Cable Fault Location

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Cefster

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Hi All

I’m a newbie to this site and I am just starting out and need some help. 

I am am trying to find the location of a fault in a lighting circuit. I am loathed to by a cable fault located but can’t remember the sum. I know in GN3 it gives you the sum to work out the length but not the rough location of the fault in the circuit. 

 
The only sure fire way to fault find lighting circuits are to break them down

Is it the MCB or RCBO/RCD tripping?

Are the neutrals at the lights or switches?

 
if its a dead short then you can use resistance per metre to get an aproximate distance to the fault but dont forget this is cable distance, not a straight line distance

but for the best part, that wont work and youll need to keep splitting the circuit til you find the faulty cable

 
Hi All

I’m a newbie to this site and I am just starting out and need some help. 

I am am trying to find the location of a fault in a lighting circuit. I am loathed to by a cable fault located but can’t remember the sum. I know in GN3 it gives you the sum to work out the length but not the rough location of the fault in the circuit. 


Unless the circuit is just a single radial cable with no branches off it AND no other parallel resistances, (other lights / extractor fans etc. still in circuit), AND you are 100% sure that the fault is a dead short across conductors, then any resistance values you may have read/calculated are a bit hit and miss. Most lighting circuits end up with numerous branches going off to switches or lights from the main supply cable. Resistance per meter for the common CSA copper conductors are in the Appendix of the On-Site-Guide. Are you sure your symptoms do actually relate to a short of some sort?

Doc H 

 
I have a very expensive cable fault locator ( Time Domain Reflectomoeter if anyone cares 😊) it can show a fault position to within about 2m in 1 km.

However it becomes a very expensive paperweight if you don't know the cable route!

ask the bloke with the JCB on the golf course!....I told him the distance to fault, he KNEW the cable went in a dead straight line. Changed his tune when he dug up the 14th Green and had forgotten there was 20m cable coiled up midway in case they fitted another irrigation  head

whoops!

 
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every cct is a collection of shorter bits of cable with joins in at accessories such as switches and pendants. Occasionally you will find  spider wired lighting system with a big junction box, usually somewhere like the top of the stairs. So your best bet, is pick a point such as a pendant, disconnect outgoing feed and see if fault has gone. If it has fault is further up the cct, if it hasn't it's between that point and the board,. Keep repeating the process until you have a good idea of which cable length is at fault, then guess the route of said cable. 

 
Thanks for all the responses. 

Regarding the issue it is a strange one. The MCB has 2 line’s in it and 2 neutrals. There was no lights on in the house apart from the hallway and the new bedroom extension. I found that the 2 areas that the lights worked had different line’s. When I disconnected a line and removed a neutral from the bar the hallway lights went off but the bedroom light stayed on. I then put the line back in and the hallway lights came back on and the neutral became live. All the switches when checked were permanently live. 

 
Two lines & neutrals into the MCB?  I think that MCB sounds like and RCBO not an MCB.  What EXACTLY has happened before the "fault"?  Is this an existing installation you have been asked to investigate a fault on? Or have you done some work and then found you can't get the power to stay on?  If you have done some work what exactly have you done? My gut feeling, with no further info but my assumption we are talking RCD's (RCBO's) and light circuits is a crossed/borrowed/shared neutral somewhere.

Doc H.

 
concurr, defo sounds like a borrowed neutral, hence 2 lines in MCB, don't think it's an RCBO as he mentions  a 'bar' I'm thinking neutral bar? Still don't know what the 'fault' was that sparked an investigation.

 
I've been fault finding a lighting circuit for some of this afternoon ....almost dead short L-N; L-E, N-E

So after taking some of the circuit apart which cover part of the ground floor and part of the first floor,  (some lights looped at lights, some looped at switches) and hidden junction boxes (in a loft space with no access)  - I'm going back on Friday, with the agreement of the clients, to cut a hole in the ceiling to investigate the loft space .......... then there is the small matter of a huge wasp nest removed last year in the void between the ground floor and first floor ................ wonder if this has had an impact!

Happy days !

 
well that could just be something wired to the circuit, like a TV aerial booster in the attic, a light you left switched on, or 12v transformer. WHAT WAS THE ORIGINALL FAULT????  :shakehead

 
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