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Characteristics of Type B and Type C MCBs for motor circuit.

Alan B

From my reading and pervious discussions on this forum my understanding is that for very short term overloads the tripping characteristic for type B and type C devices is the same.

i was then thinking why do LED manufacturers recommend type C protective devices due to inrush current, which I would assume is very short term.

The reason I am asking the question is that I am wiring up a large compressor which requires a 32A circuit breaker (don't have access to the instructions at the moment). My plan was to use a type C device because of in rush currents, but struggling to get hold of one.

If the difference between type B and type C MCB's/ RCBO's  is only for longer term overloads would using a type C device actually make any difference and am I wasting my time trying to get hold of one.

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  • 0

    Alan, I think that you have that the wrong way around. See Fig 3A4 and 3A5. (In the Big Brown Book they are on the same leaf, but in the Big Blue Book they sit nicely one above the other when opened.)

    Type B will trip "instantaneously" at 5 x nominal, whereas type C will trip at 10 x. The rest of the curves are similar.

    Typically a motor's start up current is 6 x nominal (at the least the ones which I have), which would cause "nuisance tripping" with a type B MCB assuming of course that the rating of the MCB matches exactly that of the motor.

    I feel sure that the manufacturer's instructions (which you do not have at the moment) will clarify matters.

    (I am surprised that you cannot easily find a type C MCB.)

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  • 0

    Alan, I think that you have that the wrong way around. See Fig 3A4 and 3A5. (In the Big Brown Book they are on the same leaf, but in the Big Blue Book they sit nicely one above the other when opened.)

    Type B will trip "instantaneously" at 5 x nominal, whereas type C will trip at 10 x. The rest of the curves are similar.

    Typically a motor's start up current is 6 x nominal (at the least the ones which I have), which would cause "nuisance tripping" with a type B MCB assuming of course that the rating of the MCB matches exactly that of the motor.

    I feel sure that the manufacturer's instructions (which you do not have at the moment) will clarify matters.

    (I am surprised that you cannot easily find a type C MCB.)

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