Most effective or correct way to fault find?

[PEG]

I am just itching for a fault to come my way now!! It's not going to stand a chance!

...3 storey victorian multi-occupancy dwelling,all floors on one circuit,laminate in every room,joiner,plasterer,decorator all left this morning...enjoy :conehead:

 

[Lucien Nunes]

If it's a hard fault like a screw penetration, you can find it without any opening or splitting. Ensure everything is disconnected, separate the ends at the DB and connect a small regulated current source, say 1A, across the faulted cores of one end only. Go round the points reading the voltage across these cores. As you move towards the fault from the energised end the voltage will drop, when you reach the fault it will be near zero and all points beyond it will be identical.

In effect you are doing manually what a continuity tester does, but because you are using a four-lead connection to the fault (current injected through the circuit conductors, voltage read via the outlet) you eliminate the relatively high and variable contact resistance and get a much more precise indication.

If the CPC isn't picking up any parallel paths, the drop for 1A will be about 20mV per metre of 2.5 T+E, so with an ordinary DMM that will read in mV, you can localise to within a few inches.

Alternatively, if you pump in an audio frequency tone, provided the wiring is not in metallic containment you can find the fault with a search coil, if it happens to be buried in the wall rather than at a point. Standard practice in comms wiring.

 

[davesparks]

Now there's a good idea !
And constant current sources are pretty commonly available these days in the form of LED drivers. Might have to use 750 mA rather than 1A but that shouldn't make a huge difference I wouldn't have thought?

Or just knock up a 1A supply with a LM317 or similar

 

[Lucien Nunes]

Precisely. Unfortunately you can't just use a modern SMPSU and put it into current limit as it will fold back to next to nothing or go into burst mode.

Of course, if the fault is much more than an ohm or so it will tend to get lost amongst the cable resistance unless the circuit is very long, making it harder to locate. Also, if the test supply has much voltage compliance, you could dissipate significant heat and make the fault worse. Limiting it to a couple of volts is a good idea, or use a lower current like 100mA if you don't need pinpoint accuracy.

 

[PEG]

If it's a hard fault like a screw penetration, you can find it without any opening or splitting. Ensure everything is disconnected, separate the ends at the DB and connect a small regulated current source, say 1A, across the faulted cores of one end only. Go round the points reading the voltage across these cores. As you move towards the fault from the energised end the voltage will drop, when you reach the fault it will be near zero and all points beyond it will be identical.

In effect you are doing manually what a continuity tester does, but because you are using a four-lead connection to the fault (current injected through the circuit conductors, voltage read via the outlet) you eliminate the relatively high and variable contact resistance and get a much more precise indication.

If the CPC isn't picking up any parallel paths, the drop for 1A will be about 20mV per metre of 2.5 T+E, so with an ordinary DMM that will read in mV, you can localise to within a few inches.

Alternatively, if you pump in an audio frequency tone, provided the wiring is not in metallic containment you can find the fault with a search coil, if it happens to be buried in the wall rather than at a point. Standard practice in comms wiring.

I have an older type BT Tone Generator,that i have played with,although i have to admit,not having actually diagnosing a fault using it. It is in a box with some Yankee gear that we experimented with years ago,in an attempt to apply a traceable signal to an underground water main. If i remember,its' success rated alongside that of a capable divining rod exponent...:conehead:

 

[davesparks]

Divining rods work pretty well so I assume that was successful?

 

[ferg]

I recently did an EICR on a small 3 bed property where it had been DIY rewired at some point.

The socket rings were a bit awkward to fault find.

1 socket in lounge, 1 in kitchen, 1 in bed 1, 1 in bed 2 ...


Next ring similar.

Lighting similar.

Immersion cct also fed a socket in living room and 1 in kitchen.

You get the picture.

A nightmare to map out never mind fault find.

In the end my best friends were pen, paper and patience.

 

[SirKit Breaker]

Before answering this, i want more info, such as How has the fault come to light, what are the symptoms, and what are the protective device arrangements. I would then decide how to find the fault.

A recent example i had.

Lighting circuit trips RCD when any light on circuit is switched on, no fault with lights switched off, it has a 6a mCB and 30mA RCD. It is wired in 1.5mm T&E, and is brand new rewired circuit loop in/out. So straight away i know switch wires are OK, so i IR each leg of the loop and find that bed 3 to bathroom loop has 0 meg N-E, Replaced cable job done.

Cheers.............Howard

 

[TJ Anderson]

Before answering this, i want more info, such as How has the fault come to light, what are the symptoms, and what are the protective device arrangements. I would then decide how to find the fault.

A recent example i had.

Lighting circuit trips RCD when any light on circuit is switched on, no fault with lights switched off, it has a 6a mCB and 30mA RCD. It is wired in 1.5mm T&E, and is brand new rewired circuit loop in/out. So straight away i know switch wires are OK, so i IR each leg of the loop and find that bed 3 to bathroom loop has 0 meg N-E, Replaced cable job done.

Cheers.............Howard

That's bad on a newly wired circuit. What had happened to the cable on the Bedroom to Bathroom Leg to give to N-E fault?

 

[David M]

That's bad on a newly wired circuit. What had happened to the cable on the Bedroom to Bathroom Leg to give to N-E fault?

Not sure if any of you guys have come across this, but a few years back while doing a rewire during the final test we found a similar fault. It turned out to be a length of faulty t&e. I took the cable back to the suppliers as it was a manufacturing fault.