DIY Doctor UK DIY Forums

Hi every one,
looking for some assistance from the more experienced ones of you out there. Changed a consumer unit the other day, tested what i could and left it at that for the day. That evening got a phone call saying the electric had gone off,asked the customer what they had done, they told me it was when they turned on the toilet light.Electric was reset and left till the morning. Next day started to fault find, found that both RCD'S were tripping which led me to look at the cpc. Found a junction box and seperated every thing done a continuity check form MCB to j/b to find live feed ,turned on power to j/b then started to probe around the rest of the disconected wires.
I found that when i conected the cpc from the c/u to one of the disconected cpc's at j/b i was getting 36 volts and 50 htz, on another i was getting an even higher reading still with 50 htz. Then there were a couple of cpc's that had low voltage with zero htz, like wise when tested across the neutral so now i know why the RCD'S were tripping, or so i thought. Took off all the light switches looked at all the ceiling roses and everything seems fine.running out of time i decided to put evey thing back togeather,now this is where i'm really confused between the cpc and neutral threre is no longer any voltage.Am i missing something here,could someone explain why i no loner have any voltage across cpc and neutral now its all reconected .
Many thanks in advance
Remelad

Without seeing it first hand impossible to say.
I am supprised by some of your actions though is this a job you do for a living.
You changed a CU didn't do a PIR prior to change, did what testing you could for the day, then energized the system and left it?
Is that the action of a responsible person?

Hi kb,
everything you have said in your reply is absolultly right. Leaving it for the night might not be the right thing to do but sometimes time gets the beter of us all. Everything tests okay when all is connected its only when this circuit is broken down the problems begin,i will endevor to find the cause and when i do i will post my findings
Regards remelad

hi remelad......i feel your pain, ive had exactly the same thing happen to me yesterday! how did you sort the problem??
i managed to prevent the rcd tripping, by removing the offending lights from the circuit...but im getting strang test results R1 + R2 over 100ohms, but zs at 1.56....
your help would be greatfully recieved!!
cheers

Hi po5hscott...the question had me thinking for a while,so i decided to investigate on my own house. Once again i was coming up with some strange readings. voltage on earth to earth. this did not make sense to me,as to measure voltage it has to be done in parallel... I decided to change from a digital meter to a coil meter and the needle just twitched. I can only assume that the readings that i was getting was induced voltage.... The customer in question i was working for had a damaged neutral wire which was causing me the problem, although this did not show up when i did an i/r test, and was hard to trace in a false ceiling....... As kb quite rightly said 'tested what you could and went home'.bad mistake on my behalf.Lesson learnt for me...... As for your own question i'm not sure of the answer as i,m not good at giving advise, thats why i come here to seek advise myself, from people like kb and sparx. Hopefully they will look at this thread and offer you some advise. Sorry i can not be of any help Remelad

Hi po5hscott,
check the batteries in your meter!
r1+r2 uses them to power circuit under test, Zs uses mains power to do test, once caught etc.
regards Sparx

remelad. well done! I was going to mention high impedance digimeters not good on fault finding on circuits disconnected due to induced voltages, always use a moving coil meter for this- if that needle moves something made it! unlike digi which will give a reading on volts waved in the air...
We once had 4 or 5 degree qualified power engineers in a nuck station spend 3 days tracing a fault such as this when they all stated a core in a multicore cable was live when it should have been dead.
Took away their pocket digimeters and went to plant with big AVO model 8
about size of 2 bricks and just as heavy. lo no movement....wonderful for a lowly foreman to prove, painful for them when chief electrical Eng. heard about it!!!
Opposite occurs when testing electronics BTW as M-coil meter overloads circuit and high impedance digimeter doesn't,
like this trade horses for courses!
regards SPARX