Miniature Circuit Breakers (MCB)
Miniature Circuit Breakers (MCB) are designed to protect cables and electrical equipment from two main types of fault.
Overloads
This is the gradual build up of heat caused by an increase in the normal current flowing through a circuit. Miniature circuit breakers contain a thermal device which detects this build up and trips the MCB, the speed of this depends upon the severity of the overload as can be seen by the trip curves indicated below.
This is a sudden increase in the current flowing through a circuit which causes the Magnetic device within the MCB to operate within a fraction of a second, so protecting the installation and any person who may have come into contact with the supply.
MCBs have a maximum current rating which can range from 1A upto 63A therefore selection of the correct version will depend upon the circuit it is protecting. They also have three 'Trip' characteristics which are categorised as follows:
C Curve: this trips at 5 - 10 times its rated current
D Curve: this trips at 10 - 20 times its rated current
The correct trip curve is selected depending upon the type of equipment being protected. For example flourescent lighting and motors can cause an initial surge on startup which may cause a B Curve MCB to nuisance trip, therefore a C Curve MCB should be installed. If this is still inadequate then a D Curve should be selected.
Miniature Circuit Breakers also have a maximum short circuit capacity which is usually expressed as it’s KA rating, CPN MCBs will withstand a short circuit current of 6KA and 10KA.
Click Here to see the CPN Range of MCBs
If you are unsure as to the suitability of any MCBs please contact Direcelec technical department.
MCB Trip Curve Tables
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