Dead test fail?

[ryan77]

Evening all ,
Called out to short circuit on cooker circuit assumed that cupboard fitted above isolater switch screwed through cable , disconnected at switch then dead tested back to board
Continuity l- cpc .22
l-n .16
n-cpc .22
Ir test l-n 999
l- cpc 999
n-cpc 999
All passed yet still shorted out any ideas plan to replace whole cable.

 

[HandySparks]

How do you know that it's 'shorted out'?

 

[ryan77]

How do you know that it's 'shorted out'?

MCB tripped

 

[HandySparks]

How do you know it's a problem with the cable and not the appliance?

 

[ryan77]

How do you know it's a problem with the cable and not the appliance?

Sorry could be clearer disconnected appliance just cabling back to board tested from isolater switch assumed short would show on ir test

 

[westward10]

If the cable has tested out okay, why not test the appliance with all parts of the cooker switched on.

 

[ryan77]

If the cable has tested out okay, why not test the appliance with all parts of the cooker switched on.

The circuit tested ok ,reconnected at isolater switch left switch turned off turned circuit back on mcb tripped cooker turned off .Is it possible that ir test passed yet cable still pierced by screw ?

 

[westward10]

I would have thought the IR test would have shown any screw through the cable. There is nothing shorting within the isolator when you screw it back is there. Why don't you redo the IR test with the isolator connected.

 

[ryan77]

I would have thought the IR test would have shown any screw through the cable. There is nothing shorting within the isolator when you screw it back is there. Why don't you redo the IR test with the isolator connected.

I thought IR test should have picked it up most confused that it did not , left cable free to air just choc blocks not connected at all thinking isolater problem still tripped mcb . must be cable pierced a fraction ?

 

[HandySparks]

What voltage did you do the IR testing at?

 

[westward10]

Why have you decided the fault lies with this cable, there is more to the circuit than the cable to the isolator. Test it all.

 

[ryan77]

Why have you decided the fault lies with this cable, there is more to the circuit than the cable to the isolator. Test it all.

What voltage did you do the IR testing at?

240 and 500 volts

What voltage did you do the IR testing at?

Why have you decided the fault lies with this cable, there is more to the circuit than the cable to the isolator. Test it all.

IR test at 240 500 volts , New cupboard fitted above isolater switch led me to think cable damaged from isolater back to board .

 

[HandySparks]

Think I'd try IR at 1000V with the cable disconnected from L and N at both ends but leave the cpc connected.

 

[ryan77]

Think I'd try IR at 1000V with the cable disconnected from L and N at both ends but leave the cpc connected.

Damn fault finding cheers for your input lads .

 

[Electric Magic]

Could be a problem with the circuit breaker

 

[Pete999]

Evening all ,
Called out to short circuit on cooker circuit assumed that cupboard fitted above isolater switch screwed through cable , disconnected at switch then dead tested back to board
Continuity l- cpc .22
l-n .16
n-cpc .22
Ir test l-n 999
l- cpc 999
n-cpc 999
All passed yet still shorted out any ideas plan to replace whole cable.

Disconnect at the cooker itself and the board leave cpc connected to earth bar and test again at 500v

 

[telectrix]

still think you may be correct in assuming a screw in cable. i once had a short on a socket RFC L -E caused by nicked L shorting to back box. MCB tripping but showed clear on 500V IR test.

 

[AndyL]

I would test from the CCU with L&N disconnected but CPC in the MET, disconnect the cooker and have the isolator on. Test at 500v

 

[Pete999]

I would test from the CCU with L&N disconnected but CPC in the MET, disconnect the cooker and have the isolator on. Test at 500v

Copycat LOL

 

[AndyL]

Sorry Pete just re read your post,
My post should read " I agree with Pete"

 

[bigspark17]

What mft are you using and is it in calibration.? Ive never heard of a 240v IR test. You seem certain it is a damaged cable by the screw but test results youve put up would indicate the problem lies elswhere, but you are the one on site so you have the benefit of being there. What was zs at cooker isolator.?

 

[AndyL]

What mft are you using and is it in calibration.? Ive never heard of a 240v IR test. You seem certain it is a damaged cable by the screw but test results youve put up would indicate the problem lies elswhere, but you are the one on site so you have the benefit of being there. What was zs at cooker isolator.?

Hard to measure this with MCB banging out

 

[Pete999]

Sorry Pete just re read your post,
My post should read " I agree with Pete"

Only messin

 

[Gnasher]

Your IR test should show up any issues. However If you can see the Heads of the screws on the new cupboard you could disconnect the cable connect all 3 conductors together and using a long lead check for continuity between screw heads and connected end of cable.

 

[Midwest]

To add my morsel;
what history of this fault, did the customer give you?
what range of insulation resistance are you using, 999?
as others have suggested, mcb might be faulty, could you try another mcb temporarily in the CU?

PS, the continuity results, do their correlate with the suspected length of cable?

 

[johnduffell]

Must be a short of the live to another earth or neutral. Ir test the live to the MET and the neutral bar.

 

[colshaws]

Try IR testing between L and and earth of another circuit, had a fault on lighting circuit where testing cable alone showed no fault but when testing the live to an earth on the rfc it showed a short, turned out new boiler had been fitted and screw had just nicked the live and was shorting through boiler case.

 

[AndyL]

^^^ This is why IR test should be done to MET with all protective conductors connected