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Energy let-through for Type 2 MCB's ?

justinneedham

I'm nursing an elderly installation containing MK brand Type 2 breakers.

Does anyone have let-through energy curves for MK Type 2 32A MCB ? - Specifically LN5932.

Without any better data I'm likely to expect it to have I^2t no higher then a type C BS60898, and (based on 7x multiplier for instantaneous trip..), perhaps somewhere between a B and  C? Is that a safe assumption? 

I have curves for Wylex BS60898 and for one or two other brands. 

It's a bit of hand-waving I'd sooner not do if there are measured curves somewhere out there?

Parents
  • 0

    The 16th edition has the curve for the 32 Amp breakers to B.S. 3871. It runs very close to the 30 Amp M.C.B. curve. Fig. 3.5.

    Time current characteristics for B.S. 3871. 

    0.1 to 5 seconds. 224 Amps. For a 32 Amp. M.C.B. Type 2.

    Unfortunately what you need for energy let-though is the "instantaneous" response - which is mostly off the scale of those graphs - which flat-line at either 0.01s or 0.02s depending on which edition (which seems dubious in itself). If you did try to apply those graphs directly the energy let-though would be simply proportional to the square of the fault current without limitation - so you'd often get implausibly high values - e.g. 3kA fault current for 0.02s gives 180,000A²s - compared with say 18,000A²s for a 32A B-type according to BS EN 60898.

        - Andy.

Reply
  • 0

    The 16th edition has the curve for the 32 Amp breakers to B.S. 3871. It runs very close to the 30 Amp M.C.B. curve. Fig. 3.5.

    Time current characteristics for B.S. 3871. 

    0.1 to 5 seconds. 224 Amps. For a 32 Amp. M.C.B. Type 2.

    Unfortunately what you need for energy let-though is the "instantaneous" response - which is mostly off the scale of those graphs - which flat-line at either 0.01s or 0.02s depending on which edition (which seems dubious in itself). If you did try to apply those graphs directly the energy let-though would be simply proportional to the square of the fault current without limitation - so you'd often get implausibly high values - e.g. 3kA fault current for 0.02s gives 180,000A²s - compared with say 18,000A²s for a 32A B-type according to BS EN 60898.

        - Andy.

Children
  • 0 in reply to AJJewsbury

    Indeed. I find that if I apply a 30K A^2s (which would be the approximate case for 32A type C in this location), then I'd be just OK. On that basis I'm assuming that the type 2 32A (Yes it is 32A not 30A) will be fine. 

    Regards

  • 0 in reply to AJJewsbury

    Anything above 3kA fault current will probably blow the breakers up. They are M3 rated.

    And prey tell please, why do we need the energy let through info. on these domestic sized M.C.B.s?

    Z.

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