MCBs or Miniature Circuit Breakers are intended to give protection
against overloads and short circuits, which can cause damage
to cables and equipment.
MCBs have current ratings (6A, 10A etc.) above which they
will start to open or trip and give protection to the equipment.
The first characteristic is the overload which is intended
to prevent the accidental overloading of the cable in a no
fault situation. The speed of the MCB tripping will vary with
the degree of the overload. This is usually achieved by the
use of a thermal device in the MCB. The second characteristic
is the magnetic fault protection, which is intended to operate
when the fault reaches a predetermined level and to trip the
MCB within one tenth of a second. The level of this magnetic
trip gives the MCB its type characteristic as follows: - ·
Type B - trips between 3 and 5 time full load current
Type C - trips between 5 and 10 times full load current
Type D - trips between 10 and 20 times full load current
Apart from compliance with the Wiring Regulations, which
is the subject of another sheet, the importance of the "types"
is to ensure that the MCB does not give unwanted tripping
when a device it is protecting is started up.
For instance, for electric motors or low voltage lighting
the use of a type B MCB may give unwanted tripping and
the choice of a type C MCB will probably solve the problem.
In some instances the type D MCB may have to be selected but
in this case it may be advisable to discuss with a qualified
engineer before there may be other problems created.
The third characteristic is the short circuit protection,
which is intended to protect against heavy faults maybe in
thousands of amps caused by short circuit faults. The capability
of the MCB to operate under these conditions gives its short
circuit rating in Kiloamps (KA). In general for consumer units
a 6KA fault level is adequate whereas for industrial boards
10KA fault capabilities or above may be required.
MCBs are offered in types B , C and D characteristics. Where
normal resistive loads are being fed the type B characteristic
may be sufficient but in many industrial application motors
and special lighting and data equipment is being fed which
may adversely affect the MCBs during starting and it will
be necessary to use type C characteristic breakers.
In some cases such even these MCBs may not be sufficient
when Compressor motors are used and a type D MCB may be necessary.
However, caution should be practised because the characteristics
for these MCBs may make compliance with the Wiring Regulation
difficult without a change in the specifications of the other