100A main breaker tripping with low loading?

[ChevyKevv]

Hi all.

I have been asked to price up remedial works for a fault which I haven't traced yet........

Our local village hall has two Crabtree C50 3 phase boards on of which controls the ring mains, lighting cooker etc.

This board has 3 RCCB's for the 3 ring main circuits.

The POC said they suffer from nuisanse tripping which I could understand if it was the ring RCCB's but she says "it trips the lot" sometime just by turning the lights on.

Now she hasn't been there when it's tripped out so doesn't exactly know which breaker needed resetting, but if it goes with only the lights on does she mean the main 100A breaker goes? if so what could cause this with such low loading?

Two of the ring mains are the red phase and 1 on the yellow (old colours) with the cooker being also on the red.
With the blue having a water heater 15A and a couple of heaters.
Could it be down to an imbalance?

I don't want to turn down local work but how do you price up for unknowns as I haven't tested anything yet?

 

[RISElectrical]

You could offer a price for a days investigation and upon taking the job agree to deduct this from the cost.

Failing that you could offer a PC Sum and take a stab at what needs doing. Either way dont offer a quotation, just an estimate.

 

[Engineer54]

Your not going to know anything, until you get your test kit out and start going through the system. Just wondering how dedicated circuit RCD'S can trip out the whole DB?? Another reason why you don't always take notice of anyone that has unreliable knowledge of what's happening and when... At the moment no-one can advise you on what's causing the fault(s), the permutations being too many!!

Those old bakerlite C50 set-up's were the pit's when they first brought them on to the market, and now can be expensive to replace the individual breakers, RCD's and the later RCBO's...

 

[HandySparks]

Until you can get an accurate description of the symptoms (ie exactly what needs resetting), or have the fault demonstrated to you, I wouldn't even even offer an estimate. You need to make the customer understand this. If it's really local I might pop in and spend half an hour trying to re-create the fault.

It's a bit like saying her car makes an odd noise and expecting a price to fix it with no further information.

 

[ExArmy]

Your not going to know anything, until you get your test kit out and start going through the system. Just wondering how dedicated circuit RCD'S can trip out the whole DB?? Another reason why you don't always take notice of anyone that has unreliable knowledge of what's happening and when... At the moment no-one can advise you on what's causing the fault(s), the permutations being too many!!

Those old bakerlite C50 set-up's were the pit's when they first brought them on to the market, and now can be expensive to replace the individual breakers, RCD's and the later RCBO's...

exactly! i bet the fault is nothing like that poor description

 

[johnboy6083]

If it is tripping an MCB that feeds the board, then it could be a discrimination problem.
Rule: never feed MCBs from MCBs if losing power is a problem.
las others have said, you really need to witness the tripping, and do some testing!

 

[ChevyKevv]

Thanks all, I do think I'm not getting the full picture, as you say the POC hasn't witnessed any tripping so it's only word of mouth about the lights "blowing everything" I'll ask for more info and offer a couple of hours investigation ( charged of course) before proceeding.

If it were just the ring circuits it would make more sense and give a better starting point.

 

[telectrix]

could it be lamps blowing and causing tripping due to the instantaneous high fault current caused by the ionisation of the gas in the lamps when they blow?

 

[1shortcircuit]

The POC said they suffer from nuisanse tripping which I could understand if it was the ring RCCB's but she says "it trips the lot" sometime just by turning the lights on.

Now she hasn't been there when it's tripped out so doesn't exactly know which breaker needed resetting, but if it goes with only the lights on does she mean the main 100A breaker goes? if so what could cause this with such low loading?

Sounds to me like there may be discrimination issues?

If it's using 30mA protective devices but doesn't have a time delayed upfront then this could well be the case?

Sounds like you need to pop by and find out what setup the installation uses. Without seeing it, it could be anything.

:thumbsup

 

[RISElectrical]

Also when someone wants a job done quick they always exaggerate and tripping everything actually means just1 circuit but of they say that it doesn't sound urgent

 

[Clive P]

In the past I have found C50's to be a pain in the neck. I remember working on an office once where all the rings were protected by these and they were forever going on nuisance tripping, once we had changed them out for a different make there were no further problems. Also with these once one has tripped through short circuit they do not reset with the same tolerance. I know this is a biggy, but they are pretty much obsolete now, would the customer consider a change out?

 

[ChevyKevv]

I've explained the problems with trying to find a fault like this without actually testing anything.
have now been asked to price up replacing the RCD's back to regular breakers and replacing the sockets in the hall with RCD'S sockets.

And PAT test all their equipment as it's never been done.

They have two of these boards so don't know whether they'd go for a full replacement but I can mention it.

Thanks for all the replies.

 

[Engineer54]

Why not just find the fault that causing the tripping, rather than replacing god knows how many socket outlets with RCD units. ...Probably be cheaper to replace those old boards than to supply all those RCD socket outlets ...lol!!!

 

[i=p/u]

could be when heating kicks in???????????? waterheater????????????????????????// insulation resistance knocking breaker o at a time

 

[Snapester]

Have you had the clamp meter out and checked the loadings on the phases? Sounds like discrimination though, but it would be good to see the loadings.