Isolate circuit but trips fuse board

[Ellsa100]

Hi

Weird problem.

I’m replacing plug sockets. I isolate the circuit for the particular sockets at the fuse board so it is safe to swap out. All good. Suddenly the RCCB for all sockets - not just the circuit of the sockets I have isolated - trips. Why is this happening?

The RCCB doesn’t trip when all the circuits are live. It is as if it won’t let me isolate an individual circuit as it trips the whole consumer unit shortly after I isolate a circuit

Thanks

 

[Risteard]

So is it dangerous to work on a circuit when I use the circuit breaker? I always check with a non-contact voltage tester there is no voltage on the circuit before commencing work on that circuit. Turning off the main switch seems a bit over the top just to work on one circuit? Plus I need lighting. Thanks for your thoughts.

What you have to realise is that a neutral conductor is classified as a live conductor, because it is intended to carry current in normal use.

 

[Ellsa100]

What you have to realise is that a neutral conductor is classified as a live conductor, because it is intended to carry current in normal use.

Thanks - appreciate that- so should I avoid working on a circuit when I have used a circuit breaker to turn off live for that circuit and tested live and neutral with non voltage contact tester (but not turned off the main switch) and if so why? Thank you

 

[AJB Temple]

Think of it this way Elisa 100. Live current flows out of your meter, via the consumer unit, through all of your circuits and flows back through the neutral wires. This flowing current will be used when you have an appliance or light switched on.

All the neutral circuits are directly connected together in your consumer unit using a metal bar in that consumer unit. Same applies to all of the earth wires (which modern electricians may call a CPC).

Your Circuit Breaker (MCB M being for Miniature) in your case is interrupting the live circuit that it is attached to, but is NOT interrupting the neutral circuit. Current is flowing (returning) through the entire neutral circuit in your house as you will have lights on as you said and maybe appliances in use (eg washing machine) on different circuits.

So even on the circuit you think you have isolated, your neutral remains connected to the rest of the neutrals on live circuits.

If you touch a neutral on your isolated circuit to earth, the residual current circuit breaker (RCCD) in the consumer unit will detect this as a leakage of current. As a leakage could for all it knows be current flowing through you to earth (ie you are getting an electric shock) it plays safe and trips all power off everywhere.

I am a DIYer too and I expect qualified professionals could explain it better than I have. Personally I do not work on circuits without isolating the neutral as well. My consumer unit has a type of switch that does that and I can also isolate the consumer unit totally from the meter if I need to with a simple switch as well. In your case I would turn everything fully off at the main switch and use a battery powered light to assist you if that is needed. Electricity is a killer so don’t take risks.

By the way, non contact voltage protectors are less than ideal for identifying when something is live or not (unreliable and give a false sense of security at times), and will not identify anything that is only potentially live. There are more suitable testers that will be much safer - but obviously that incurs expense and you need to know how to check them and use them.

 

[timhoward]

Thanks - appreciate that- so should I avoid working on a circuit when I have used a circuit breaker to turn off live for that circuit and tested live and neutral with non voltage contact tester (but not turned off the main switch) and if so why? Thank you

It's linked to why the RCD tripped. The N conductor is connected to other N conductors inside the consumer unit.
It is possible (should any of a number of things happen upstream, inside or outside your house or with the supply) for the current path of other circuits in your house to go via the socket circuits N wire, via you, to earth.

This reason was why I said "or better use the main switch". @Risteard is spot on that really it should be the only approach in your situation.
You might get away with it 99.9% of the time, but if the supply N happens to break at the wrong moment, it's curtains time!

(This is one of a number of reasons why new consumer units that I fit have double pole isolation on each circuit and separate RCD protection for each circuit. They are much easier to do maintenance on!)

There are other reasons too related to the average experience of DIY people doing this, that would not necessarily apply in your case.

 

[cireland]

Non-contact voltage detectors are NOT to be used to prove a circuit is dead.
I would strongly advise getting a set of two pole voltage detectors, test before isolation to prove working,
isolate, prove dead, then retest detector to prove still working.

