RCD tripping - can I test if the MCB is damaged?

[charlesh]

Hi,

One of the RCDs in my consumer unit (Volex VCU6) is tripping and won't stay "on". This is for a downstairs lighting circuit comprising dining room light, hallway, outside light and 2 lounge lights. The MCB for this is rated as 6A. The house is less than a year old and I've not added to any of the circuits in any way. So, first question, is this an appropriate rated MCB for this circuit?

Last weekend I had a power outage which was traced to an isolating switch inside my meter box. One of the cables into this switch had not been properly clamped, and so over time had arced and melted. The engineer from the electricity board did not replace the isolating switch, but just wired things back up, as he did not favour the use of such switches for precisely this sort of problem.

Since then, I cannot get the RCD for the lighting circuit to stay on. Is it possible that the problem with the isolating switch has caused this particular MCB to become damaged? Can I test this? Even with the power off at the main switch of the consumer unit, the MCB will not stay in the ON position. Is this normal?

Or, as I've read on other posts, am have I got some kind of residual Earth leak somewhere in the circuit?

Thanks in advance for any advice offered.

Charles

 

[philrosenberg]

If it were me (although i'm just a diyer not a spark) I'd try disconnecting the lighting circuit at the CU (with everything turned off obviously) then stick the power back on (again, with all ppropriate covers replaced) and try to see if the RCD will reset when not connected to anything. If it won't then I guess it's faulty, if it will then it seems likely you have a fault in your wiring. You can get RCD testers, but I have no idea how they work or what they do as I've never used one or seen one used.

 

[RF Lighting]

Is it actually an RCD that is tripping or is it really an MCB?

What is written on the device?

You say that the MCB will not reset even with the main switch OFF. This to me sounds like the MCB has gone faulty. Does it flash or make a popping sound when you try and reset it?




Good ol volex

 

[charlesh]

Hi,

Thanks for the replies chaps.

To describe more precisely what is happening, it is the MCB that is tripping, not the RCD. This particular MCB is marked as being of type VB06 (see Pg 55 of this PDF catalogue: http://www.electrium.co.uk/Volex2006(2).pdf).

So I have tried turning the main swtich off the CU off, with the MCB in the off position, then turning the main switch on. No problems, all other circuits work normally, but then if I try and put the MCB in the ON position it just won't have it. Some times there's no physical resistance to the switch so it goes up and then flops back down again, other times it makes just a single "click" sound and if I've put the light switches "on" for that particular circuit, then the bulbs momentarily come on before the MCB springs itself back into the OFF position.

And to confirm, even with the main switch to the CU off, the MCB will not stay in the ON position.

Incidentally, the dining room light is not on this circuit as I previously thought it was. I have checked all the bulbs, including the outside light, and they work fine.

Can I replace the MCB myself if it is faulty, or is this a job for a professional? Am I likely to find a suitable replacement at my local DIY store do you think?

 

[Lectrician]

Sounds most deffinantly like a dodgy MCB.

Not sure on the reference you keep making to RCD?

They are readilyavailable, and you could change it yourself if you felt confident. You would need to work in the CU which would have live cables upto the main switch with the cover removed

Having a main switch in the meter cupboard would allow you to turn the supply to the CU off, but you no longer have one.

I personally am against additional DP swithes before the CU - I see no point at all in them, they are just a loose link in the chain - as proved.






I am not surprised the MCB has broken - Volex is shocking kit.

http://www.screwfix.com/app/sfd/cat/pro.jsp;jsessionid=4SMWCNSM4GIWYCSTHZOSFFY?id=44464&ts=31993

 

[charlesh]

Thanks for the confirmation Lectrician. The references I've made to RCD are probably down to ignorance I'm afraid - I'm not a professional as you may have guessed.

I have already had the cover of the fuse box off (with the main switch of the CU in the off position) and checked that the brown wires (2 of them) which are secured at the top of this particular MCB were secure. I appreciate there will still be live cables in here from the meter box.

Presumably replacing the MCB just involves loosening the screw which clamps these two in place, and the lower screw, and then levering the MCB out somehow? I tentatively tried this already, but I think I must be missing something as it refused to budge.

 

[Lectrician]

Once the two terminals are undone, you should be able to lift the MCB 'upwards' on that make of CU I recall. Other makes require you to release a din-rail clip with a flat blade driver, but the Voles ones don't I dont think.