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Topic Title: RCD malfunction
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Created On: 19 November 2012 02:31 PM
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 19 November 2012 02:31 PM
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DMcG1978

Posts: 15
Joined: 22 December 2011

Hi all, long story short. Went to see a domestic TT installation last week, 6mm Earthing conductor had been cut, bonded on to water then continued on its merry way back to the consumer unit. 100mA RCD upfront worked like a treat, so I asked another contractor for a new spike to be installed with 16mm straight to MET which was done. Returned today and now the RCD won't trip, test button won't even work.

Anybody have any ideas what would cause this?

Regards

D
 19 November 2012 02:40 PM
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AJJewsbury

Posts: 11374
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New N-PE fault within the installation -- just co-incidence that it happened between your two visits?

BTW - why a new spike?
- Andy.
 19 November 2012 02:49 PM
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DMcG1978

Posts: 15
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Thanks Andy, couldn't find the old spike under the patio somewhere, so much easier using the 6mm as the bonding conductor and fit a new spike nearer the CU with a new earthing conductor. I'm not following your coincedence theory, to be honest, RCD stays on, it's just been rendered useless as an RCD. Could the other contractor have damaged it somehow?

Regards

D
 19 November 2012 02:51 PM
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vesuvius

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Pee around the Spike BADDUM TISH!!

In all seriousness, the trip button just checks the mechanical mechanism within the RCD for tripping.

Have you performed a full RCD test? what readings did you get? or are you saying theres just nothing happening at all now? bit more info please.
 19 November 2012 03:02 PM
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DMcG1978

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Thanks Vesuvius, RCD tested last week at 27.3mS at 1x and 19.8mS at 5x, test button worked fine. Now nothing at all. No tripping when testing and test button doing nothing.

The only change in the last 5 days is a new spike being installed.

Regards

D
 19 November 2012 03:13 PM
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vesuvius

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Your original results look fine. This seems like a bit of a funny one.

If it were me i would whip the old RCD out and throw another one in just to see if the same happens. (Bit of nightmare i know especially if you dont have a spare RCD hanging about).

I can honestly say ive never seen an RCD that hasnt tripped under test, the only time ive seen the test button not working is when theres no power but this is obviously not the case.
 19 November 2012 03:19 PM
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OMS

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The only change in the last 5 days is a new spike being installed.


You have checked where that new spike connects to have you - if it's been stuffed in the neutral bar by any chance - or not connected at all, to anything and the water bond has been eliminated as well ??

Regards

OMS

-------------------------
Failure is always an option
 19 November 2012 03:26 PM
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AJJewsbury

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I'm not following your coincedence theory, to be honest, RCD stays on, it's just been rendered useless as an RCD.

With the relatively high impedance of a TT system, an N-PE fault can result in a proportion of the "leakage" current finding its way from PE back into N on the load site of the RCD - so desensitising the RCD - sometimes to the point of it failing to trip when expected on test (and sometime on T button, depending on the internal wiring).

Again due to high earth loop, a N-PE fault won't necessarily trip the RCD when loads are applied (unlike TN systems) - e.g. with say a 100 Ohm spike and a 100mA RCD, you'd need 5 to 10V N-PE to cause a trip.

It's just one possibility. Duff RCD is another (have you tested it with the load site totally disconnected?)

What's the resistance of the new spike?

- Andy.
 19 November 2012 03:27 PM
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alancapon

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Did you test the RCD correctly - with all outgoing conductors disconnected from the device?

Regards,

Alan.
 19 November 2012 03:33 PM
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DMcG1978

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Thanks OMS, I (well to be strictly true, my guy) have checked all of the above, water(6mm)and gas(10mm) bonded and connected to MET, 16mm Earthing conductor connected to MET. Getting 51ohms Ze doing a loop test on the 16mm.

All I can think of is the meter (fluke 1653)isn't functioning correctly, "my guy" not functioning correctly or the RCD has been damaged.

