There is a 200A distribution circuit supplying a DB with a 400A MCCB.
The 200A supply is fed from a 200A MCCB.

I can't see anything here that breaches the regualtions.
Am I missing something?

is the 400A Moulded Case Circuit Breaker the main switch of the other DB or is it feeding and protecting a circuit from the DB?

In either case it means on overload the 200A MCCB will open before the 400A one even thinks about operating so you have no discrimination and you have the inconvenience of losing the supply to that DB and all circuits fed from it

The 400A MCCB is the panel's main breaker.

In the event of a trip you are going to lose all power to the boards, reagrdless.
It just means that the 200A braker at the feeder boards trips instead of the 400A one at the panel.

Discrimination shouldn't really be a concern as there is only one breaker being fed.

Not the best of solutions but it appears to be legal.
The supply cable is adequately protected by the 200A MCCB.
The 400A MCCB at the panel adequately fulfils its role of Main Switch.

Just wondering if I'm missing something.

Sounds to me as if the 400A unit is just acting as (an expensive) main switch - nothing technically wrong with that.

Does the 400A have any 'extras' on it? (e.g. an earth leakage trip)?

It might be a case of just using 'what was in the van', although it might suggest that someone changed their mind part-way through, or there's been some other cock-up (e.g. whoever did the supply end got things mixed up). Might be worth doing some measurements or max demand/diversity calculations on what the DB supplies to check that the 200A supply is adequate.

- Andy.

Hi Andy.
It is as you describe.
The max demand on the panel is roughly 160A.
The supply to the panel has been correctly sized at 200A and the 400A breaker is, as you say, an expensive Main Switch.

My understanding of the regs is that this is acceptable as no downstream protective device is required on the supply cable, as the cable is adequately protected by the upstream breaker.

I'm just checking in case in case there is a regulation breach that I have overlooked.

Do check if it is non auto - ie really just acting as a main switch

It may be there just to main the fault rating of the board (or just that the "normal" main switch is only rated at 125A - so again you need a higher rating provided by the 400A non auto)

regards

OMS

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Failure is always an option

I can't see the problem.
The MCCB is being used as a high current switch.

If the MCCB was changed and a 400A switch installed instead what would be the difference?

Depends on the switch - a lot of DB isolators would get a bit perturbed if you closed them into a fault (or even opened them on load) - above about 125A a lot of DB isolators are not fault rated (hence the use of (usually) non auto MCCB's)

regards

OMS

-------------------------
Failure is always an option

Agreed,

I don't think we've ever fitted a small panel board that doesn't have an MCCB as an incoming device. Most of them seem to specify an mccb incomer which is probably because no one makes a 400 amp 3 pole switch.


Stu