domestic failt finding

[chrisreid186]

Hi all I got called out to a fault yesterday and thought I'll share with you how I discovered the fault

I received a phone call and the customer said that the rcd was tripping and asked if I could look at it so she went out and left the keys with a neighbor.

She had a 6 way board with one rcd covering all the circuits 6A lights upstairs, downstairs and garage. 32A ring upstairs, downstairs and garage

When I arrived all the circuits were off so I turned the circuits on one at a time until the rcd tripped hopefully this would indicate the faulty circuit, however this did not happen as the rcd would trip at random regardless to which circuits where energized. To add to the confusion the rcd would hold and then trip when a light was turned on or a appliance switched on.

so the rcd is tripping on random circuits maybe it's a faulty rcd? (happens a lot) to test the rcd I did a ramp test and it tripped bang on 30mA

I noticed that they had a lot of spot lights and wondered if a lamp had partially blown and caused a fault, so my next move was to look at the downstairs lighting circuit. I decided to disconnect all the earth and neutral cables from each circuit in the fuse board and only connect the downstairs lights. I energized the circuit and after running around turning all the lights on the rcd held,

Then I decided to do the same for each circuit. when I got to the rings I briefly checked end to end continuity and also checked they were all separate just in case. This all looked to be ok so went back to energizing circuits. When I got to the upstairs ring the rcd tripped,

Next I decided to unplug everything from upstairs and look closely at the sockets, as I did I noticed a loose faceplate which I took off for a closer look. The reason it was loose was because the right lug was rounded off, a quick re-thread and the lug was sorted. Looking closer I could see a dented neutral cable was directly behind the other screw lug so I cut the neutral where the screw had pinched it and re made the connection into the face plate. This cleared the fault and the rcd didn't trip again.

I informed the customer that I found the fault and recommended that she gets a EICR. In a ideal world I would of liked to carry out a full EICR from the start to prove the circuits were safe for continued use but because the protection devices were working and the additional cost/time I gave her the option

I believe Testing this way will get you to the fault quickly however it is better to carry out insulation resistance as this gives you more accurate readings that said it would of take considerably longer to remove every lamp/plug

Now can anyone explain why the rcd tripped on a earth neutral fault when a load was applied to another circuit. I understand that the mcb is only isolating the live conductor and the fault would still be apparent.

 

[davesparks]

Faulty RCDs don't happen a lot. I think I've only had one in the 11 years I've been in the job. What I have come across is a lot of RCDs being incorrectly diagnosed as faulty and being replaced due to supposed electricians being incompetant at fault finding.

Why do you advocate a random guessing method of fault finding over a logical process of investigation and testing?
When you closed MCBs and found the RCD tripped apparently randomly regardless of which mcb was closed it should have been obvious that a N-E fault is present.
Then a simple IR test between each cct N (disconnected) and the earth bar should identify the faulty circuit.
Then use the normal process of testing the cct in halves until you identify the fault.

 

[davesparks]

Now can anyone explain why the rcd tripped on a earth neutral fault when a load was applied to another circuit. I understand that the mcb is only isolating the live conductor and the fault would still be apparent.

When a N-E fault exists then a part of the installations neutral current will divert through that fault and flow to earth. The magnitude of this diverted current will depend in the ratio of the impedance of the normal return path and that of the return path through the fault.

 

[chrisreid186]

looking at it now your right i should of tested between earth and neutral,but because the RCD was tripping under load this wrongly led me to believe it was a live/earth fault, although i wouldn't call it random guessing as i was methodically going through each circuit,if i didn't find the pinched neutral my next step would be to IR and split the circuit down, The RCD's Ive replaced in the past are normally due to people repeatedly re-setting them under fault conditions before the fault has been fixed, point taken think ill stick to the standard way of testing, starting with IR between neutral and earth then live and neutral together to earth