Characteristic Curves of MCB
Characteristic
curve is the curve between release current and tripping time. MCB have
Type B, Type C and Type D characteristic curves. They are shown below,
The
classification of Type B, C or D is based on fault current rating at
which magnetic operation occurs to provide short time protection
(typically less than 100ms) against short circuits It is important that
equipment having high inrush currents should not cause the
circuit-breaker to trip unnecessarily, and yet the device should trip in
the event of a short-circuit current that could damage the circuit
cables.
Type
B devices are generally suitable for domestic applications. They may
also be used in light commercial applications where switching surges are
low or non-existent. Tyne B devices are designed to trip at fault
currents of 3-5 tomes rated current (In). For example a 104 device will
trip at 30-50A.
Type
C devices are the normal choice for commercial and industrial
applications where fluorescent lighting, motors etc. are in use. Type C
devices are designed to trip at 5-10 times In (50-100A for a 10A
device).
Type
D devices have more limited applications, normally in Industrial use
where high inrush currents may be expected. Examples include large
battery charging systems, winding motors. transformers. X-ray machines
and some types of discharge lighting. Type D devices are designed to
trip at 10-20 times In (100-200A for a 10A device).
Normal
cable ratings relate to continuous service under specified installation
conditions. Cables will, of course, carry higher currents for a short
time without suffering permanent damage. Type B and C circuit breakers
can generally be selected to achieve tripping times that will protect
the circuit conductors against normal surge currents in accordance with
BS 7671. This is more difficult to achieve with Type D devices, which
may require a lower earth loop impedance (Zs) to achieve tile operating
times required by Regulation 413-02-08.
Surge currents
Surge
currents in domestic installations are generally low, so that a Type B
device is adequate. For example Inrush currents associated with one or
two fluorescent fittings. Or the compressor motor in a
refrigerator/freezer are unlikely to cause unwanted tripping.
Fluorescent and other discharge lamps produce surge currents and while
one Or two fluorescent lamps are unlikely to cause a problem, the block
switching of a number of fluorescent lamps In a shop, office or factory
can produce substantial inrush currents. For this reason Type C devices
are recommended for these applications.
The
magnitude of the surge current will depend on the lamp rating, starting
system and type of control gear used in the luminaires. Reputable MCB
manufacturers produce tables listing the number of fittings of a
particular make and type that can be used with their devices.
Unwanted Tripping
Sometimes
failure of tungsten filament lamps can trip Type B circuit breakers in
domestic and retail environments. This is caused by high arcing currents
occurring at the time of failure and is generally associated with
inferior quality lamps. If possible the user should be encouraged to use
better quality lamps. If the problem persists then one of the
measurement listed below should be considered.
A
Type C device may be substituted for a Type B device where unwanted
tripping persists, especially in commercial applications. Alternatively
it may be possible to use a higher rating Type B MCB say 10A rather than
6A. Whichever solution is adopted, the installation must be in
accordance with BS 7671.
A change
from Type C to Type D devices should only be taken after careful
consideration of the irstallation conditions, in particular the
operating times required by Regulation.
Other Considerations
The
importance of selecting circuit-breakers from reputable manufacturers
cannot be over emphasised. Some imported products, claiming .to have a
6kA short-circuit capacity, had failed dramatically under test. In
contrast the test procedures followed in British ASCTA (Association of
Short Circuit Testing Authorities) laboratories are among the most
suitable in the world.
Type B
devices should only be used in domestic situations where high inrush
currents are unlikely and Type C devices should be used in all other
situations.
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