Mcb tripping

[lightning]

Hello

We seem to have delevloped a fault in the down stairs ring.
We can use the tv, hair dryer standing lights sky but soon as we plug in say kettle , lawn mower it will trip.
We had a out side light wired up through the garage consumer unit all work done by a qualified sparky and also a 3 way light switch all working correctly
Check all sockets for loose wires but no problems

Sparky is coming ack to test but like me busy till Nx week

 

[John D v2.0]

Sounds like a small neutral fault to earth somewhere on the same rcd as the socket circuit. You'll likely need a tester to find it

 

[ericmark]

Does not make sense. For a MCB to trip it means over the 32 amp normally used for ring finals, although a lawn mower or kettle could be faulty, it seems unlikely both have become faulty together.

So do you mean a RCD or RCBO? Now with a RCD then it would make some sense for high loads to cause it to trip and low loads not to affect it.

With a RCD a neutral to earth fault may not cause a RCD to trip as the voltage is the same with both earth and neutral so no current can flow. However as the load increases the voltage in the neutral raises, compared with earth, so when large loads are used then the difference is enough to cause a trip.

So if we for example have a bit of damp bread stuck in the toaster this may produce a neutral to earth fault even when the toaster is not switched on, it only needs to be plugged in. However not enough current will travel to earth to trip the RCD, however when the kettle is used, then there is enough current going through the damp bread, so it trips.

So step one is unplug everything you can, and switch off all you can with double pole switches, and see if kettle still trips the RCD.

Lights are a problem as you can't turn off the neutral, however they should be split, and also the sockets should be split, so you test kettle in a socket which is not from the same RCD.

This is why outside lights normally use double pole switches, so you can turn off the neutral.

 

[lightning]

Sorry meant rcd , kettle doesn’t trip with up stairs ring or in kitchen.
Have unplugged everything and removed fuses from fuse spur but still trips .

Thanks for posts

 

[lightning]

Have tested every socket with a tester all ok
Lights are on 2 circuits up and down stairs
Thanks

 

[John D v2.0]

Have tested every socket with a tester all ok
Lights are on 2 circuits up and down stairs
Thanks

Was it an insulation resistance tester? If not it won't find it. You may be able to find with a multi meter but either way you'd need to understand what the problem is and have some fault finding experience.
Removing fuses won't help as they are in the live not neutral. But you'd need to basically isolate, break down the circuit and test in half until you find the problem.

 

[Taylortwocities]

OK. Turn of the main switch in teh garage consumer unit. Does the problem still happen? If not, then its likely the fault is something attached to the garage CU.
If the outside light was a recent thing + all the rain we have had, then I would suspect that first. How long ago was it fitted.

 

[ericmark]

Sorry meant rcd , kettle doesn’t trip with up stairs ring or in kitchen.
Have unplugged everything and removed fuses from fuse spur but still trips .

Thanks for posts

This is the problem, the fuse removal does not help, as fuse is not in the neutral, it is in line, and likely problem is a neutral earth fault. It is hoped the switch on a switched FCU is double pole.

 

[lightning]

Thank you for reply when I turned off garage cu the problem still exist

Thanks

 

[lightning]

Yes light was fitted a while ago on a plug into socket in garage , got sparky round to ride up wiring as a lot of fuse spurs taken of sockets.
Now light goes into double pole switch then wired back to cu

 

[bernardgreen]

Some electricians cannot understand how a fault on circuit whose MCB is switched off can cause an RCD to trip when a load on a different circuit is switched on. This lack of experience leads them to believe the fault is on the circuit that is supplying the load and not on the circuit whose MCB is switched OFF

 

[JohnW2]

Some electricians cannot understand how a fault on circuit whose MCB is switched off can cause an RCD to trip when a load on a different circuit is switched on. This lack of experience leads them to believe the fault is on the circuit that is supplying the load and not on the circuit whose MCB is switched OFF...

I'm sure that, over the years, your oft-posted diagram will have helped a lot of people to understand this issue, but I wonder if you have ever considered re-drawing it so as to make it even clearer?

As I need not tell you, there is nothing wrong with what the diagram shows (in terms of current paths etc.), so I am merely talking about 'aesthetics'/appearance/clarity. Whenever I look at it, it usually takes me two or three 'looks' before I grasp what the diagram is telling me. I think one of the problems is that, at first glance, the solid grey line showing current flow looks like another conductor.

I also wonder if some people might be a little confused by your depiction of 'the point' at which you show the partial neutral current 'rejoining' the neutral conductor - since, other than in TN-C-S installations, that 'point' will be at the DNO's transformer.

Just a few thoughts!

Kind Regards, John

 

[EFLImpudence]

I have often thought that and wondered if DIYers would understand or even distinguish between the parts which apparently touch each other.

Also, I always think it is upside down.

 

[JohnW2]

I have often thought that and wondered if DIYers would understand or even distinguish between the parts which apparently touch each other. Also, I always think it is upside down.

Indeed. As I said, although I fully understand what it is intending to show, and despite the fact that I have looked at that diagram countless times, I still usually have to 'take a second look' (if not also a third) before I fully 'understand' the diagram - so, as you say, I think that many DIYers (and maybe others) would probably struggle to fully grasp what was being depicted.

Kind Regards, John

 

[SUNRAY]

I have often thought that and wondered if DIYers would understand or even distinguish between the parts which apparently touch each other.

Also, I always think it is upside down.

It's the first time I've seen this diagram and I confess my first reaction was to read it upside down, I thought the RCD was individual to the outgoing circuit. Then when I realised my error I didn't find it easy to follow the words and diagram together. Ironically it is completely correct.
Even more ironically this is a situation I have encountered several times and even witnessed 2 CU's being replaced due to exactly this problem, which did not cure the problem of course.
We had this issue at home with a faulty dishwasher but the kitchen ring RCD only tripped when the electric shower was used which was on the other RCD.