
high cost branch breakers may be necessary.
Selective-Interruption(Discrimination)
Selective Interruption requires that in the event of a fault, only the device directly before the fault will trip, and
that other branch circuits of the same or higher level will not be affected. The range of selective Interruption of
the main breaker varies considerably depending on the breaker used.
Cascade Back-Up Protection
This is an economical approach to the use of circuit breakers, whereby only the main (upstream) breaker has
adequate interrupting capacity for the maximum available fault current. The MCCBs downstream cannot handle
this maximum fault current and rely on the opening of the upstream breaker for protection.
The advantage of the cascade back-up approach is that it facilitates the use of low cost, low fault level breakers
downstream, thereby offering savings in both the cost and size of equipment.
As Mitsubishi MCCBs have a very considerable current limiting effect, they can be used to provide this ‘cas-
cade back-up’ protection for downstream circuit breakers.
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