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Main and sub distribution boards - Circuit Breakers

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?

Parents

0 mapj1 over 3 years ago

I hope I have this right, given the lack of schematic capture on this forum is this about it ?

If so, if there is a fault on cct Q2, then it is a toss up if Q1 or Q2 will fire or both, if neither side has programmable delay.
Even if one had a higher rating than the other, say Q1 was 50A, for a high current fault they will still race.
What loads are present, rather than what breakers, will influence how this behaves and if you can fiddle the ratings a bit.. Knowledge of the loads - will they all be on at once for example, affects the ratio of outgoing circuits to incoming cable capacity. Many industrial processes have things that are never on together, and we can rely on that knowledge to avoid over sizing the supply.
Do we care if the ‘many MCB 2’ go off it there is a fault on the Q2 circuit ?
For the other question, in general on TT 3 phase systems you break the neutral, but otherwise 3 pole breaking is preferred.
Really more info is needed to put this in context before we solve the wrong problem.
Also do you really mean 230V 3 phase in the sense of 230V live to live, or 230V live to neutral and 400v live to live. The latter is the UK and EU norm.
Mike.
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0 mapj1 over 3 years ago

I hope I have this right, given the lack of schematic capture on this forum is this about it ?

If so, if there is a fault on cct Q2, then it is a toss up if Q1 or Q2 will fire or both, if neither side has programmable delay.
Even if one had a higher rating than the other, say Q1 was 50A, for a high current fault they will still race.
What loads are present, rather than what breakers, will influence how this behaves and if you can fiddle the ratings a bit.. Knowledge of the loads - will they all be on at once for example, affects the ratio of outgoing circuits to incoming cable capacity. Many industrial processes have things that are never on together, and we can rely on that knowledge to avoid over sizing the supply.
Do we care if the ‘many MCB 2’ go off it there is a fault on the Q2 circuit ?
For the other question, in general on TT 3 phase systems you break the neutral, but otherwise 3 pole breaking is preferred.
Really more info is needed to put this in context before we solve the wrong problem.
Also do you really mean 230V 3 phase in the sense of 230V live to live, or 230V live to neutral and 400v live to live. The latter is the UK and EU norm.
Mike.
Cancel
Vote Up 0 Vote Down

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Main and sub distribution boards - Circuit Breakers

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