Possibly drilled through a cable?

Tonight I may have drilled through a cable but at no point did I notice it, in terms of drill feel, reaction, or anything dramatic happening. I merely have the coincidence of drilling into walls and an electrical problem. Perhaps you guys can help me figure out if I should rip the walls open or not?

The RCD on one of the "groups" of circuits tripped and will not come back on unless the red "MAIN SWITCH" is turned off. Even if I switch off ALL of the black switches in this group, if the MAIN SWITCH is on then the RCD immediately trips back off. The black switches include such things as the cooker, kitchen sockets, and upstairs lighting as well (I don't know why). The house has several such groupings, duplicated on day/peak, and night/off-peak CU's (it has storage heaters, your see).



Now for what I was up to: a cupboard tore off the wall in the house and the original fixings were done for, as was the plasterboard in the immediate vicinity. As a patch, I screwed some ply to the plasterboard and attached the cupboard back up to that. It was at some point during this work that the electrical issue appeared - I only noticed when my saw wouldn't go back on. I had indeed been drilling during that time but I can't say for sure that's when it happened as nothing e.g. the lights were affected right where I was.

The kitchen was new in 2010 and all done from scratch, as in from the bricks up - all new plasterboard (on battens), all new electrics, etc. I was not there for any of this but I know it took place, as part of a whole-house renovation.

The fixings on one side of the cupboards were right in the middle of one of the vertical permitted cable zones (i.e. 4ft or so above a socket), so theoretically cables would have been permitted there. But the presence of the original cupboard fixings suggested to me that when the kitchen was installed it was known that there were no cables in that zone (it all being done at the same time), so I imagined it safe. Also, all of the kitchen sockets etc run along in a rim above the counter top and it would be an arbitrary place for one to go up vertically. So I decided it would be alright - maybe foolishly.

What shall I do tomorrow to check this out? There are other RCDs I can try switching around into that slot, that's an obvious first step I can try tomorrow.

Since the RCD stays tripped even with ALL grey circuit switches off, does that tell us anything? That seems to me like it would isolate all of the circuit(s) already, so if I had drilled through something it would be cut off and no longer troubling the RCD?

Thanks for any ideas

smp50 said:

Since the RCD stays tripped even with ALL grey circuit switches off, does that tell us anything? That seems to me like it would isolate all of the circuit(s) already, so if I had drilled through something it would be cut off and no longer troubling the RCD?

Click to expand...

Yes, that does possibly "tell us something", assuming that your drilling were responsible. At first sight, it sounds as if your drilling activities might have resulted in a short between the neutral and earth conductors of one of the cables on a circuit protected by that RCD. The 'grey circuit switches' [do you mean the ones with black levers?] (MCBs) only disconnect the live/line conductor, not the neutral, so would not isolate a fault between the neutral and earth.

The only problem with that explanation is that for such a fault to make the RCD stay tripped would require that something somewhere in your installation was drawing current, which should not be the case if all of the MCBs were switched off.

If the problem is of that nature, I suspect that it may be necessary to disconnect all the MCBs in that 'section' (disconnecting both L & N) in order to determine which of the circuits is causing the problem. It's the sort of work for someone who is comfortable and competent doing things within a consumer unit.

Kind Regards, John

JohnW2 said:

The only problem with that explanation is that for such a fault to make the RCD stay tripped would require that something somewhere in your installation was drawing current, which should not be the case if all of the MCBs were switched off.

Click to expand...

No it would not, it just requires a potential difference between neutral and earth. On a TN-S or TT supply you are likely to have such a potential difference even with everything in your installation switched off.

plugwash said:

No it would not, it just requires a potential difference between neutral and earth. On a TN-S or TT supply you are likely to have such a potential difference even with everything in your installation switched off.

Click to expand...

Correct.

I'd say that it may not have happened during drilling, especially if you have simply srewed the cupboard to the ply with longer than needed screws.

If you have a meter it's worth testing all the exposed screw heads.

SUNRAY said:

If you have a meter it's worth testing all the exposed screw heads.

Click to expand...

Ah, nice idea - yes I do have a multimeter, could you specify what/how I should test them? What setting and where will each electrode go?

I will return to the house in a few hours and do this. I'll also remove the cupboard and ply, and see if I can see anything through the drill holes.

Unscrew the sockets from the wall directly below where you have been working, with the power off if you've managed to restore it since. Have a look behind them and if any of the cables go up directly below where you have drilled or put any screws into studs then it's safe to assume you've hit a cable. If you have hit a cable then you'll need to open up the wall in the vicinity of the offending screw/drill hole and repair/replace the cable.

plugwash said:

No it would not, it just requires a potential difference between neutral and earth. On a TN-S or TT supply you are likely to have such a potential difference even with everything in your installation switched off.

