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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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    AJJewsbury: 
    Your only chance is to arrange things so that a fault downstream of the second MCB would be too small to operate the first MCB - but that rather goes against the principles of ADS/fault protection where a fault just before the 2nd MCB needs to reliably open the first MCB, and since the 2nd MCB doesn't add significant impedance to the circuit, a fault just after the 2nd MCB is likely to produce a very similar fault current and so still risk opening the first MCB (as well as the 2nd one).

    I think that you have to get lucky. I think that I may just get some discrimination between a 63 A MCB in the DB at the intake and a 32 A one in the house DB by virtue of the Ze being sufficiently high and the cable being long enough. As Andy says, the PFC at the load end of the distribution circuit needs to be high enough to give instant trip, but by the time that we get to the sockets, the PFC should be below 315 A.

    In practical terms, any fault is much more likely to occur in an appliance than in the fixed wiring, at least in my house.

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  • 0

    AJJewsbury: 
    Your only chance is to arrange things so that a fault downstream of the second MCB would be too small to operate the first MCB - but that rather goes against the principles of ADS/fault protection where a fault just before the 2nd MCB needs to reliably open the first MCB, and since the 2nd MCB doesn't add significant impedance to the circuit, a fault just after the 2nd MCB is likely to produce a very similar fault current and so still risk opening the first MCB (as well as the 2nd one).

    I think that you have to get lucky. I think that I may just get some discrimination between a 63 A MCB in the DB at the intake and a 32 A one in the house DB by virtue of the Ze being sufficiently high and the cable being long enough. As Andy says, the PFC at the load end of the distribution circuit needs to be high enough to give instant trip, but by the time that we get to the sockets, the PFC should be below 315 A.

    In practical terms, any fault is much more likely to occur in an appliance than in the fixed wiring, at least in my house.

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