Types of MCCB

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There is no type A.

Type B is commonly used for domestic & light commercial.

Type C is used for heavy commerce and industry.

Type D is used for things like X ray machines in hospitals.

Circuits attached to each type of breaker must not exceed a specified EFLI value.

That figure is half that of the preceding type, ie Type C maximum EFLI is half that of the type B value & Type D maximum is half that of the Type C value.

So, it is acceptable to use a certain type of circuit breaker only if the maximum EFLI measured on that circuit does not exceed the tabulated value for that size and type of breaker.

In practice in a dom situation you would not use Type D (and only Type C if you have something like heavy motors, welding gear or nuisance tripping on a lighting circuit).
 
EFLI = Earth Fault Loop Impedance = The total impedance of the Live from the Substation to your house, to the fault and back through the Earth to the substation.

I think you mean MCB not MCCB

MCB = Miniature Circuit Breaker as found in a consumer unit.

MCCB = Moulded Case Circuit Breaker as found in an industrial submain board.

The B, C, D curves govern how quickly the breaker trips on moderate short term overloads
B curve will withstand upto 5x the rated current for upto 10s
C curve will withstand upto 10x the rated current for upto 5s
D curve will withstand upto 20x the rated current for upto 3s

But as the loop impedance has to be small enough to operate the MCB under fault conditions C (or worse D) curve MCBs limit the maximum length of your circuit wiring or require thicker wiring.

In short don't use C or D types unless you have to and the circuit is designed accordingly.

Sailbadthesinner.
 
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That doesn't look right to me, certainly not for MCBs :confused:
Type B = x3-5 magnetic trip
Type C = x5-10 magnetic trip
Type D = x10-20 magnetic trip
The magnetic trip (which looks after fault current) causes operation within 0.1s at the maximum current required to operate it i.e. B=x5, C=x10 and D=x20, hence the EFLI needs to be low enough to ensure enough current flows in the circuit to ensure this happens.
The thermal trip (overload) is the same for all 3 types of MCB, they just cut off at different positions in the graphs in the appendix of BS7671 as the line meets the magnetic trip current which is in a different position for all the 3 types.
 

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