DIYnot
Local | Network
   DIYnot > Forums
Local | Network
DIYnot Network Local DIYnot Network Local  
  Forum IndexForum Index     RulesRules    HelpHelp     Join FREERegister Free     About CookiesCookies     SearchSearch     LoginLogin 

Non protected MCB tripping RCCB !! How?


 
Post new topic   Reply to topic    DIYnot.com Forum Index > Electrics UK
Search this topic :: View previous topic :: View next topic  
Author Message
sidewinder

from United Kingdom

Joined: 07 Jul 2009
Posts: 4
Location: Lancashire,
United Kingdom

PostPosted: Tue Jul 07, 2009 12:13 pm Reply with quote

OK here goes.
I have a split consumer unit that is wired normally. One of the MCBs on the non protected side (I have checked that the neutral is on the non protected neutral block) trips the RCCB when ever the MCB is reset. The MCB does not trip itself, it just trips the RCCB! So I am unable to have the MCB (remember its on the unprotected side!) and the RCCB set at the same time.

The MCB is question is on the non protected side so how can it be tripping the RCCB?

Do I have an earth leak on that MCB?

Any other help would be much appreciated.

Thanks.
Back to top
 Alert Moderators

If you do not want to see this advert, click here to login or if you are new click here to join free.
rebuke

from United Kingdom

Joined: 04 Nov 2008
Posts: 872
Location: Cambridgeshire,
United Kingdom
Thanked: 25 times

PostPosted: Tue Jul 07, 2009 12:34 pm Reply with quote

Have you checked all the neutrals (you mention the one for the circuit protected by the MCB in question, but not the others), as if any neutral is on the wrong side, then you could see strange issues.

A picture of the inside of the CU might be useful - it may allow people to spot some obvious mistake or whatever...
Back to top
 Alert Moderators
dannyboi2003

from United Kingdom

Joined: 11 Jul 2006
Posts: 502
Location: West Glamorgan,
United Kingdom
Thanked: 16 times

PostPosted: Tue Jul 07, 2009 12:40 pm Reply with quote

Whats on the circuit that wont reset, lights, sockets....?

My money would be on a shared neutral taken from one of the RCD protected circuits somewhere in the house.

Has the CU just been fitted or have you had any other electrical work done recently?
Back to top
 Alert Moderators
sidewinder

from United Kingdom

Joined: 07 Jul 2009
Posts: 4
Location: Lancashire,
United Kingdom

PostPosted: Tue Jul 07, 2009 1:02 pm Reply with quote

I have checked all the other neutrals and they are correctly wired.
This circuit seems to run some sockets but it is not a ring.

In desperation I have contaced the previous owners of this house and managed to get some history. Here is the story. The MCB that trips the RCCB was on the protected side and worked fine except that Occationally the RCCB tripped. In an attempt to do some fault finding the previous owner moved one MCB at a time to the unprotected side and left it for a while to see if the RCCB tripped. I asked how occational it was and he said once or twice a month. This MCB was the second MCB out of 4 that he had moved (putting the first one back to the protected side when the RCCB tripped again).

I have just moved the MCB in question to the protected side and everything seems to work fine. I guess the RCCB will probably trip once or twice a month as it did for the previous owner. Atleast I can now test the circuits.

I am still confused why a non protected MCB would trip the RCCB! Is this circuit a good one to start with from a testing point of view. Can I infer that there is a fault in this circuit due to its non protected side history or am I missing the point somewhere?
Back to top
 Alert Moderators
ericmark

from United Kingdom

Joined: 27 Jan 2008
Posts: 7614
Location: Flintshire,
United Kingdom
Thanked: 644 times

PostPosted: Tue Jul 07, 2009 1:32 pm Reply with quote

If you look at first diagram I show an earth neutral fault.

Now on second diagram I show same fault with an extra load going to a item which is not faulty

This shows how a device on the non protected side can cause a neutral earth fault on protected side to exceed the level where the RCD will trip.

I have used a program called Multisim which simulates electronic designs but also shows what happens with simpler circuits I have given each wire a resistance under an ohm and loads with ohm resistance.

