Main service fuse blowing...

[SSE]

Have been reluctantly roped into checking out a job for someone, more as a favour. It’s a hall that is used every week but not every day. Basically one of the three main fuses has blown twice now so I’m told. It’s a 3 phase supply each with a 63A fuse. The fuse is housed in one of those old 3 phase isolator switches with the big handles, it then from there supplies a 3 phase heating contactor then from there one of the three consumer units. Now the consumer unit that it’s affecting supplies roughly about 20 under seat heaters & two wall heaters. All the heaters under the seats are switched off/on by a 13A fused spur & the two wall heaters are on 20A DP switches. Basically the caretaker noticed the fuse had blown so he replaced it & everything seemed to be working for a few days, went back & it had blown again. No MCB’s in the consumer unit had tripped, these are all type C’s though. I went a quick look last night & did IR’s to all the circuits which all came back >299 on eveything. None of the fuses for the heaters have blown, just the main 63A. Put a fuse in today & hey presto it’s all working again...for now. What could be causing this? Contactor? Loose connection on circuit? I’ve left the circuits all off at MCB’s for now but consumer unit is still powered up, gonna keep an eye on it for a few days to see if the fuse goes again, if it does it surely has to be the contactor. I’ve not much experience with contactors to be honest!

 

[Pete999]

Have you bothered to take a current measurement on the offending circuit? What type of fuse is it that blows? Could be nothing more than an overload condition, clamp the L supply feeding that particular board.

 

[MDJ]

so 22 electric heaters on a 63 amp supply?

 

[Pete999]

so 22 electric heaters on a 63 amp supply?

That's how I see it Mate, hence my suggestion to take s.ome current readings

 

[WillWX]

That's how I see it Mate, hence my suggestion to take s.ome current readings

Would’ve been my first port of call to be honest.. make life easy for yourself.

 

[marconi]

What is the type of fuse? BS88 HRC gG? Or something else?

 

[telectrix]

insert a "farmer's fuse, charge £200. move on.

 

[Nigel]

Overload or loose connection.

Next job.

 

[marconi]

SSE - you ought to ask if there was ever a trouble-free period before you were called in. What controls the contactor? - a thermostat, timer, caretaker's switch? Measure too the three line - N voltages and compare them - off load and on-load. Measure N-E voltage on and off load. Not only measure running load currents but also (if you have such an ampclamp) measure the peak inrush. Last measure the voltage across the fuse and knowing the current flowing through it calculate the power it is dissipating - do this at switch on, after a minute, 5 minutes and 15 minutes. Do the heaters have integral thermostats - I suspect the seat heaters do not; but the wall heaters might - are they functioning correctly? - Have they been turned to end stop maximum? What size are the feeds to the CUs?

 

[AceRimmer]

Had a problem like that before but was on a bigger supply isolator 300amp I think but sounds like same type isolator what happens was a slack connection on outgoing L2 fuse and burnt through the paxilen what was round metal bar that moves the fuses

 

[telectrix]

deleted

 

[Pete999]

Had a problem like that before but was on a bigger supply isolator 300amp I think but sounds like same type isolator what happens was a slack connection on outgoing L2 fuse and burnt through the paxilen what was round metal bar that moves the fuses

Paxolin