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?

 

[Marcus Vaughan]

Hello.

It sounds like you have a neutral to earth fault - possibly in the junction box you have put in - or at the end of the new cable you have put in. There will be some current in the neutral if it is connected into the consumer unit - and if that current leaks to earth then you have an inbalance and the RCD will trip.

Check the neutral and earth are separated. MCB being turned off will make no difference.

If the junction box has screw terminals - which it probably has for that sized cable - then it needs to be accessible and for future testing and inspection - and not buried under floor boards.

Is this a DIY job?
Testing?
Certification?

 

[Pete999]

Hello.

It sounds like you have a neutral to earth fault - possibly in the junction box you have put in - or at the end of the new cable you have put in. There will be some current in the neutral if it is connected into the consumer unit - and if that current leaks to earth then you have an inbalance and the RCD will trip.

Check the neutral and earth are separated. MCB being turned off will make no difference.

If the junction box has screw terminals - which it probably has for that sized cable - then it needs to be accessible and for future testing and inspection - and not buried under floor boards.

Is this a DIY job?
Testing?
Certification?

Don't be silly Marcus, clue "cheap joint box"

 

[Deleted member 9648]

Have you insulated and separated the cut end of the new cable you have used to extend from the JB ?? even with the MCB off the N and E will still be connected and if they are shorted on the cut end out will go the trip.
Easy electrical work isn't it? who needs an electrician, I can do that.

 

[Marcus Vaughan]

Don't be silly Marcus, clue "cheap joint box"

I’ve only ever used the Click junction boxes for 60 amp. I think they are the Mutts. I can’t imagine introducing a fault in one of them - that would take real - errr - skill?

In my head I can see the cut end of the cable causing the fault.

 

[chris301up]

Hi Marcus

Thank you for your response. Yes I was thinking that it could be a neutral to earth fault, but has there isn't anything connected yet, I con only assume at this point that it could be the junction box?

I have emailed the suppliers and perhaps they will replace this? It certainly wasn't cheap. It is a DIY job I'm afraid and I understand the box should be accessible. I always put the earth connection between live and neutral in boxes and chocs, but perhaps I just need to wire the earth by the live and see if that helps?

 

[chris301up]

I've just checked and confirm this is a Click screw type 60 amp junction box

 

[davesparks]

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.

What testing have you carried out on the circuit in question?

Do you understand how an RCD works? Are you a qualified electrician?

Don't just rely on switching off an MCB to make an unterminated cable safe, it should be properly isolated at the supply end.

 

[Marcus Vaughan]

Hi Marcus

Thank you for your response. Yes I was thinking that it could be a neutral to earth fault, but has there isn't anything connected yet, I con only assume at this point that it could be the junction box?

I have emailed the suppliers and perhaps they will replace this? It certainly wasn't cheap. It is a DIY job I'm afraid and I understand the box should be accessible. I always put the earth connection between live and neutral in boxes and chocs, but perhaps I just need to wire the earth by the live and see if that helps?

I’d check the cut end first.

 

[Deleted member 9648]

It's blindingly obvious he's left the cut end open

 

[chris301up]

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.

 

[ChrisElectrical88]

Haha faulty joint box.

 

[buzzlightyear]

It sounds like you have a neutral to earth fault

Yes I was thinking that it could be a neutral to earth fault

I always put the earth connection between live and neutral in boxes and chocs, but perhaps I just need to wire the earth by the live and see if that helps

boy o boy ,and I wonder why people burn the houses down .

 

[ChrisElectrical88]

boy o boy ,and I wonder why people burn the houses down .

Where do you put your earth?

 

[buzzlightyear]

Where do you put your earth

me I'm floating around it .

 

[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.----------/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.