Any electricains out there?

[Tractor Man]

I've got a bit of a problem with the electrics to my workshop and wondered if anyone could help.

I've just had a dedicated consumer unit installed from the mains power coming in to supply power to the workshop, before the workshop power came from a 40A MCB in the house consumer unit. I had this new unit fitted as when I was running the welder the house lights flickered. The details of the new installation are shown in the diagram.

However the RCB circuit breaker in the workshop consumer unit and the unit in the house are tripping (shown in red). I have isolated all the MCB's in the workshop CU so no power is being drawn and this is still happening. Yesterday I was using the compressor in the day and neither breaker was tripping, however I went into the workshop in the evening and turned the lights on and both breakers tripped.

Should an MCB circuit breaker only be fitted at the workshop end and not at both ends, with just an on off isolating switch and a 40A MCB in the workshop CU in the house?

I'm probably being a bit confusing, but I would ideally like to know what the problem is before calling the electrician in again!

Any help would be greatly appreciated.

Will

 

[madkayaker]

havening two RCD the same rating and time is going to cause issues as they fight each other but thats normal. If the RCD is tripping when all the MCB are off it due to a N to Earth fault. The cable to the garage should be protected by a time delayed RCD. The MCB at the supply end protects the sub main cable and the individualize ones protect each circuit. Has the workshop been working since the change? is it occasional or regular?

 

[Tractor Man]

It started to work ok after the change, but then started to trip fairly regularly (mostly in the evening), however sometimes it does work normally for a period of time but then suddenly trips for no apparent reason. This is why I can’t understand what is causing the problem.

Thanks for the help.

 

[madkayaker]

what kind of light have you got?

 

[Tractor Man]

3x 500w halogens and 2x 100w bulbs

 

[madkayaker]

not that then. if it had been flouro that can be a pain in ****. time to get a sparkey to test insulation resistance that could be down. or check for water an condensation in equipment

 

[Tractor Man]

I had a feeling it could be condensation as it only seems to trip in the evnings when its colder.

Thanks again

 

[rtbcomp]

One problem a mate of mine had was that he connected the live feed to the lights to the protected live on the RCD but took the neutral back to the unprotected neutral. Everytime he put the light on the breaker tripped.

This was a solid fault though, yours sounds intermittent.

 

[madkayaker]

RCD work by balancing what goes out with what comes back. If there is a difference they trip.

 

[Tractor Man]

Somtimes everythings works perfectly, other times the main breaker just wont turn on!

It seems to happen more in the evening, would the main breakers be affected in a drop in temperature or a very small amount of condensation?

 

[Woody.v8]

Somtimes everythings works perfectly, other times the main breaker just wont turn on!

It seems to happen more in the evening, would the main breakers be affected in a drop in temperature or a very small amount of condensation?

How old is the RCD?

 

[madkayaker]

you would need enough for there to be a path between two parts e.g. N to Earth. Electricity is energetic stuff and can jump 3mm at 230V. Radio's and the like are normal candidates if left plugged in.

 

[Tractor Man]

The RCD is probably about a year old. Its a Wylex 100A 30ma.

http://www.screwfix.com/prods/77076/Electrical-Supplies/Consumer-Units/Wylex-MCBs-Incomers/WYLEX-100A-30mA-DP-RCD

 

[madkayaker]

You can sometimes get condensation across the top of light bulbs on pendant (thats why you can't use them in bathrooms) As its occasional live with it and it will either clear or big bang then you have found the fault. Unless you have a friendly sparky then get him to give it once over.

 

[Tractor Man]

I think that I will have to see what happens when it starts to get warmer (hopefully soon!), if the problem still persits then I think I will have to call out the spark and get him to do some tests!

Thanks everyone for your help

 

[piman]

Hello Tractorman,

residual current breakers are very sensitive so condensation as you suggest could be the problem. Have a close look at the terminations at both ends, for as you say it trips with no load so there must be some leakage somewhere in the installation. Is the cable buried and if so could it have been damaged, although that would probably give a permanent fault.

Just to add to this, although too late for you, it is better, although more expensive, to use a breaker for the main supply and RCD's for individual circuits which minimises nuisance tripping.

Alec

 

[Tractor Man]

Thanks piman

I'm believe that there is a fault with one of the the RCB's so I will get it tested and may install a new one, or as you suggested use a breaker for the main supply and use and RCD for the workshop supply.

Will

 

[madkayaker]

to comply with the regs you need a RCD at the mains end. It can be up to 300mA.

 

[piman]

Hello MK,

why is that?

Alec

 

[piman]

Helloa TM,

as both are tripping, I don't think that one of them is faulty. I still think there is a leakage somewhere in the main circuit?

Alec