RCD and Mcbs tripping on new wiring for Shed

[SteveM423]

Hi. I have just wired up my shed and an outside lantern.

I have run a circuit using Swa from a new MCB on the house to a New consumer unit in the shed with an RCD and 1 16A MCB for a socket and 1 6A MCB for lights. Because the shed is 50m away, I have not connected it to the house earth but have used a new earth stake at the shed and earthed it there.

On switching on the house MCB with the Shed RCD on and MCBs on all is ok.

Once I start to draw any current the appropriate MCB and RCD trip in the shed CU. IF I switch on a light the 6A MCB and RCD trip or if I plug something into the socket and switch on, the 16A MCB and RCD trip.

All wiring looks ok. Any ideas?

Many Thanks

Steve

 

[PrenticeBoyofDerry]

All wiring looks ok.

And what did the test results look like?

Any ideas?

State the results of the tests and I am sure we can diagnose the fault.

 

[ban-all-sheds]

Get an electrician.

You've clearly not done any testing, have no ability to do any testing, and I'd wager no understanding of testing.

And somewhere you have ****ed up big time with your wiring which "looks OK".

Without a shadow of a doubt you fall woefully, quite possibly dangerously, short of the level of competence needed to design, install and test sub-mains, outside cables, CUs, final circuits, to sort out earthing systems, to safely install armoured cable running between two different equipotential zones, etc etc.

Did you also break the law by not applying for Building Regulations approval?

Please just turn off the MCB supplying the mess you have made, and let someone who actually knows what they should know sort it out for you.

 

[SteveM423]

Hi Ban all sheds.

I will try and respond to each of your points:

Whilst I am not a fulltime Domestic House Electrician, I trained as a Aircraft electrician 35 yrs ago and have a HNC in Electrical Engineering.
I have since worked on just about every electrical system you can think of Including a dozen or so full rewires and spent 3 yrs as an electrical instructor (except HV). I have also studied the 15th, 16th and 17th regs but not had much chance to practice all the various nuances.

Now I know that doesn't make me an expert, but I do know how to test, have the ability and have indeed tested this installation ("which looks ok", appreciate could have been clearer on this point), It was on the functional testing it failed.

Suffice to say the only thing on your lst of competences that I am not very familiar with is Earthing between two different equipotential zones, but I understood that it was safer to install a TT system rather than try and carry the PME through over such a distance - any thoughts ??

Clearly I have missed something, probably cos I'm an amateur at this.
I am off for a weeks holiday now (currently at airport, trying not to get wound up) but will provide test results to simonzx when I return if a week in the sun doesn't get the Brain cells working.

Regarding the law, I usually get a Part P qualified friend to test my work when I am happy with it but have lost touch with him as recently moved away. ( I know thisnt strictly legit, but it happens).

MCB is switched off you will be glad to know.

Don't feel you have to reply, if you don't have anything constructive to say.

 

[Taylortwocities]

I know exactly what you have done. All that reading, HNC and regs bashing has not taught you any basic skills.

Somewhere in that simple shed CU you have connected the incoming neutral to the neutral on the load side of the board. usually it is because there are two neutral busbars
The incoming and the outgoing. There is sometimes a link between the two. It is a very common basic error to leave that link in.

Otherwise, post a photo of your wiring in the CU and I'm sure we can spot it in a nanosecond.

 

[JohnW2]

I know exactly what you have done. ... Somewhere in that simple shed CU you have connected the incoming neutral to the neutral on the load side of the board. usually it is because there are two neutral busbars ... The incoming and the outgoing. There is sometimes a link between the two. It is a very common basic error to leave that link in.

Forgive me for being dim (it is Friday night ) but, although what you describe is, indeed, a classic way to end up with RCDs tripping, I think I need some help in understanding how it could explain his MCBs operating.

Kind Regards, John

 

[SteveM423]

Thanks TTC I can visualise that I have done that.

Will sort that out when I get back, and sure it will sort out the RCD problem. but as John says is that why MCB trips also?

I will be offline for rest of week as Mrs M not happy with me!

Thanks again now hopefully can relax for a few days,

Rgds

Steve

 

[Taylortwocities]

mmm I missed that.

Anyways, a photo may well tell us more!

 

[viewer]

OP is away so I can show my ignorance by asking a question: phrased as logic Y/N
Nothing happens when the circuit is energised so there is no permanent short circuit
If the MCB and RCD trip as soon as the power is used then there must be a direct short
The trips only apply to the circuit in use (socket or light)
The only common factor is the RCD or its connections
Conclusion that it is a faulty RCD or it has been incorrectly fitted.
???

