Distance Calculation to find fault

[Mr.Black]

Does anyone know or where to find the calculation you do to find a fault/break in a circuit that can't be physically traced.I'm sure i've seen it somewhere in one of my books but can't remember.

Cheers

 

[yellowvanman]

If its a break then to find the fault you need a TDR (Time domain reflectometer), it basically sends a signal down the wire and at the break the signal gets 'reflected' back, based on the time it takes the signal to return, it can tell you how far down the cable the fault is.

If the fault is a dead short between cores then by doing a resistance measurement between the shorted cores and knowing the resistance of the conductor (based on CSA) per meter, you maybe able to gauge how far away the 'short' is. If the 'short' is not a 0 ohm short then you've got a problem!

 

[Mr.Black]

Yes i vaguely remember this.The conductor resistances are near the back of GN 3.........please correct me if i'm wrong.

 

[johnboy6083]

Is it called the murray loop?

 

[BillyTheSparks]

I have just brought one of these:
Ethos 4700 Transmittor & Cable Locator - Ethos Security Products

Although I have never used it yet, the instruction booklet does say it can be used for tracing cables and breaks in said circuit.

Hope this helps. Perhaps the local wholesalers could let you "try" one before buying??

 

[alarm man]

tdr is the thing to have but at 3k each its a luxury,do you know any bt engineers,they use them

 

[pushrod]

If it is a dead short say on a radial caused by a nail connecting two of the conductors(if it is a ring disconnect one end) test resistance between conductors at CU and sockets - the position with the smallest reading is the closest. Identify the conductor size - and look up the resistance per metre in table 9A of the OSG for the size of the conductors involved. Divide that value into the lowest resistance reading and you have the distance of that reading from the fault.

Only works really well if you have a very good contact between conductors, but even if it is not perfect can still get you close to it.

 

[Mr.Black]

Cheers man,pencil and paper is always cheaper and keeps the brain working!!!!!!