Fault Finding Help

[peck06]

Please Help...

Here's the issue

Got a customer with a fault on the lighting circuit. Done tests on the circuit and all checks out ok, IR is ok, Zs is ok etc

When the breaker is turned on it stays on for about 5 minutes then trips out. Does the same with no lights switched on, all lights switched on or each individual light switched on. Checked all connections in fitting and switches all look good and tight.

Swapped it onto a different MCB does the same thing so it's not the MCB at fault.

Unfortunatly the bloke has just laid new carpets upstairs with underlay that he has stapled down, I'm at the point now where I need them up and to get into the floor boards where the cables run.

Am i going along the right lines or have I missed something glaringly obvious? My thinking is he's put some new floor boards down and nicked a cable with a nail, but why would the MCB hold in for 5 or so minutes before tripping?

Any help/advice greatly recieved and appriciated.

Circuit is 1.5 twin but is the old stranded type cable. On a 6A Crabtree type 1 BS3871 MCB.

 

[Chr!s]

Is the circuit RCD protected?

Have you got a amp clamp?

 

[andyb]

What were your IR test results?

 

[Bradzz]

have you check all light fittings could be a lose connection, by him putting down the underlay, the banging of the staples might have dislodge wire, or the same could be a junction box? check lights first then go down the route you have already said.

 

[SirKit Breaker]

See if you can find the end of the circuit, and start the laborious job of disconnecting it bit by bit till you identify the part causing the problem. Time consuming and a pain in the butt, but if you need floors up it may save you the problem of taking up too much. Lighting circuits are a pain at times. keep us posted as well, when you sort it.

Cheers.........Howard

 

[peck06]

IR's are fine around 700-800 Mohms from what I remember. Checked all light fittings, connections all good. I'm thinking either nail in a cable or dislodged connection in JB but why would the MCB not trip straight away??

Nothing to do with Ampage as trips even when all lights are off. Unless next door have managed to tap off it somehow I suppose.
No RCD just MCB.

Thats the other issue the end of the circiut is under the bathroom floor which is laminate on top of hard board on top of floor boards, talk about nightmare! Only good thing is they are away for a month in April so don't have to go back until May to get it sorted.

Thanks for the suggestions so far, keep them coming....

 

[andyb]

IR's are fine around 700-800 Mohms from what I remember. Checked all light fittings, connections all good. I'm thinking either nail in a cable or dislodged connection in JB but why would the MCB not trip straight away??

For the mcb to trip there must be a fault, so if he put a nail through a cable that will show up when testing. Are you reading between l-e & l-n, 700/800M seems high to me in an old installation, my meter would not even give a reading like that, it would say >500 if on 500 volts.

If it's an intermittent fault then you just do as Howard says.

 

[Chr!s]

So IR L-N ok, L-E ok and N-E ok?

Did you disconnect the earth of the circuit when carrying out the test?

Have you done a test for separation of circuits?

 

[peck06]

Yeah my mistake my meter only goes up to >199 I meant 70/80M, its been a long week so far.

 

[Guest 004]

700 ohms is less than the 1 meg permitted by BS7671, which would give a fault current of 0.32 amps, which when energized slowly builds up over time to trip the MCB

 

[telectrix]

70M is his IR. suggest disconnect cpc in CU and see if it still trips. this will narrow it down as to whether it's a L/N or L/E fault. if it's L/N remove all lamps the IR across L and N. ( make sure to disconnect any fluorescent fittings ). then, as sirkit suggests, dis. each fitting at ceiling roses from the end one back till the fault disappears. take sandwiches with you.

 

[Guest 004]

I would find the midpoint of the circuit , not start at the last

 

[Amp David]

Is there anything else thats been tapped of the lighting circuit that shouldn't be. If all the tests are fine then find it hard to be a cable fault.

If all lights or off and the MCB still trips, there has to be something putting a load on it.

As already said, clamp meter the cable at the breaker to see

 

[Guest 004]

dimmer switch maybe?

 

[andyb]

If there was something tapped off, that would show up when testing.

I've only got the time fault characteristic chart for mcbs to 60898, but I'm sure that it's not a million miles away from a bs 3871 and that shows that it would need about 11 amps to trip a 6 amp mcb in about 5 minutes.
That means a resistance of about 20 ohms.