A question for the sparkies on here.

cddc

To cut along story short....recent extension done including new kitchen and all new electrics. Yesterday, circuit breaker for Combi/Microwave trips whilst in use. Turn off isolating switch for the appliance, try switching the circuit breaker back on. Trips again. Nothing else on this circuit.

Call the sparky. He comes out, and eventually traces fault to a poor piece of wiring on his part where he has connected the cable from the appliance into the main circuit. This is beyond the isolating switch. Surely the whole point of an isolating switch is that if you turn it off, no power should get past it?

Sparky finds it is still letting 50 volts through with the switch off and blames induction....this after he has changed the switch for a new one.

I am thinking poor wiring, especially after seeing the effects of his efforts on wiring in the microwave....am thinking now of getting someone else in to check all his work. He has given us a part P certificate and supposedly checked everything.

Has he got a valid point or am I being thrown a line here?.

Rockingit

OK, couple of parts to this:

1) Poor initial connection of appliance. TBH, it can happen, and especially in a kitchen environment where access to terminals is rarely perfect. It shouldn't, but it does. It's also hard to test for as there's no 'onward leg' that we can plug a tester into. If your sparks has looked, owned up and fixed it then I'd say that's a fair call all round.

2) Isolation. Well, this is a little more tricky. For starters, I'm assuming that we're talking about a double pole isolator (so that's a break in both the live and neutral wires). So yes, normally you would expect it to do just that and end up with 0v on the downstream side. Sometimes a switch might leak a tiny amount, but generally speaking they do what they say on the tin. HOWEVER... what he said about induction DOES (sometimes) happen. Basically, this is the natural phenomenon (and how electricity is made in the first place) that a current flowing in a loop of wire will create a magnetic field around it. Then, if you take a length of wire and move it through a magnetic field you will create an electric current in that wire. So....what this means in practice is that if you have a 'dead' cable running adjacent to a live one for a distance then it is possible for the magnetic field created in the first to induce a current flowing in the other. The effect of this is most commonly seen in houses where you have two-way lighting circuits powering LED lights - they will still flicker when supposedly off.

This is why the voltage being measured is only around 50V - it's not actually connected to any part of the system so isn't a 'true' power source (although energy is energy and your meter will still be picking it up).

If he's changed the switch, looked at the connections and is happy with it then there's not really a huge amount anyone else will be able to do unless you want to go ripping cables out and redesigning where things are going in the kitchen. There are things called snubbers which can be fitted (basically a way of electrically shorting the induced voltage back down to zero) but I wouldn't worry in this case.

So.....given that I'm not stood in your kitchen with my test kit and inspecting his work, it does sound quite plausible.

If you have any concerns, speak to his Part P scheme (will be on the cert) and they can advise you on the best course of action.

cddc

Thanks for your full and thorough response... I know we are all human and mistakes happen, and was grateful he came out of hours to sort the fault out. Do understand about induction as sell K&B for a living so know the theory and practice. Just never thought you would still trip the circuit breaker with the isolating switch off. What worries me it has been far from the only fault. Thanks for the advice. I may well have a word with part P scheme if I get any more...

Rockingit

cddc wrote: »

Thanks for your full and thorough response... I know we are all human and mistakes happen, and was grateful he came out of hours to sort the fault out. Do understand about induction as sell K&B for a living so know the theory and practice. Just never thought you would still trip the circuit breaker with the isolating switch off. What worries me it has been far from the only fault. Thanks for the advice. I may well have a word with part P scheme if I get any more...

Ah....did I misread your OP? As in, that with the isolator 'off' there was still an upstream protective device (RCD at a guess) not happy? It's not completely impossible according to physics and induction etc, but it's far more likely to be a simple ordinary fault that he's not found.

What type of isolator is it? If it's only a single pole one then you might have a neutral-earth fault which is why an RCD wouldn't reset when it was isolated. It could, also, in fairness to the Sparks be a faulty appliance.

If you post up whereabouts you are I might know some 'trusteds' in your area you can contact if you do want to get a second opinion - I'm also a moderator on an electricians forum hence how I know.

vaio

if a circuit breaker is still tripping with the isolator off then it's nothing to do with induction, it's a fault.

Might be cable or might be circuit breaker but should be easy to find with a tester

Rockingit

vaio wrote: »

if a circuit breaker is still tripping with the isolator off then it's nothing to do with induction, it's a fault.

Might be cable or might be circuit breaker but should be easy to find with a tester

At a practical level I'm in 100% agreement, it's certainly a fault, but my further reference to not impossible re induction is simply that we played around with the physics of collapsing fields, induction and RCD's and it is, on paper at least, possible to upset an RCD given how they actually work.

cddc

Once the sparky had fixed the wiring fault, the RCD was not tripping. My concern was that when the fault occurred, the RCD was tripping despite the fact that the isolating switch was in the off position, and that the fault was past the switch.

It is not still tripping now! Are you saying there is still a fault?

vaio

Rockingit wrote: »

At a practical level I'm in 100% agreement, it's certainly a fault, but my further reference to not impossible re induction is simply that we played around with the physics of collapsing fields, induction and RCD's and it is, on paper at least, possible to upset an RCD given how they actually work.

I thought the OP was talking about a breaker rather than an RCD but, that not withstanding, I'd be interested to hear how induction can trip an RCD with no load on it. Do you have a link?

Rockingit

vaio wrote: »

I thought the OP was talking about a breaker rather than an RCD but, that not withstanding, I'd be interested to hear how induction can trip an RCD with no load on it. Do you have a link?

It was a good couple of years ago and I now can't remember exactly where I found it - though I'll happily have a look when I get a few. If memory serves it basically took the form of the energy of a collapsing field / back emf creating enough disturbance in the balance within the RCD coil to trip it. It was a little unlikely, though on paper plausible. A bit like the chances of my kids clearing up after them.

vaio

I *think* the old voltage type rcd could drip if you switched off an inductive load it was supplying

fluffpot

Also if the switch is not a double pole one (ie switches off both Line and Neutral) then any N to earth fault will still be detected by the RCD