C or D type MCB?

Dear all,

I've a small machine running in a single phase 240V. Its FLA is

17.88A. The Mfr has recommended a 32A MCB with a main device interrupt of 10kAIC and the machines SCCR is 5kA.

The in rush current at power on is 25Arms, 20ms ; 50A peak.

Should I use a C or D curve type MCB?

How can I find this info... is there a curve etc?


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I don't understand why the manufacturer gives the breaking capacity -- that depends on the supply impedance and PSSC (and in some cases on the up-stream protection), not the load. Simlarly, I don't understand how the machine can have a SCCR (unless it's a generator).

I don't have the tables on me, but I think even a type B MCB will handle that, although medium power motor loads are more often protected by Type C.

They are in the back of the IEE wiring regs. Roughly, a type B will allow a 2-5 times surge, a type C will allow a 5-10 times surge, a type D will allow a 10-50 times surge.

Note that this isn't just an issue of motor load. The MCB is primarily protecting the cable run, and issues such as cable size, length of run, supply impedance, earth fault loop impedance, type of earthing, and type of up-stream protection are all factored in to the choice of protective device.

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Andrew Gabriel

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