A MCB 1200+ amps tripped .. help is needed urgent

[apk2000]

Hi

I'm supervising maintaining a building powered by a 1250 Amps and i had a total blackout twice within a week.

Checking the MCB and it's healthy and checking the REF is also fine and it has been reset normally.

What would make such MCB trip?

Sorry but no photos are allowed to post by the policy!

We follow the BSI

Urgent help is needed!

 

[James]

2 ways a circuit breaker can trip

1 over current
2 faulty breaker

Have you checked the current at all?

 

[littlespark]

By saying the MCB is healthy, that means it is staying on for a time before tripping.
From the mcb, how does it split up to feed the building? Is there other distribution boards? Are they protected by smaller breakers?


As above, there are 2 ways for it to trip…
Overcurrent… short circuit or faulty…….
That’s three

There are three way……
Overcurrent, fear, surprise, ruthless efficiency….


That’s four


No one expects the Spanish Inquesition!

 

[pc1966]

You really need to get some clip-on current transformers and a logger and see what current is being taken. A logger will show if there was a fault with a massive current leading to a rapid disconnection, or some period when a lot of equipment was in use at the same time so overloading the supply and tripping in seconds/minutes on the thermal part of the MCB characteristics.

Also it is probably not a MCB (miniature circuit breaker), it might be a MCCB (moulded case circuit breaker) but at 1250A more likely ACB (air circuit breaker). MCB are usually only to 63A (occasionally 125A) and fixed characteristics, MCCB are usually up to 630A and I guess roughly half of them have some degree of adjustable settings, ACB go up to several thousand amps and virtually always are adjustable.

The electronic control units add complexity and can show odd failures. Also some of these devices also have earth-fault detection and you can end up with unfortunate "small" fault in the tens of amp region, where the installation design lacked RCD protection on the faulted segment (or RCD is faulty as well), and that can trip things without a big BANG! as many thousands of amps have a party.

The breaker ought to have some status indication to show what tripped it, more so if fancy modern unit, maybe even with some plug-in diagnostics option.

 

[pc1966]

Maybe @Julie. has some better advice, more her area of expertise.

 

[plugsandsparks]

Need to get some photos of the make model of the breaker. I would expect it to have a number of settings that may or may not have been set appropriately, fault current size and duration immediately spring to mind and also any earth fault settings, that may have been implemented as PC1966 mentions above.

 

[Julie.]

As above, it's likely to be a MCCB (or ACB/OCB...) and not a MCB.

These all have a wide range of protection applied to them, so the type of protection fitted is important in order to analyse the problem.

Most offer additional earth fault protection, so it could be a relatively small leakage current - hence no big bang pointing to an obvious fault.

Some show a trip reason/code/flag.

But in general, either faulty (unlikely but it does happen), a genuine fault is being detected, or the settings are inappropriate and it's detecting a "fault" from normal service conditions.

We would need to know the type of protection fitted and the settings to give a reasonable guess of the problem.

 

[apk2000]

Thank you all for your kind support.

It has been agreed with the management to replace the ACB since such incident occur on other locations!

 

[pc1966]

Thanks for the update. Hopefully there will not be any more problems, but check any new MCCB/ACB is correctly configured after installation.

 

[apk2000]

Thanks for the update. Hopefully there will not be any more problems, but check any new MCCB/ACB is correctly configured after installation.

Sure the new ACB will be compatible with the old one and thanks for reminding