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nuisance tripping of MCB

Postby skin » Sat Sep 06, 2008 2:20 am

hi all,
i was called out to a friends house today because her 32amp MCB on the downstairs ring main was tripping whenever it felt like it, sometimes after 5 mins and sometimes after an hour. i think it may be a bad connection in one of the sockets (please correct me if im wrong). i have a megger MFT1553 meter and i wanted some professional advice on how to find this fault without having to physically take off every socket as there are about 20 of them.
thanks for reading
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Simply Build It

Postby TOPSPARK » Sat Sep 06, 2008 9:26 am

First things first have you unplugged all appliances and any other thing electrically for mthe sockets?
If you have done this then take your tester and put i on insulation resistance 500megaohms switch off electric and then test between Line anand neutral and between Line and earth.In order for each test to read clear the reading should be 200megaohms or above depending on make and model of equipment.If reading is say0.5megaohms then only way to find fault is to test between each sockt one at a time.A tedious job but the only way to find the fault.
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Postby ericmark » Sat Sep 06, 2008 1:50 pm

Finding faults of this kind can be very time consuming. The meter may show faults but it can also completely fail where lost connections are the problem. I would use Clamp on amp meter to see if it is a real overload those able to hold highest reading are very helpful. Fridges etc always suspect as they have auto restart after stall overload and any fault can re-appear every 5 minuets as it auto resets.
A little detective work is most likely to help.
What you need to find out is what work has been carried out.
There are traditional problem areas. I always check is the consumer unit itself as many come populated but the MCB’s are loosely placed in the unit by the whole sale outlet and should be tightened by person fitting the unit but the person fitting the unit seeing them already fitted has never checked them.
Sockets by doors which when banged shake the wall.
Units where far too many wires have been pushed into small spaces like the 20 amp grid switches found in kitchens.
This is the jump methods where you stab at likely causes.
The other is logic method remove MCB and temp replace with two 20 amp MCBs then split ring approx half way. (Where no room for second MCB then test one half at time.)
Once faulty half located split that half way and reconnect at first split point. and so one. Long winded but must work.
In reality one starts with method one and if that does not work then method two.
In both cases the consumer unit is normal start point. Swapping two 32A MCB’s around also a good start point. Also testing it is a ring main at same time.
But I have one large reservation in that one can’t always isolate consumer units elsewhere which means there are live components in the box in which you are working. Great care is required I would assume having a megger you will understand the dangers but unless you have received some training then it may be better if you don’t go into a consumer unit.
Second is Part P and technically to split the ring into two radials requires the go ahead from building controls even though you intend to return it to original state once completed.
You have stated in a previous post that you are Part P registered so I am sure there is no problem in your case but I still need this note for any other readers of this post.

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