WYLEX RCDO

Trefor · 19 Jul 2014

Hi
I have just built a new workshop in my garden and I am wiring it up.

I used to work as an electrician many years ago but I am not up to date with the current regulations. However I spoke to an electrician friend who passed my plans for the permanent installation and is going to test the installation when it is complete.

The shed is supplied by a three cored armoured cable, live, neutral and earth with the armouring also connected to earth. I have been using the buried armoured cable as a temporary 'extension lead' fed via a 13 amp plug and an RCD whilst fitting out the internals. I have had no problem with this arrangement.

I am now connecting the electrics permanently. I have fitted a WYLEX NH series metal distribution board with an MCB for the lighting and an RCDO for the ring main. I have connected the RCDO as per the WYLEX instructions and the circuit powered up correctly when switched on. Wishing to test the RCDO I pushed the 'test' button, there was a flash and a burning smell and the RCDO will not energise the ring main. I had already tested the ring main with an old megger before connecting the RCDO, I have since re-checked my wiring and it conforms with the WYLEX instructions.

Unfortunately my friend is away on a well earned holiday so is unavailable. Can anybody help?

AndyPRK · 19 Jul 2014

Where did you get the rcbo from?

Where did you connect the white wire?

We're you a qualified electrican ?

JohnW2 · 19 Jul 2014

AndyPRK said:
Where did you get the rcbo from?

I would think that's the most crucial question. No matter how it was wired (correctly or not), including the question of what, if anything, the (white) functional earth was connected to, and particularly since it worked OK when energised, for it to 'go bang' when the test button was pressed is surely an indication of something fundamentally wrong with the RCBO, isn't it?

If it cost a sensible amount of money and came from somewhere reputable, then I would say that it should be returned for replacement. If it was cheap and came from somewhere iffy, then a lesson has hopefully been learned!

Kind Regards, John

Trefor · 19 Jul 2014

Hi
Yes I was a qualified electrician but as I posted that was a long time ago, I went on to do a degree etc. and I'm now an instrumentation and control systems engineer. Because my electrical installation experience was a long time ago I asked a qualified friend to check over what I was doing and to approve this. He is also going to test the installation when it is complete.

The RCDO is a WYLEX one from TLC and cost, if I remember correctly, about £23.00.

The white functional earth was connected to the earth bus bar. It says in the instructions 'route functional E lead to the chosen E bar connection'.

JohnW2 · 19 Jul 2014

Trefor said:
Because my electrical installation experience was a long time ago I asked a qualified friend to check over what I was doing and to approve this. He is also going to test the installation when it is complete. ... The RCDO is a WYLEX one from TLC and cost, if I remember correctly, about £23.00. The white functional earth was connected to the earth bus bar.

In that case, as I said, I think you almost certainly have a duff RCBO which needs to be returned to TLC for replacement. As I said, no matter how incorrectly you had wired it (and I'm not suggesting that you did), there's no way that it should 'go bang' when you press the test button. Was this perhaps the first time you had pressed that button whilst the circuit was energised?

Kind Regards, John

ericmark · 19 Jul 2014

I would phone TLC as likely it's one of a batch and they would likely post a replacement without the need for it to be returned.

Trefor · 19 Jul 2014

Thanks everybody. I will get in touch with TLC ASAP.

Adam_151 · 19 Jul 2014

JohnW2";p="3155669 said:
Trefor said:

As I said, no matter how incorrectly you had wired it (and I'm not suggesting that you did), there's no way that it should 'go bang' when you press the test button.

Click to expand...

Its possible on a three phase board to acheive that through... wasn't me, and the guy was lucky that only an RCBO got energised before problem spotted!

As to the OPs issue, it has been known for them to fail in this way, I had one go on me the other month while doing an EICR, RCD tester determined that it had failed, was just setting the tester on a surface ready to make note of the fact and the rcbo tripped off with a bit of a flash... It appeared to me like it was still trying to trip from the RCD test, something failed inside resulting in a dead short which tripped the over current part of the device

Trefor · 19 Jul 2014

Hi John
Yes I connected the wires, switched on and everything was OK. I plugged a small lamp into one of the sockets on the ring main, it lit and seemed OK, so I pressed the test button and then bang etc. The circuit couldn't then be powered up again.

