RCB Tripping Advice Please

[chris301up]

My Son is in the process of redesigning his kitchen and one of the issues is that the electric cooker needs to be moved, approximately another 2 to 3 meters or so.

The current wiring is the old red and black 6mm running from a consumer unit situated some 60 feet away in the front room. This is a small terraced house and we didn't want the problem of ripping up all the upstairs floorboards and chasing out the plaster to replace the wiring, so we thought the easiest way was to purchase a 60amp junction box and extend the wire accordingly.

Today we ran the old wiring through the ceiling joists and then joined a new length of 6mm twin & earth to the exit point where the new cooker socket will be located. Now, what we cannot understand, is that the RCB keeps tripping and disconnecting the main ring main socket circuit.

At this point I will say that there is nothing connected to the end of the new cable, and the MCB for that circuit is in the off position, but this doesn't stop the RCB from tripping.

We initially thought the problem was with the kitchen ring main circuit, but I disconnected this completely and the RCB still tripped. Just as a matter of course, I thought I disconnect the junction box, and 'bingo' all was well. Why on earth would this cause a problem when it's not actually connected to anything and the MCB for that circuit is off? I will say at this point the cooker was working perfectly up until a few weeks ago when we stripped the kitchen out completely.

Just wondered if anyone has had similar issues and maybe have an answer?

 

[Deleted member 105166]

@chris301up you may understand the need for isolation, but do you have the knowledge and tools to practice safe isolation?

The existing 6mm cable is already 20m long, have you referenced BS7671 to check suitability of the new extended length in 6mm, taking account of installation method, rating factors, voltage drop, and maximum earth fault loop impedance to meet required CPD disconnection times, for the design current?

Please call a sparky in before you hurt yourself, or someone else.

 

[Charlie_]

FAQs - IET Electrical - https://electrical.------.org/bs-7671/building-regulations/part-p-england-and-wales/faqs/

This might clear a few things up for you sir

 

[SparkyAndGeorge]

Please get a sparky in to sort this for you

 

[davesparks]

I understand the supply must be isolated at supply, and yes, a RCD is a sensitive safety device that switches off electricity automatically if there is a fault, and designed to protect against the risks of electrocution and fire caused by earth faults.

All I was confused about is why the RCD should trip when nothing is connected to that circuit, except the new short length of cable and a junction box.

In other words no, you don't understand how an RCD works, just a rough idea of the protection it provides.
An RCD trips when there is an imbalance between the current flowing through the circuit conductors, generally this is as a result of current flowing to earth, this can be directly from L to E or by neutral current being diverted via a N to E fault.

So an RCD will trip with nothing connected to the circuit if there is a low resistance from N to E.
Can you confirm exactly what you have done to isolate this circuit? If you have not isolated both L and N then it is easy for the open end of the cable to present a N to E fault an dtrip the RCD.

 

[Andy78]

Short version.
It worked before. You touched it. It now doesn't work and you don't know why. Electricity can kill you. Get an electrician in.

 

[chris301up]

........It worked before. You touched it. It now doesn't work and you don't know why.....

My point exactly. All I have done is added a 60amp junction box with a short 6mm cable extension, which isn't connected to anything.

 

[Bobster]

My point exactly. All I have done is added a 60amp junction box with a short 6mm cable extension, which isn't connected to anything.

The fact that when you disconnect this, the fault is gone, points to you either installing a damaged cable, or incorrectly.

I presume as you are DIY you have nothing available to test with?

 

[chris301up]

Yes I am DIY. I am just going to have another look at this shortly. Have a multi-meter if that helps?

Could the junction box be faulty is also a question at the back of my mind?

 

[Bobster]

Highly unlikely.

 

[Deleted member 9648]

Yes I am DIY. I am just going to have another look at this shortly. Have a multi-meter if that helps?

Could the junction box be faulty is also a question at the back of my mind?

Can you confirm the cut end of the cable is properly insulated/safe and the cores separated?

 

[chris301up]

Okay everyone. Have had another look at the problem this morning and can confirm that it looks like the junction box is the cause. When connecting just the junction box to the existing wiring the RCD trips. No extension cable attached at this stage.

Just a bit of trial and error! I connected a standard 30amp junction box, together with the new extension cable, and all is well. RCD not tripping. Nothing else connected to the cable. I'll order another junction box and go from there. Thanks everyone for your kind advice

 

[Andy78]

Well if you have no idea what you are doing and have no way to test if it will be safe then I suppose trial and error is your only way to diagnose a fault. Not that you have done that for sure.
Don't forget to cross your fingers when energising to replace basic test procedure.

 

[DPG]

Could you post a picture of the junction box - I'm intrigued to see what the issue is.

 

[Deleted member 105166]

Yes I am DIY. I am just going to have another look at this shortly. Have a multi-meter if that helps?

Unlikely... in fact probably unhelpful in that if you don't know what you are doing, you could end up hurting yourself. Read this thread. If you value yours and your family's lives and your home, you will call an electrician.

 

[Deleted member 9648]

In 40 years working I have never had a faulty joint box.