Power supply turned off but cable still live.

[Dajoma]

I was intending to take off a spur from the mains cable in my garage to provide extra plug sockets. I turned off the power to the garage at the main consumer unit by switching off the trip switch for the garage. On checking in the garage all lights were not working and all appliances (e.g. freezer and fridge) had turned off. I assumed that power to the garage had therefore been disconnected. I cut through the cable, fortunately with a rubber handled pair of cutters, only to discover that power was still flowing and tripped the RCD in the consumer unit. I subsequently discovered that there is an isolator switch for the garage power supply (new build house and had not previously seen it) which of course I had not used. Four questions:
1.Should the power have been disconnected even if I had not used the isolator switch, as I had supposedly turned it off at the consumer unit?
2.Does this means that the polarities are wrong at the consumer unit?
3.If answer 2 is “Yes”, would having turned the isolator off have made any difference?
4.Does an isolator switch interrupt both live and neutral connection?
Any advice would be greatly appreciated

 

[securespark]

Was there a loud bang accompanied by damage to your cutters?

If not, the power was off, but the short you created when cutting through the cable tripped the RCD.

If the above assumption is accurate, should this have happened? Yes.

Are polarities wrong? Not necessarily, but to check this from a socket outlet, buy a plug-in tester.

If you have RP (Reversed Polarity), then turning off the MCB (or any single pole switch) would have broken only the neutral. The line conductor would still be live.

Only double pole (DP) switches break both line & neutral conductors.

 

[JonoPashley]

This is the reason we practice safe isolation religiously and always always check for dead using an approved method

http://niceic.org.uk/inc/file-get.asp?FILE=BPG2_March07(sec).pdf&RURL=/en/contractor/section.asp||13

http://www.sjib.org.uk/documents/NWR_SecI_App4.pdf

 

[Lectrician]

How are the lights in the garage fed? Is there a small consumer unit in the garage? If there is, you are not trying to spur from the cable before this consumer unit are you?

It sounds like you simply cut through the cable and shorted N to E. This would cause the RCD to trip. An MCB only isolates the live, not the neutral.

 

[Dajoma]

Hi securespark
Thanks for your quick reply.
There was no loud bang or damage to cutters. Just a rather surprised me at the top of a ladder.
If the power was off when I cut through the cable i.e. live was disconnected, how does a trip occur. I’m not doubting your expertise at all I’m just curious.
I will get a plug tester as you suggest.

 

[securespark]

It will be an NE short, then.

Can you confirm it was the RCD that tripped out when you cut the cable?

If the device that tripped was feeding other circuits and current was flowing in those circuits, then shorting out neutral and earth would cause an imbalance to occur and the RCD would trip.

 

[Dajoma]

securespark
Yes it was the RCD that tripped out and it controls several other devices e.g. downstair lights and sockets etc. So a NE shorts sounds like the answer.
First time I’ve used a forum for anything. Bloody amazing! Will certainly know where to come next time. Thanks for your help.
Thanks also to Lectrician and JonoPashley (advice taken but can’t get either link to work).

 

[ban-all-sheds]

This is the reason we practice safe isolation religiously and always always check for dead using an approved method

http://niceic.org.uk/inc/file-get.asp?FILE=BPG2_March07(sec).pdf&RURL=/en/contractor/section.asp||13

http://www.sjib.org.uk/documents/NWR_SecI_App4.pdf[/QUOTE]
You could do all that and still create a NE fault in exactly the same way the OP did.

 

[Adam_151]

Worse still, you could do all that and still get a shock off a borrowed neutral ... perhaps we should put inverted commas around the word 'safe' ...

*awaits NotHimAgain to comment*

 

[ericmark]

This may seem basic but to Isolate one should use the Isolator not just switch of the miniature circuit breaker (MCB) which is unfortunately even shown as a method of isolation in the PDF that "JonoPashley" directed us to.

Some boards like the one shown in the PDF with insulation tape on have sliders to allow you to isolate the line supply and if one is careful one can work on circuits live and this is common (If one remembers a neutral is considered as live) however it is not recommended and the Electrical Safety Council should no better than to tell people they can work on "live" circuits where only the phase supplies have been removed.

With a TN-C-S supply the 17th Edition does permit the switching of only the line conductors but this is not extended to other earthing systems.

An isolator in domestic looks like this

with some boards you may have a RCD which can also isolate

and some RCBO's also isolate

but RCBO's and MCB's also come in single and triple versions (single and three phase) which do not isolate these

even though you connect both line and neutral to the device only switch the line not the neutral.

Because earth and neutral are bonded together in a TN-C-S system at the head which will normally be very close to the consumer unit single pole switching is considered good enough to work on although it can still cause the RCD to trip. But with both TN-S and TT the voltage between earth and neutral can be quite different although you can still switch just the line supply the neutral must be considered live and for isolation it also must be isolated.

Sorry unable to complete right now

 

[ericmark]

What I had intended to include was procedure for isolation.
1) Test it is live.
2) Remove supply.
3) Test it is dead.
4) Retest tester.
1) For one two main reasons one is to test the tester which should be of a type where it can't be switched off or to wrong range like this

used instead of using proving unit

The second is to insure it is not supplied though something which has temporary switched it off which could switch on again later like a photo cell with lights.
2) This should be done so that it will require a tool (That includes a key) to re-instate the supply. This may be anything from dropping tails to locking the door to room with switch in.
3) It is surprising how many people rely on a label. And also how many people don't walk around with replacement labels in their pocket always test it is dead.
4) Even using battery less and switch less testers like shown it can still go wrong if there is an alternative supply you can use that to test the tester otherwise a proving unit is required. I use the 12 to 230 volt inverter I use for charging things in car when there is no supply.

As with many Electricians I am guilty of not always following the rules. And not always disconnecting the neutral however I am careful and don't touch any neutral wire before checking it is dead and even then I treat it as a live wire and re-test after disconnection. I have a clamp on amp meter which always goes over neutral wire before disconnection not 100% but it has saved me a few times with borrowed neutrals and I wonder how I ever managed without one.