Regards

D
 19 November 2012 03:44 PM
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DMcG1978

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The RCD was tested with no load, the whole installation was isolated. The dwelling is the end house of 4 terraced houses, overhead supply to 1st then 2 wire clipped direct along the back of the houses. The water and gas as far as I can tell is metallic between the houses. Andy, Could this have any bearing on results. Would I need to disconnect all bonding conductors before perfoming RCD tests.

Regards

D
 19 November 2012 03:55 PM
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vesuvius

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Instrument manufacturers can supply loop testers that are less likely to trip and RCD. Tow common techniques

1. testing at limited current.

Some instruments limit the test current to below 15ma. this should mean that RCDS with a rated residual operating current of 30ma and greater will not trip.

2. D.C biasing the RCD.

Loop testers using a D.C biasing technique saturate the core of the RCD prior to testing so that the test current is not detected. This technique can usually be expected to be effective for both type A and type AC RCDs.

Though im assuming the tester you are using is the same tester that carried out a perfectly acceptable set of test results before.
 19 November 2012 04:07 PM
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DMcG1978

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Yeah vesuvius, same meter, same RCD, different plonker at the wheel and different earth path is the only differences. No one seems to be suggesting that an RCD can be destroyed, so at least I can rule that out when I go speak to the customer tomorrow. Thought it was going to turn out to be one of those blame games that we are all so fond of these days! End of the day, they have to pay for a new RCD and spike and the other contractor got the work so the customer is safe and there is an other electrician with a days wage in his pocket...but I'm still none the wiser how it happened.

Regards

D
 19 November 2012 04:12 PM
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AJJewsbury

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Andy, Could this have any bearing on results. Would I need to disconnect all bonding conductors before perfoming RCD tests.

No, just both N and L on the load side (i.e. just turning off downstream MCBs isn't adequate).

- Andy.
 19 November 2012 04:14 PM
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vesuvius

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RCD's can be broken, but in my experience it is quite hard to achieve, at least with modern day ones with a BS number.

I cant see your installer, unless hes done something really crazy, breaking your RCD.

During practice testing ive thrown everything but the kitchen sink at RCD's and the only thing ive ever seen one do is trip outside 40ms on the 180 degree X5 test. I keep it now to show a faulty RCD, quite rare.

All of this doesnt help you with your problem of course
 19 November 2012 04:48 PM
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jcm256

Posts: 1861
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I confess I destroyed one by not using the none- trip feature on the loop tester, puff of smoke came out of the RCD. It certainly would not operate with the test button afterwards. Is it an 'Electronic' RCD as opposed to the electro-magnetic-mechanical type?
Regards
jcm
 19 November 2012 04:53 PM
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KFH

Posts: 205
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I just did a PIR on a caravan site and two of the pitch RCDs would not operate on the test button but did test OK via the tester. In both cases there was a neutral fault on each supply caused by me moving a junction box (one for each RCD) that had poor connections inside, in one case the neutral had fallen out altogether. When testing with the tester I had been testing to earth. Had already done Zs readings and they had been OK. The dangers of moving stuff while testing :-)
 19 November 2012 05:28 PM
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DMcG1978

Posts: 15
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This is one of those old Wylex wem 80Amp numbers, I have come across faulty RCDs in the past but just put it down to being seized because these things are rarely tested, who does that? I have 2 RCDs in my own CU and can't say hand on heart that I practice what I preach i.e. test the button quarterly. maybe once a year or two when I do some "DIY" and inadvertently have a N and CPC touching.

On this occasion, on this job, it was fine one day, then a few days later it needs replacing. So can't really use the seized argument.

Regards

D
 19 November 2012 05:51 PM
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OMS

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You almost certainly have a N-E fault on the installation - tests OK when on the water pipe (metal, serves several properties) and when you have now added an electrode I suspect it has substantially higher impedance than the water pipe - get an IR test done - whilst you are there test the RCD from the incoming neutral and outgoing line - see what you get.

Regards

OMS

-------------------------
Failure is always an option
 19 November 2012 05:57 PM
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mossep

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Joined: 05 December 2010

Maybe it is just broken? If your not sure if you tester is working, short out N-E and see if that trips it.

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www.mossep.com
IET » Wiring and the regulations » RCD malfunction

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