 

[Ellsa100]

Non-contact voltage detectors are NOT to be used to prove a circuit is dead.
I would strongly advise getting a set of two pole voltage detectors, test before isolation to prove working,
isolate, prove dead, then retest detector to prove still working.

thanks. I have a multimeter - can I use that instead? And what would the test be, just that there is no voltage from either Live or Neutral when compared to Earth or something else? thanks again.

 

[Ellsa100]

It's linked to why the RCD tripped. The N conductor is connected to other N conductors inside the consumer unit.
It is possible (should any of a number of things happen upstream, inside or outside your house or with the supply) for the current path of other circuits in your house to go via the socket circuits N wire, via you, to earth.

This reason was why I said "or better use the main switch". @Risteard is spot on that really it should be the only approach in your situation.
You might get away with it 99.9% of the time, but if the supply N happens to break at the wrong moment, it's curtains time!

(This is one of a number of reasons why new consumer units that I fit have double pole isolation on each circuit and separate RCD protection for each circuit. They are much easier to do maintenance on!)

There are other reasons too related to the average experience of DIY people doing this, that would not necessarily apply in your case.

thanks, appreciate advice. the consumer unit is about 15 years old. would you recommend I upgrade the consumer unit so it has double pole isolation and separate RCDs as it is not very practical to turn off the mains each time i want to work on a circuit? or is there another solution? thanks.

 

[Ellsa100]

Think of it this way Elisa 100. Live current flows out of your meter, via the consumer unit, through all of your circuits and flows back through the neutral wires. This flowing current will be used when you have an appliance or light switched on.

All the neutral circuits are directly connected together in your consumer unit using a metal bar in that consumer unit. Same applies to all of the earth wires (which modern electricians may call a CPC).

Your Circuit Breaker (MCB M being for Miniature) in your case is interrupting the live circuit that it is attached to, but is NOT interrupting the neutral circuit. Current is flowing (returning) through the entire neutral circuit in your house as you will have lights on as you said and maybe appliances in use (eg washing machine) on different circuits.

So even on the circuit you think you have isolated, your neutral remains connected to the rest of the neutrals on live circuits.

If you touch a neutral on your isolated circuit to earth, the residual current circuit breaker (RCCD) in the consumer unit will detect this as a leakage of current. As a leakage could for all it knows be current flowing through you to earth (ie you are getting an electric shock) it plays safe and trips all power off everywhere.

I am a DIYer too and I expect qualified professionals could explain it better than I have. Personally I do not work on circuits without isolating the neutral as well. My consumer unit has a type of switch that does that and I can also isolate the consumer unit totally from the meter if I need to with a simple switch as well. In your case I would turn everything fully off at the main switch and use a battery powered light to assist you if that is needed. Electricity is a killer so don’t take risks.

By the way, non contact voltage protectors are less than ideal for identifying when something is live or not (unreliable and give a false sense of security at times), and will not identify anything that is only potentially live. There are more suitable testers that will be much safer - but obviously that incurs expense and you need to know how to check them and use them.

thanks - useful advice.

So am I right in understanding there is always a current flowing through the neutral wires - including the circuit which has been broken - as long as at least one circuit is live and/or unless the main switch is off?

 

[Ellsa100]

also - is using a mains wiring socket tester - the one which plugs into a socket which checks for mis-wiring (I have the Kewtech KEWTECH 103 - OK instead of using a voltmeter / multimeter to show the lines are not live?

 

[cireland]

The reason for using a test lamp is it's easier to prove that they are working correctly. If you isolate the RCD, none of your sockets will be working , so unable to prove correct operation.

 

[Ellsa100]

stumbled across great video here

which summarises what most of you have said

 

[Ellsa100]

The reason for using a test lamp is it's easier to prove that they are working correctly. If you isolate the RCD, none of your sockets will be working , so unable to prove correct operation.

Yes, but my question is about whether a mains wire tester is (together with turning off the circuit at the MCB) a safe alternative to turning off the main switch (when changing a plug socket etc)? I am not sure if it is?

 

[cireland]

It isn't. As has been pointed out, the mcb only isolates the line conductor.
Isolating the RCD or main switch isolates both line and neutral.
Is your D.B. the same as the one in the video?