Mains tripped when working on disconnected circuit?

[coldshoulder]

I had to replace our immersion switch today, and switched off that circuit prior to doing this of course (and double checked with my test meter that it was definately then off).

Whilst pushing the three wires through the old casing, the trip for the whole house went plunging everything to OFF. I presume that the wires had touched each other, but surely that shouldn't have caused a short circuit?

Anyway, I carried on and installed the replacement switch and now all is fine - BUT why did this happen? Any thoughts, please

 

[Taylortwocities]

Well done for following isolation procedures!

To isolate the immersion you just turned off the contact breaker (or removed the fuse) at the consumer unit. This only disconnects the live/line conductor leaving the neutral still connected.

The RCD (main switch) looks at both the live and neutral and checks for an imbalance. It will have seen something it did not like as the earth touched the neutral. This is normal behaviour.

Edit - addition:
It is bad practise for there to be a single point of failure in an electrical installation. For this reason a single RCD should not be used as the incomer for a whole house. One day the whole house will be plunged into darkness as your visiting great aunt is standing at the top of the stairs......

 

[bernardgreen]

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

 

[coldshoulder]

Well done for following isolation procedures!

To isolate the immersion you just turned off the contact breaker (or removed the fuse) at the consumer unit. This only disconnects the live/line conductor leaving the neutral still connected.

The RCD (main switch) looks at both the live and neutral and checks for an imbalance. It will have seen something it did not like as the earth touched the neutral. This is normal behaviour.

Edit - addition:
It is bad practise for there to be a single point of failure in an electrical installation. For this reason a single RCD should not be used as the incomer for a whole house. One day the whole house will be plunged into darkness as your visiting great aunt is standing at the top of the stairs......

Thanks for the prompt response. This means then that whenever any work is undertaken on a circuit, the whole system on that RCD is vulnerable to tripping (and our computers too!). I have learnt something today! Many thanks.

 

[Alarm]

One day the whole house will be plunged into darkness as your visiting great aunt is standing at the top of the stairs......

I know people that would pay for that information

 

[ban-all-sheds]

 

[pjcomp]

It is bad practise for there to be a single point of failure in an electrical installation. For this reason a single RCD should not be used

Perfectly correct - but it's also in the regs to protect a TT installation (spike for earth) with an RCD, so there are lots of 'em out there.

PJ

 

[JohnW2]

Perfectly correct - but it's also in the regs to protect a TT installation (spike for earth) with an RCD, so there are lots of 'em out there.

There are certainly a lot of 'em out there, but primarily in relation to (or leftovers from) 'old' installations (RCDs on none, or only some, final circuits). If all final circuits are protected with RCDs or RCBOs, then there surely is no requirement for an additional 'single point of failure' RCD at the origin of a TT installation?

Having said that, I do have (100mA S Type) RCDs at the origin of my TT installation despite all final circuits having protection - left-overs from 'split load' days but also useful as (a) a means of isolation and (b) back-up RCD protection.

Kind Regards, John

 

[xr4x4]

If it had a FCU in the immersion circuit and this was turned off, should the rcd in the cu still trip?

 

[bernardgreen]

If it had a FCU in the immersion circuit and this was turned off, should the rcd in the cu still trip?

The FCU would be single pole so the neutral would still be connected. Therefor a neutral to earth short would create a bypass past the RCD sensor for some of the load current of any equipment on that neutral bar.

 

[xr4x4]

Arrr... I thought FCUs are double pole.

 

[JohnW2]

The FCU would be single pole so the neutral would still be connected.

I thought that most (maybe all) switched FCUs now have DP switches - although that may well not have been true in the past.

Kind Regards, John.

 

[kai]

Use a MK K330WHI DP switched 13amp fuse-connection unit for immersion of 3kw or below - I have one on mine, it is definitely double pole switched with a 20 year guarantee. The information sheet states that it has a Live break first, and neutral break last feature in its mechanism.

 

[securespark]

One day the whole house will be plunged into darkness as your visiting great aunt is standing at the top of the stairs......

One day the whole house will be plunged into darkness as your visiting great aunt is standing at the top of the stairs......

Sorry. Alarm beat me to it on the Great Aunt/ Joke angle and I missed it. Again. Must read thread fully before operating KB.

 

[holmslaw]

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