So you are looking for a earth neutral fault on the protected side. I would look at heating applances first as many use mineral insulated cables which tend to absorb mosture and many only have switches on the line so although switched off can still give you an earth neutral fault.

Switch the isolator off on cooker and imersion heater for example.
Back to top
 Alert Moderators
sidewinder

from United Kingdom

Joined: 07 Jul 2009
Posts: 4
Location: Lancashire,
United Kingdom

PostPosted: Tue Jul 07, 2009 7:18 pm Reply with quote

Ahh I see, I think!
Great info. I think I need some specialist sparky test gear really.

Thanks for the help.
Cheers
Back to top
 Alert Moderators
ColJack

from United Kingdom

Joined: 16 Feb 2007
Posts: 11773
Location: West Midlands,
United Kingdom
Thanked: 408 times

PostPosted: Tue Jul 07, 2009 8:38 pm Reply with quote

if the breaker tripped the RCD straight away when not on the RCD side, but didn't once returned to the RCD protected side then it sugests that it has a shared neutral with a circuit on the RCD side or that you have a crossed neutral, but that ought to be obvious and more than 1 circuit would have caused the RCD to trip ( the one that it's mixed up with would get no current back on the neutral )

pull all the neutrals out ( and take this opportunity to put cable numbers on them if they're not already on ) and test for something close to continuity between them.. if you find a pair that have contunuity ( or close enough ) that aren't two ends of a ring, then it's time to do a little digging, or more accurately a little bit of taking socket / switch / light fittings off..
Back to top
 Alert Moderators
sidewinder

from United Kingdom

Joined: 07 Jul 2009
Posts: 4
Location: Lancashire,
United Kingdom

PostPosted: Tue Jul 07, 2009 9:31 pm Reply with quote

Thanks ColJack that gives me a good way forward and I fully understand that. Might be a few days though cos have to go away to work till Monday. I will update as soon as I get a chance next week.
Cheers.
Back to top
 Alert Moderators
securespark

from United Kingdom

Joined: 11 Jan 2004
Posts: 25151
Location: Cheshire,
United Kingdom
Thanked: 436 times

PostPosted: Tue Jul 07, 2009 10:36 pm Reply with quote

ericmark wrote:
If you look at first diagram I show an earth neutral fault.

Now on second diagram I show same fault with an extra load going to a item which is not faulty

This shows how a device on the non protected side can cause a neutral earth fault on protected side to exceed the level where the RCD will trip.


eric- if you have a moment, would you be so good as to redraw your diagrams in the context of a split-load board?

I can't quite follow your diagrams! icon_redface.gif
Back to top
 Alert Moderators
ColJack

from United Kingdom

Joined: 16 Feb 2007
Posts: 11773
Location: West Midlands,
United Kingdom
Thanked: 408 times

PostPosted: Tue Jul 07, 2009 11:06 pm Reply with quote


too many resistors to follow.. hurt my head the first time I traced it out too... icon_wink.gif icon_lol.gif
Back to top
 Alert Moderators
Search this topic :: View previous topic :: View next topic  
Post new topic   Reply to topic    DIYnot.com Forum Index > Electrics UK All times are GMT
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
Similar Topics   Replies   Views   Posted 
Non-RCD protected circuits 51 1460 Sat Oct 30, 2010 8:45 am
New RCD protected CU and Supplimentary Bonding in Bathroom 4 200 Tue Nov 23, 2010 9:37 pm
Am I fully protected? 5 320 Sat Feb 05, 2011 11:15 am
Extending an RCD protected bathroom circuit 3 480 Mon Jul 12, 2010 7:29 pm
Electrics fault with rcd protected circuit 8 200 Mon Oct 18, 2010 9:33 am


 
DIYnot
Find an Expert | Find a Supplier | Search DIYnot.com
Network | Advertising | Newsletter
DIY | DIY How To | @home | DIY Wiki | DIY Forum
By using this site you agree to our Terms of Service / Disclaimer.
Please read our Privacy Policy. Copyright © 2000-2014 DIYnot Limited.