I will contact TLC

Thanks for your help.

JohnW2 · 19 Jul 2014

Adam_151 said:
JohnW2 said:

As I said, no matter how incorrectly you had wired it (and I'm not suggesting that you did), there's no way that it should 'go bang' when you press the test button.

Click to expand...

Its possible on a three phase board to acheive that through...

Possibly. I'd have to think about that, but I still find it hard to believe. Let's face it, all the test button should do is connect one of the inputs of the device to one of the outputs of the devise through a relatively high value resistor (designed to not allow much more than 30mA to pass even with full supply voltage across it). I suppose if one managed to get P-P voltage between the L&N of the RCBO, it might possibly be enough to blow up the resistor, but I would have thought that unlikely.

Adam_151 said:
As to the OPs issue, it has been known for them to fail in this way ...

Indeed. It can certainly happen with RCDs. You may recall a year or two ago that I reported here that when my daughter pressed a button on an RCD (which she'd done many times before, without incident), there was a flash and a bang which took out the DNO fuse. As I illustrated in photos, the problem there was a (presumed) manufacturing fault which allowed the insulation of a live conductor to get lacerated when the mechanism operated. I presume that the same bang/flash would have occurred if the mechanism has operated in response to a fault, rather than pressing of the test button. Here's some of the relevant photos, the last of which shows the culprit insulation damage ('held open'):

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JohnW2
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JohnW2
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Kind Regards, John

JohnW2 · 19 Jul 2014

Trefor said:
Hi John ... Yes I connected the wires, switched on and everything was OK. I plugged a small lamp into one of the sockets on the ring main, it lit and seemed OK, so I pressed the test button and then bang etc. The circuit couldn't then be powered up again. ... I will contact TLC.

Indeed. I really cannot think of a plausible explanation other than a 'duff' RCBO. See previous post for an illustration of what happened with my daughter's RCD a year or two ago.

Kind Regards, John

garlicbread · 19 Jul 2014

Depending on your istallation method, the lighting may also need RCBO protection for a certificate.

Trefor · 19 Jul 2014

garlicbread said:
Depending on your istallation method, the lighting may also need RCBO protection for a certificate.

The new workshop is fully weathertite, Braced 4 x 2 timber with board then covered with a breather membrane with tongue and groove over, insulated inside with plasterboard. Roof is boarded and covered with two layers of felt, insulation inside and also plasterboarded. Its very snug and dry.

The light are fluorescents that are all internal to the workshop, there is no external lighting on the workshop light circuit which is wired in 1.5mm twin and earth, all fittings are correctly earthed.

JohnW2 · 19 Jul 2014

Trefor said:
... Roof is boarded and covered with two layers of felt, insulation inside and also plasterboarded. Its very snug and dry. ... The light are fluorescents that are all internal to the workshop, there is no external lighting on the workshop light circuit which is wired in 1.5mm twin and earth, all fittings are correctly earthed.

Fair enough, but the point being made was that if any of the wiring in the lighting circuit (including that to switches etc.) was 'buried' <50mm below the surface of the plasterboard, then the regulations would require the circuit to have RCD (or RCBO) protection (and also routed in the prescribed 'safe zones'). If the wiring were entirely on the surface, and visible, there would be no such requirements.

Kind Regards, John

Adam_151 · 19 Jul 2014

If the lighting cabling is concealed less then 50mm from the finished surface and does not have an earthed metallic covering (e.g. SWA or MICC) then in a domestic environment (your garden workshop is still classed as domestic!) then it'll have to be RCD protected to comply with the 17th edition. If you had wired it with the cabling surface in trunking or conduit then RCD protection would not be necessary

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WYLEX RCDO

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