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Help needed: home electrics

Community Veteran
Posts: 2,106
Registered: 06-02-2008

Help needed: home electrics

Evening all, I had my house entirely rewired earlier this year as part of my renovations with an earth spike in the garden (no earth in the supply). All has been fine until recently when I replaced a pendant with a decorative fitting. The wiring in question is your standard loop in, loop out with an additional spur and 2 way switching. I replaced the connection block as the old one was part of the pendant fixing and most of the connections were twisted together due to the extra spur which connects in. I checked and double checked all of the connections and the light works perfectly. However, the breaker now occasionally trips. It can trip at anytime, regardless of switch position but seems particularly prone to tripping when the shower or cooker are turned on (both high load, 30 or 50A cable IIRC). I thought that the problem might be the earth (as the new fitting is big and metal) so I disconnected the fitting's earth connection which does improve the situation. I'm going to swap back to the pendant, but with the new connection block to try to isolate the problem but thought I'd post here to see if we have any sparkies who can give any advice!
Possibly useful info: the breaker that trips is non-rcd, one of two (downstairs lights and upstairs lights). There is an rcd labelled 'lights' but that doesn't trip. The electric supply in this village isn't great, namely the lack of earth but my 2kw mitre saw will crash my computer if it's plugged into a socket nearby (on the ring main) - presumably due to back emf.
Edit: forgot to add that I'm considering soldering all the twisted cables together to make a better connection as the old block was linear and could connect more cables as opposed to the new one which only has two places to push cables in and screw them down to join them together.
Edit2:
The old fixing is like this:
http://www.howtowire.co.uk/images/pendant_terminals.JPG
The new fixing is like this:
http://www.saunashop.co.uk/res/user/2871_terminal-strip-600x443.jpg
50 REPLIES
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: Help needed: home electrics

dont know if you have had any rain where you live, but not much here.
Earth spikes arnt much good in dry soil, a bucket of water might help
anniesboy
Grafter
Posts: 385
Thanks: 2
Registered: 06-01-2011

Re: Help needed: home electrics

As an ex electrical inspector (very ex 40 years ago). My advice is given it was rewired recently get the electricians back to check what is causing this.
After all its tripping for a reason. Thats what trips are supposed to do. Earthing is on metal fittings etc for a good reason.
I wont guess what is causing the tripping ,please get it checked.
And please dont waste a bucket of water.
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: Help needed: home electrics

agreed that earthing is normally to metal  pipes, but my water pipes are plastic, my earth is to the Electricity board earth (about 45 years old)
oldgeezer
Grafter
Posts: 206
Registered: 02-02-2010

Re: Help needed: home electrics

As a recently retired electrician I would strongly advise you to get a qualified (part P certified) electrician to check this out pronto.
If  the breaker that trips is a MCB rather than a RCD or a RCBO ( combined MCB & RCD ) then several kilowatts are being disappated unless the breaker trips instantly. It is unusual nowadays to just rely only on an earth spike. Normally the supply company supply a PME (protective multiple earth) connection of the main earth conductor to the incoming neutral at the supply cut-out. Nowadays all gas pipes, central heating pipes and water pipes are bonded to the main earth in order to reduce the hazard of potential differences causing shock hazard. These must not be used as the main earth. If an earth spike is the only protection then ALL circuits must be RCD protected as an earthing system with a low enough impedance to cause the biggest MCB  to trip on an earth fault would be of huge proportions.
In my experiance  random tripping of RCD's is often caused by neutral to earth faults. Normally very little potential difference exists between these conductors but volt drop due to heavy loading on other circuits can cause enough current (20/30 mA) to trip a RCD.
In any event the situation could cause fire or shock hazard and needs urgent attention.
I remember investigating an incident 30 years ago in a similar situation where a live/earth short was causing the entire plumbing system to stand at 240 volts above true earth for an hour at a time until a fuse ruptured. The entire system was earthed (by the supply company) to the incoming water pipe.Investigation showed that this was only copper for about a yard into the front garden and then continued in polythene. As it happens only the worms in the garden perished and not the owners family.
Get it checked out !
oldgeezer
Community Veteran
Posts: 2,286
Thanks: 219
Fixes: 1
Registered: 04-08-2009

Re: Help needed: home electrics

Is this any help? Bit lengthy but useful info. As far as I'm aware using an RCD with a sensitivity of less than 100mA is not recommended with an earth spike because of nuisance tripping!
http://www.marcspages.co.uk/pq/3342.htm
Edit: This is a lift from the end of that article... A word about 'Electronic' RCDs; These devices, as opposed to the electro-magnetic-mechanical types, are extremely prone to nuisance tripping because they are designed to not only trip during imbalance but also trip if a fault is detected with the incoming supply. Unfortunately, being electronic, these devices have very fast reaction times and will trip even on extremely sharp events such as switching transients. Many a case of nuisance tripping has been resolved by replacing the RCD with the more basic type.
Edit 2: Ignore all that! Just spotted that it's a fuse-switch that's opening not an RCD. I'll go away and make a cuppa!  Cheesy
oldgeezer
Grafter
Posts: 206
Registered: 02-02-2010