Click to expand...

That's true. Ironically, I only added that second paragraph as an edit because, having read what I had just posted, I was thinking that some clever clogs might otherwise try to find a 'flaw' in what I had written about N-E faults. My only excuse is that it was the middle of the night My apologies.

Having said that, the phenomenon you describe will probably only apply in the case of 'negligible impedance' L-E faults (i.e. conductors actually touching) - such as the OP may have. My experience is limited, but, other than when I have accidentally touched L&N together, I'm not sure that I have ever seen an RCD operate because of an L-N imbalance 'in that direction' - i.e. more current through the N side than through the L side (the latter zero when everything is switched off).

What I have usually seen is the opposite, with less current through N side than L side of the RCD, because of some 'N current' having leaked to earth through some 'fault' on a switched-on (and loaded) circuit. That often results in considerable confusion, because the RCD then usually only trips if a significant load on one of the circuits protected by the RCD is switched on. That situation is most common when the N-E fault is of relatively high impedance (not low enough to trip the RCD solely on the basis of a small L-N PD in a TN-S or TT installation), usually due to water/moisture (which is easily the most common situation in which I have seen RCD trips).

Kind Regards, John

SUNRAY said:

If you have a meter it's worth testing all the exposed screw heads.

Click to expand...

I was going to suggest that, until I realised that it probably wouldn't help much. Measuring resistance to N from any screw head on any accessory connected to any of the relevant circuits (protected by the same RCD) would confirm, in general, the existence of an N-E fault - but I think we already know that, since the RCD trips with all MCBs 'off' and can only be reset with the Main Switch off. It won't help to identify the affected circuit, since the Ns and CPCs of all relevant circuits will be joined in the CU.

That's why I suggested, instead. that the only way to narrow the problem down to one circuit would be by disconnecting Ns and/or CPCs from individual circuits at the CU.

Kind Regards, John

I assumed sunray was suggesting testing screwheads NOT associated with electrical accessories.

plugwash said:

I assumed sunray was suggesting testing screwheads NOT associated with electrical accessories.

Click to expand...

It's not my day. is it? ... yes, that's probably what he meant! Apologies again.

Kind Regards, John

I would take the cupboard down and dig around the fixing holes with an old driver to see if there's anything there.
Turn the power off first!

smp50 said:

AI do have a multimeter, could you specify what/how I should test them? What setting and where will each electrode go?

Click to expand...

smp50

As you have a multimeter, I would check the heads of the new screws and see if any have continuity to earth.
My assumption being that the new screw that has hit the wire 'might' be in contact with the earth wire (and multimeter will show LOW resistance when testing for Earth).
Screws that have not hit a wire should have no connection with the earth wire. (and multimeter will show INFINITE resistance)
(Of course the screw might of only hit the L and/or N and so this method will not work if screw is not touching the earth).

So using a multimeter set it to measure 'resistance' (best if set to a low value of resistance).
Put one multimeter probe onto the metal screw of a nearby light switch or plug socket (as these metal screws should be connected to the earth wires that are in your house).
Put the other multimeter probe onto the head of a new screw that you put in the wall.

You should see infinite resistance (often a line or E or same display as when you hold probes apart) showing that the two are not connected, indication that that scrtew has not hit a wire.
If you see a resistance value on the mutimeter (similar display as when you touch probes together) then that suggest that that new screw has hit a wire and is touching an earth wire.


As a quick practice test, put one proble onto the screw of a plug socket, and one proble on the bare metal of oven/hob. Both should be connected to earth and Multimeter should (correctly) show a low resistance measurement.

SFK

If you have used a plastic or fibre wall plug, the screw itself may not actually be touching the damaged wires.

sparkwright said:

If you have used a plastic or fibre wall plug, the screw itself may not actually be touching the damaged wires.

Click to expand...

Indeed. In fact, there may no longer be anything still touching the damaged wires - the conductor of a damaged (insulation-damaged) neutral may have been pushed into contact with the bare CPC/earth conductor.

Hence.one could not draw any confident conclusions as a result of not finding continuity between one of the screws and something earthed but one could 'draw conclusions' if one did find such continuity.

Kind Regards, John

Ahem!

securespark said:

I would take the cupboard down and dig around the fixing holes with an old driver to see if there's anything there.
Turn the power off first!

Click to expand...