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

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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  • Further to that - the slow thermal part of all breakers B,C D W Z of a given rating is pretty much the same - a 10A breaker will carry 10 A all day, and the thermal part trips off at twice that current after a what seems like an eternity but is in fact some minutes. This emulates an old fashioned hot wire fuse pretty much - the higher the overload ratio the faster it operates.
    What varies by letter group is the near instant operating magnetic part - that of course the fuse does not have.

    So looking again at that 10A slow bow part, a B10 adds a magnetic part set up for about 30A and may go anywhere between 20A and 50. The C10 type has a coil with fewer turns, and is set to instant trip at between 50 and 100A, the D type higher still (100A to 200A )etc. Then there are the trick sizes W K and Z that no-one uses if they can avoid it that have the wholesaler sucking through his teeth and  wondering how much he can mark up the price- these are just slightly tighter specs  and in most cases a B will do for a Z and a D for a K, but not always.
    So for a motor with inrush, or indeed lamps with inrush issues, a C type is less likely to pop out when unexpected, and a K or D type less so still.

    note that another way to look at this is to realize that the magnetic bit of a 32A B type is more or less the same as that inside a 16A C type and the same coil could also be fitted to an 8A D type if such a thing existed...

    regards Mike.
    This graph with time up the side and overload multiple along the bottom may help.

    PS having said that an 8 amp D type does not exist, blow me the Eaton catalog has it ,and also  breaker type 's' I have never heard of before.. Live/learn and all that. A designation just asking to be confused with an S-type RCD is you ask me but there you go!

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  • Further to that - the slow thermal part of all breakers B,C D W Z of a given rating is pretty much the same - a 10A breaker will carry 10 A all day, and the thermal part trips off at twice that current after a what seems like an eternity but is in fact some minutes. This emulates an old fashioned hot wire fuse pretty much - the higher the overload ratio the faster it operates.
    What varies by letter group is the near instant operating magnetic part - that of course the fuse does not have.

    So looking again at that 10A slow bow part, a B10 adds a magnetic part set up for about 30A and may go anywhere between 20A and 50. The C10 type has a coil with fewer turns, and is set to instant trip at between 50 and 100A, the D type higher still (100A to 200A )etc. Then there are the trick sizes W K and Z that no-one uses if they can avoid it that have the wholesaler sucking through his teeth and  wondering how much he can mark up the price- these are just slightly tighter specs  and in most cases a B will do for a Z and a D for a K, but not always.
    So for a motor with inrush, or indeed lamps with inrush issues, a C type is less likely to pop out when unexpected, and a K or D type less so still.

    note that another way to look at this is to realize that the magnetic bit of a 32A B type is more or less the same as that inside a 16A C type and the same coil could also be fitted to an 8A D type if such a thing existed...

    regards Mike.
    This graph with time up the side and overload multiple along the bottom may help.

    PS having said that an 8 amp D type does not exist, blow me the Eaton catalog has it ,and also  breaker type 's' I have never heard of before.. Live/learn and all that. A designation just asking to be confused with an S-type RCD is you ask me but there you go!

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