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Main and sub distribution boards - Circuit Breakers

Nick Parker

a. I have a UPS (200kVA) at 230V 3 phase output, the ampere rating is 500A which feeds the main distribution board.
b. The Main Distribution Board has an Incoming MCCB with 630A and there are several outgoing circuit breakers at the Main distribution board, out of which 1 circuit breaker is of 25A MCB (Q1) 3P powers the secondary distribution board. 

c. The cable between the the Main distribution board and the secondary distribution board is protected against overloads and short-circuits by the mentioned circuit breaker (Q1)

d. At the arrival of the Secondary Distribution Board I have a 25A 4P Switch Disconnector (S1)

e. There are several outgoing circuit breakers at the secondary distribution board, out of which one  circuit breaker is of 25A MCB (Q2) 3P

1. How do I check manually that circuit breaker (Q1) is discriminated against the fault at the secondary of the outgoing breakers at the secondary distribution board?
2. Should I install 6A or 10A MCB in place of 25A MCB (Q2) to have a better discrimination
3. Do any standard limit the number of circuit breakers in the secondary distribution board? If no standard states it, what is the general engineering practice?

4. Should I have 4P MCB in place of 3P 25A MCB (Q1). When do I need to have 4P MCB?

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  • 0

    The first thing you need to decide is whether you need to have discrimination and, if it is only a “nice to have”, how much are you prepared to pay for it.

    Yes, 2 to 1 is a good rule of thumb but it is not guaranteed to give you discrimination and with a breaker that reacts quickly to an earth fault, seldom does.

    Faults tend to happen in the final user equipment so discrimination here is useful. If you have a gentle overload then any difference in rating should give discrimination. If you have a fault in a main feeder then maybe it does not matter if everything goes off, better that than an even bigger bang.

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  • 0

    The first thing you need to decide is whether you need to have discrimination and, if it is only a “nice to have”, how much are you prepared to pay for it.

    Yes, 2 to 1 is a good rule of thumb but it is not guaranteed to give you discrimination and with a breaker that reacts quickly to an earth fault, seldom does.

    Faults tend to happen in the final user equipment so discrimination here is useful. If you have a gentle overload then any difference in rating should give discrimination. If you have a fault in a main feeder then maybe it does not matter if everything goes off, better that than an even bigger bang.

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