RCD Tripping with load since adding spur.

[Jonathan777]

I had a single double socket radial in the lounge and thought I would join it to the existing downstairs ring via a junction box hence making it a spur.  I identified the ring cable and cut it, rejoined it with a junction box and connected the existing cabling from the radial (leading to the socket).  Both the ring and the radial were on the same side of the consumer unit. Since doing this any heavy load (oven, kettle, vacuum etc) trips the RCD covering the ring side of the consumer unit - even if the load is from the other side of the consumer unit. I'm at a loss as to what is going on. Can any one point me in a direction to figure out what's happening?

 

[Murdoch]

Dare I ask you if you did any dead testing or have any test kit?

 

[Jonathan777]

Stupidly I didn't test before going ahead as I made the assumption that as both were working as they should prior, and I had no other issues, that it wouldn't be a problem.  I've got a multimeter so tomorrow I'll start testing, but I really can't figure out what's causing the issue. I've replaced the RCD thinking it was old and may be faulty but still the same. I didn't realise I had an issue till a few day after (having replaced the floor and bath) hence it's not an easy option to just switch it back to try!!

 

[Murdoch]

Stupidly I didn't realise I had an issue till a few day after (having replaced the floor and bath) hence it's not an easy option to just switch it back to try!!

whoops ....................... that is why junction boxes need to be accessible ......

 

[Jonathan777]

Tell me about it!! Do you have any ideas of where the issue could lie or things to try?

 

[Geoff1946]

RCD tripping with an increasing load  in a different circuit suggests to me a neutral to earth leakage or short. 

The load causes a small volt drop in the neutral which appears as a potential difference  between  the CU neutral and earth;  (this is completely normal).  However, IF there is then a path from the neutral of an RCD protected circuit  to earth, then  a small current will flow, tripping the RCD.

 

[Evans Electric]

cut it, rejoined it with a junction box and connected the existing cabling from the radial (leading to the socket).  

What did you do with the  radial cable  leading back to the CU ? 

 

[Jonathan777]

I've disconnected the remainder of the radial cable back to the consumer unit and that breaker has been off since.

 

[Evans Electric]

I didn't realise I had an issue till a few day after (having replaced the floor and bath) hence it's not an easy option to just switch it back to try!!

Replaced the floor  eh !    Any nails or screws hitting the cable  ?      

 

[Jonathan777]

Definitely nothing interfering with the cables - tongue and groove real wood floor only nailed at the edges (completely clear from the cable run). 

I just can't get my head around what can possibly be happening now that wasn't happening before I joined the spur (radial), so I'm not sure what to test for. I only have a multimeter - otherwise I would do an insulation test.

 

[Sharpend]

Is there a leak on the bath or damp around new JB? 

 

[Geoff1946]

If my theory of a neutral earth leak is correct a multimeter will see it. Set it on the Megohm range, isolate the circuit, and test between neutral and earth.  Any reading but infinity is a fault.

 

[Jonathan777]

No moisture or damp - the bath isn't directly over it but would need to come back out to lift the floor if you follow me. Not used the bath yet either as only just put it back in and siliconed it today. I can see the JB though the joists from the next room (about 2 metres away) and all looks sound.

 

[Sharpend]

Could be a pinched cable, or squashed cable when the lid was put on of JB. 

 

[Jonathan777]

If my theory of a neutral earth leak is correct a multimeter will see it. Set it on the Megohm range, isolate the circuit, and test between neutral and earth.  Any reading but infinity is a fault.

I'll test the circuit tomorrow like you've suggested. If this is the case though, wouldn't it trip the RCD straight away and not just when a heavy load is applied?

 

[Geoff1946]

I'll test the circuit tomorrow like you've suggested. If this is the case though, wouldn't it trip the RCD straight away and not just when a heavy load is applied?

No, because when there is no load, ( depending on your supply earth arrangements), there is no potential between your earth and neutral so no current flows to trip the rcd.  The load generates the potential to cause the trip.

Remember this is only a theory from limited knowledge of your problem and installation, but lets say I would stake a couple of pints on it!

 

[Jonathan777]

Okay, so update (for anyone who's interested!)

Completed impedance test on ring (between N and E) and all okay.

Took up bathroom floor!!!

Disconnected spur (old radial) and connected ring as was.....cooker, vacuum etc still tripping RCD!

I reconnected old radial as was and now everything works fine. Obviously there is something odd with this radial circuit. I'm guessing something else must be sharing the neutral from the other side of the consumer unit, but not sure why it works as it does now, because it's on the same side as the ring!! 

I'm not happy that it works as it does - but for the mean time it does work, until I have more time to investigate.

 

[ProDave]

So do some IR testing to see if the radial circuit connects to ANYTHING else.

 

[Geoff1946]

The ring tripping without the (assumed) radial, but NOT tripping with it connected just doesn't make sense.  There is something strange going on.

 

[Jonathan777]

I'm very confused, like you say it doesn't make sense - I'll do more investigating on the weekend