mains cable problem

[wobblymike]

i am somewhat perplexed by a cable problem and would like to seek thoughts on it from users of this forum.
my daughter in law asked me if i would replace her exterior security light which she advised me had not worked for some years with a new one she had bought. the power for it is provided from a fused switched spur outlet in her bedroom. having switched off the power and removed the old light fitting i wanted to check the now exposed cable for functionality before fitting the new light so hooked up a multimeter between live and neutral, restored power and operated the switch - no power. removed fuse from front of switch - no fuse (no wonder it had not worked for some years). inserted new fuse switched on - flash, bang rcd tripped out. removed cable from switch thus isolating it completely. carried out continuity check end to end on all 3 cores - continuity confirmed. now for the perplexing bit - bear in mind cable is totally disconnected at both ends. at remote end of cable carried out resistance check betweem live and neutral - open circuit; between live and earth - open circuit; between neutral and earth - open circuit. repeated checks at other end of cable; live and neutral - 120 ohms; live and earth - 120 ohms; neutral and earth - 120 ohms.
the cable run disappears under the floorboards in the bedroom and i am going to have to trace it as soon as possible to find out what is going on. my daughter in law has no knowledge of when the fuse was removed and by whom.
any thoughts as to possible cause welcome as stated i am somewhat perplexed as to what could cause one end of a disconnected cable to read open circuit between live and neutral and the other end to read 120 ohms.

 

[PrenticeBoyofDerry]

Could you please give more detail on where you performed the readings?
For instance was the first set of resistance readings between conductors taken from fuse spur back to the board with all conductors disconnected at both ends and was the second set of readings taken from the board, back to the fuse spur, again with all conductors connected?
Also do you have access to equipment that can test for insulation resistance ?

 

[wobblymike]

no - the cable has been disconnected at both ends and is sitting in free space. the readings were taken between live and neutral at both ends - open circuit at one end and 120 ohms at the other - defies ohms law doesn't it hence why i am so perplexed

i dont see how it can be a nail throgh the cable as any such short would show at both ends and i have confirmed the cable is not broken by doing a continuity check on each individual core. i guess i am not going to know until we get the floorboards up and follow the cable run

 

[rjm2k]

I'm no expert but, wouldn't a short like this result in your readings? Ie one wire is broken from the rest of itself and shorted to the other?

Has she had any work done in that room, maybe someone put a nail through something and removed the fuse to hide it!

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[securespark]

The OP has end-to-end continuity between all conductors...

Sounds like you need to replace the cable.

 

[PrenticeBoyofDerry]

The OP has end-to-end continuity between all conductors...

Sounds like you need to replace the cable.

I assumed the op meant end to end of each individual conductor?
Worth asking how was this test performed?

 

[JohnW2]

no - the cable has been disconnected at both ends and is sitting in free space. the readings were taken between live and neutral at both ends - open circuit at one end and 120 ohms at the other - defies ohms law doesn't it hence why i am so perplexed ... i dont see how it can be a nail throgh the cable as any such short would show at both ends and i have confirmed the cable is not broken by doing a continuity check on each individual core. i guess i am not going to know until we get the floorboards up and follow the cable run

As you say, in terms of what you've describe, it makes no sense. If you have end-to-end continuity (how low a resistance do you regard as showing continuity?) on all three cores, it would seem impossible that resistance measured between cores can be appreciably (let alone dramatically) different at the two ends. This suggests that either you or us are 'missing something'.

Kind Regards, John

 

[wobblymike]

gents
thank you for your helpful comments. you have confirmed my view that this defies logic - i will post the resolution as soon as the floorboards can be lifted and the cable run traced. to answer some specifics the end to end continuity checks read 0 ohms on my multimeter for each core. yet the reading between live and neutral at one end of the cable reads infinity on my meter and 120 ohms at the other end. bizarre or what. as stated i will post the answer as soon as i can.

 

[wobblymike]

ps secure spark asked how was the test carried out

i hooked up a 3 core cable outside at the remote end of the cable and then ran it throgh the window back to the switched end of the cable so they were close to each other and then measured for continuity between each core.

 

[securespark]

Does zero ohms suggest a connection or not?

 

[wobblymike]

zero ohms means a short circuit or in other words a direct connection