 

[bernardgreen]

Connect the output of an MCB (N) on the non RCD bus bar to the output of an MCB (R) on the RCD bus bar and then when the RCD trips the MCB (R) will feed the RCD bus bar and if the load on the RCD busr bar is high enough one or both the MCBs will trip.

Saw it when a ring final had one end into a non RCD 32 amp MCB and the other end (incorrectly ) fitted into an MCB on the RCD bus bar. A new build house, one of 6 with faults all over the place.

 

[JohnW2]

OP is away so I can show my ignorance by asking a question: phrased as logic Y/N
Nothing happens when the circuit is energised so there is no permanent short circuit
If the MCB and RCD trip as soon as the power is used then there must be a direct short
The trips only apply to the circuit in use (socket or light)
The only common factor is the RCD or its connections
Conclusion that it is a faulty RCD or it has been incorrectly fitted.
???

I'm not so sure about that. Continuing with statements of apparent logic, if the MCBs trip only when a load is connected to their output, then a current high enough to operate the MCB (considerably more than 6A or considerably more than 16A, respectively) must flow through the load? (if not, where else could it be flowing?). Unless the OPs lighting and every socket load he's tried all have L-E faults (since the RCD, also operates) I can't see how that could happen.

If the CU is wired correctly, the RCD will be upstream of the MCBs, so no sort of fault in the RCD (it would presumably have to be an L-N fault, which only showed itself when the RCD operated) ought to be able to cause the MCBs to operate.

I think a photo inside this CU is crucial - AFAICS, what is being described could only happen with some fundamental (probably multiple) wiring errors within the CU.

Kind Regards, John

 

[JohnW2]

Connect the output of an MCB (N) on the non RCD bus bar to the output of an MCB (R) on the RCD bus bar and then when the RCD trips the MCB (R) will feed the RCD bus bar and if the load on the RCD busr bar is high enough one or both the MCBs will trip.

... but as I've just written, one would still need a current high enough to operate an MCB to flow through it - what explanation for that were you thinking of? I accept that, in the scenario you're postulating, a >>6A load on the socket circuit might cause the 6A MCB to operate but (a) that's not what the OP is experiencing and (b) it wouldn't work the other way around (a normal lighting load would not cause a 16A MCB to operate).

Kind Regards, John

 

[ban-all-sheds]

AFAICS, what is being described could only happen with some fundamental (probably multiple) wiring errors within the CU.

somewhere you have ****ed up big time with your wiring which "looks OK".

 

[ban-all-sheds]

Whilst I am not a fulltime Domestic House Electrician, I trained as a Aircraft electrician 35 yrs ago and have a HNC in Electrical Engineering.

Apparently none of that has left you anywhere near competent to do the work you decided to do.

I have since worked on just about every electrical system you can think of Including a dozen or so full rewires

Despite knowing SFA?

and spent 3 yrs as an electrical instructor (except HV).

I think we sometimes get some of your students popping up here.

I have also studied the 15th, 16th and 17th regs but not had much chance to practice all the various nuances.

None of the "various nuances" have any relevance to what you've done.

Suffice to say the only thing on your lst of competences that I am not very familiar with is Earthing between two different equipotential zones, but I understood that it was safer to install a TT system rather than try and carry the PME through over such a distance - any thoughts ??

No reason per se not to export it. And if you don't then you need to deal with the fact that at one end the armour is a non-bonded extraneous-conductive-part.

Regarding the law, I usually get a Part P qualified friend to test my work when I am happy with it but have lost touch with him as recently moved away. ( I know thisnt strictly legit, but it happens).

In other words yes.

Don't feel you have to reply, if you don't have anything constructive to say.

[constructive]Get an electrician.[/constructive]

 

[bernardgreen]

Once I start to draw any current the appropriate MCB and RCD trip in the shed CU. IF I switch on a light the 6A MCB and RCD trip or if I plug something into the socket and switch on, the 16A MCB and RCD trip.

All wiring looks ok. Any ideas?

How much current are you drawing. If the lantern is a couple of kilowatts then the 6 amp will trip. If the appliance has a motor of 1 kW or more then the inrush current could trip the 16 amp.

The alternative and most likel explanation is that your statement about the wiring looking fine is incorrect.

You may have wired the light switch across the supply instead of in series with the lamp. That is something many DIYers do. All the blacks ( blues ) together and all the reds ( browns ) together looks right but is wrong.

That mistake is impossible on a socket as the switch is inside the socket.

Have you used a two pole isolator between MCB and socket. You could wire that up incorrectly so that it puts a short across live and neutral when switched on.