Re: Help needed: home electrics

On the subject of RCD's it is not generally known that the basic types can be compromised by modern electronic speed controls and the like in appliances. If the appliance draws a DC component from the mains, eg draws more current on one half-cycle than the other, then the RCD may fail to trip even if a heavy leakage occurs. This is because the DC component causes magnetic saturation of the differential current transformer ( ring balance ) in the RCD. The device will not then trip if a leakage occurs. I conducted some laboratory experiments years ago which showed that a basic 30 mA RCD would not trip with 500 mA of leakage current if it was supplying a small half wave load. The industry brought out the DC Sensitive RCD's using electronics but of course these can be affected by external influences.
My take on all this is that the occasional rogue trip is far better than possible electrocution or fire. The latest 17th edition electrical regs  insist virtually all sub-circuits are RCD protected unless the wiring is buried at least  50 mm in the wall or mechanically protected over their buried length.
oldgeezer
Community Veteran
Posts: 2,106
Registered: 06-02-2008

Re: Help needed: home electrics

Thanks for your advice guys, i'm gonna give it a go with the old fitting to see if the same problem is evident and take it from there. However, it seems to be much better since i removed the new fitting's earth from the connection block... perhaps there was a neutral-earth fault in there?
Trevor
Grafter
Posts: 124
Thanks: 1
Registered: 06-01-2011

Re: Help needed: home electrics

Ben, a neutral/earth fault would indeed trip an RCD but the reason others are (correctly) urging you to get this checked out professionally is that you said it is tripping a non-RCD device, e.g. an MCB. This implies to me a much more serious live-to-neutral or live- to-earth short with the risks identified by the earlier poster.
Trevor
Rich
Grafter
Posts: 184
Registered: 30-07-2007

Re: Help needed: home electrics

Quote from: Ben
removed the new fitting's earth from the connection block... perhaps there was a neutral-earth fault in there?

I've got agree with other posters suggestions that you need to call in a professional - If you've removed the earth connection and the tripping has improved, it suggests that any metal parts of your new fitting may be becoming Live!
Richard
Community Veteran
Posts: 2,106
Registered: 06-02-2008

Re: Help needed: home electrics

Don't worry guys, I'm close to calling the electrician, just not quite yet! I've replaced the new fitting with the old pendant to see how it goes. I've again checked the wiring and there's no obvious mistake... All neutrals together, all earths together, all loop lives connected to the feed to the switch and the switched live connected to the live of the fitting.
I grabbed my multimeter and it's showing 0ohms between neutral and earth... Does that confirm a fault? To try to narrow it down, can I flip breakers and re-test? Or do they only isolate live?
Edit: the breaker that trips is a CPN ROH106B/030
Edit2: google says this is a type b rcb...?
Community Veteran
Posts: 6,735
Thanks: 12
Registered: 02-02-2008

Re: Help needed: home electrics

Quote from: Ben
I grabbed my multimeter and it's showing 0ohms between neutral and earth...

Doesn't sound right for the system you've described.
A proper electician would do an ELI test among others.
For our system Earth and Neutral are actually the same, though ELI is 0.2ohms.
We have overhead cables and the Neutral is bonded to Earth (big spike I believe) at every 2nd (maybe 3rd) pole, according to the supply company electricians who rejigged our earth a few years ago when replacing all the overhead cables down our road with a newer design (single twin-core as oppopsed to the old multiple separate cables).
We did have it connected to the water pipe "old-style"; they just removed that and connected our main Earthing point to the Neutral.
oldgeezer
Grafter
Posts: 206
Registered: 02-02-2010

Re: Help needed: home electrics

Quote from: Ben
Don't worry guys, I'm close to calling the electrician, just not quite yet! I've replaced the new fitting with the old pendant to see how it goes. I've again checked the wiring and there's no obvious mistake... All neutrals together, all earths together, all loop lives connected to the feed to the switch and the switched live connected to the live of the fitting.
I grabbed my multimeter and it's showing 0ohms between neutral and earth... Does that confirm a fault? To try to narrow it down, can I flip breakers and re-test? Or do they only isolate live?
Edit: the breaker that trips is a CPN ROH106B/030
Edit2: google says this is a type b rcb...?

That's a RCBO a combined MCB & RCD.
oldgeezer
Community Veteran
Posts: 2,106
Registered: 06-02-2008

Re: Help needed: home electrics

Picture of the consumer unit and new block connector attached. shortly after taking these pics, the breaker tripped and wouldn't reset... I've now isolated the switches and left the ring main connected with no light fitted.
oldgeezer
Grafter
Posts: 206
Registered: 02-02-2010

Re: Help needed: home electrics

PM sent.
oldgeezer