Consumer unit RCD oddity

[Jeff Layman]

We have a 3-years old split-load consumer unit, with a separate RCD for each half.

I had to replace 3 old wall-light fittings with some new ones yesterday. I switched off the CU lighting circuit MCB, replaced the first fitting, and switched on the circuit to check the new one was ok. No problems, so changed the second one. No problems again. The third one needed a bit of fiddling, as the hole the cable exited from wasn't central, and the cable had to be moved a bit (the light fitting fixture diameter was about 10mm less than the one it replaced). So I removed about 5mm of plaster from the right side of the hole, moved the cable over, and used some filler on the left side . Of course, some plaster bits went where they weren't supposed to, and I went to get the vacuum cleaner before The Management returned from shopping. It was then I noticed that all the power was off. On checking, I found that /both/ the CU RCDs had tripped. I reset them without problem, and finished the third wall-light. It worked without problem too.

Anyone got any idea why /both/ RCDs tripped? I was working on the lighting circuit only, and not even using any power tools. I checked with neighbours, and none of them had any problem with CU RCDs tripping.

[Andy Burns]

Wall lights on one RCD, socket the vacuum was running from on the other, earthed metal pipe on vaccum, contacted earth or neutral of lighting circuit while you were cleaning-up the plaster, normal leakage from each RCD unbalances the other and they trip?

[Jeff Layman]

Good idea, but no cigar! I should have made it clear that I went to get the hand-held vacuum cleaner, and it was the absence of the "charging" light on it which first alerted me to the total power failure. I'd used hand tools only up to then to remove/replace the fittings - even rather naughtily chipping out the plaster with a screwdriver as I was too lazy to get something better suited for the job!

[The Natural Philosopher]

Neutral is common to both RCDs and so any earth neutral short can trip both

[Andy Burns]

Only on their input sides, if the split neutral buses on their output sides were common they'd trip immediately.

[Brian-Gaff]

On the face of it, no, you need to look for a common denominator for both circuits as that sort of coincidence is unlikely. Brian

[Brian-Gaff]

Most vacuums are double insulated, despite having round cables, but maybe you got one that was not! Brian

[John Rumm]

The neutral ought not be common on a split load 17th edition style CU. Each RCD should have its own separate neutral bus bar.

[John Rumm]

Have you checked that the lighting circuit does actually feed those lights?

(working on carpet etc its possible to touch a live wire and not even realise it!)

There are a few possibilities here...

as you are no doubt aware, a neutral to earth short while the MCB is off will often trip the RCD, however that ought only explain one.

The other could be a co-incidental trip for some other reason.

It could be you have some form of cross linking in the wiring you are unaware of - say a borrowed neutral that bridges the neutral side of both RCDs. (a likely candidate for this would be an upstairs and downstairs two way switch light - it is not that uncommon to find a neutral from the upstairs circuit being borrowed for for a downstairs lamp or vice versa - on a 17th edition style CU it would be likely the the circuits would not both be on the same CU)

Normally you would expect such faults to cause an immediate trip, but there are circumstances where this might not happen.

e.g.

[The Natural Philosopher]

Do RCDs isolate live AND neutral?

[Jeff Layman]

Yes. The way I always do this is to switch the lights on, then turn off the MCB switch. If the lights don't go out, I've got the wrong breaker!

Well, none of us is immune to Sod's Law!

For some background, this place is a 50-years old bungalow we moved into just over 3 years ago, To get house insurance, the company insisted on a full electrical survey. The guy we got was Checkatrade and NICEIC registered. The wiring was fine, but he couldn't update the 20-years old CU as replacement RCDs weren't available. So we had a new one fitted; that actually worked out well as we had some extra work done and more circuits added, which wouldn't have been possible with the old CU.

Of course, switching off the live supply doesn't necessarily mean that something else isn't wired incorrectly (as your link shows), which only becomes apparent when the live supply is cut. And the strange little botches I've found here (although not related to electricity) doesn't fill me with confidence. The odd thing is that I replaced a couple of other light fittings over a year ago - same MCB - and that went without problem.

[Andy Burns]

Yep they're 4 terminal devices (maybe some types have a functional earth too).

If you search for images of split load 17th CUs and trace where the neutral goes through the main switch, to a common bar, then separate connections from the common bar to each RCD and back to separate neutral bars.

While at the same time the live from the main switch goes through each RCD to multiple bus strips feeding each group of MCBs.

You have to make sure each circuit is fed from the output of the MCB and the corresponding neutral bar for that RCD.

[The Natural Philosopher]

IN which case I bet that a resistance meter across the two neutral buses with both trips off will show a reading ;-)

[dennis]

The OP said he tripped the mcb not the rcd so the neutrals could well have still been common and an earth neutral short could cause an imbalance.

[Andy Burns]

Oh no he didn't!

[ARW]

I'll bet the resistance meter will show a reading "between" the two neutral bus bars is somewhere between 0.00 ohms and infinity ohms if it is turned on.

[harry]

Shorting a neutral to earth will trip the RCD if it's still connected. Which it was. Which you did while fiddling with your lighting.

This is why supply should be isolated when fiddling. Ie both live & neutral turned off.

It's also how people get killed if there happened to be another problem on the circuit.

[harry]

[ARW]

What could be better suited to the job than a screwdriver?

Assuming that you were not hacking the plaster away with a PZ2 then job done.

[spuorgelgoog]

And I've never seen insulated chisels, which would be safer when chipping out plaster near cables.

Owain