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MCB Vs MCCB instantaneous Pick up

Nick Parker

Let's say we have a MCB and MCCB with 16A rating. MCB will trip based on the curve type, say if B curve is used, it trips between 3-5 times the 16A. Therefore, the instantaneous pickup would be anywhere between 48-80A.

For MCCB, at what current the pick up woul be? (any thumbrule?.

This is to decide to use MCB or MCCB for the final circuit as the fault current would be lower for the final circuit and pondering over if the MCCB could clear for the end of line fault.

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    You will often find that there will be no adjustability in the smaller frame, lower current sizes of T/M MCCBs, with the Im release fixed, at say 500A and thus no distinction between the Zs max at 0.1s and 5s disconnection. So whilst your 16A type C MCB will require only 80A to operate in 0.1s, the 16A MCCB will require 500 and thus a much lower Zs.

    With say a 63A MCCB with a fixed Im of 500 upfront of a 16A type C MCB, selectivity may still be difficult to achieve. I did read somewhere in Hagers technical bumf however, that whilst selectivity is desirable, it isn’t necessary for circuits up to 63A (not BS7671). 

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    You will often find that there will be no adjustability in the smaller frame, lower current sizes of T/M MCCBs, with the Im release fixed, at say 500A and thus no distinction between the Zs max at 0.1s and 5s disconnection. So whilst your 16A type C MCB will require only 80A to operate in 0.1s, the 16A MCCB will require 500 and thus a much lower Zs.

    With say a 63A MCCB with a fixed Im of 500 upfront of a 16A type C MCB, selectivity may still be difficult to achieve. I did read somewhere in Hagers technical bumf however, that whilst selectivity is desirable, it isn’t necessary for circuits up to 63A (not BS7671). 

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