How to detect leak or shortcircuit in electrical wires undeground?

Question

Recently, the circuit breaker was triggered when I activated the electricity in a cable buried in my garden. Probably water entered into a junction or I nicked the line by accident...
The line is not very deep, sometimes only 20 cm. But I don't want to dig all the line to find the fault. It would be a lot of digging. And in some areas, the line goes under concrete.

I measured the resistance. Between the primary wire and ground wire, I have 5 kΩ - which I know is very very low (should be infinite) but still not enough to create a shortcut. Between the primary and null wire, I have 20 kΩ (without touching the ohmmeter's alligators :) ). The resistance increases with 0.1 Ω per second.

How do I detect where the short circuit is? I was thinking about measuring the resistance between the primary wire and different points in the ground, along the possible path of the cable.

Naturally, the power is not connected anymore to that line now :)

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Update:

I got one of those devices that inject a radio signal into the wire and then use a detector to find where the wire is buried. If I inject the signal into the Neutral wire, I can track it through my garden. But if I inject it into the Phase wire, I cannot track the cable.

Update:

Now that the warmer weather is coming, I have done some extra measurements and I narrowed down the problem a bit more. It doesn't seem to be the cable, but the junction box where the wires are split (Y splitter) into 2 directions. I will have more measurements soon.

Answer 1

How to detect leak or shortcircuit in electrical wires undeground?

You've already detected it.

Oh, you mean its location? Why do you assume it's only one location? Whatever made it break in the first place will make it break again if you just "mend it".

And anyway, do you have proper technique and supplies to even attempt a repair in a buried cable? In your case, it'll be less work and more benefit to replace the cable. I doubt you can repair it properly, and that's assuming it's a localized damage and there are no other problems. I.e. the cable is properly rated and buried per code (deep enough, conduit if needed, etc).

Between the primary wire and ground wire, I have 5 kΩ - which I know is very very low (should be infinite) but still not enough to create a shortcut.

That 5kΩ just means that the corrosion hasn't done enough damage. It's only a matter of time. It is a short. It just hasn't been generating enough heat yet to thermally run away and burn out the wire in the hot spot. Again, the operative phrase is yet.

Between the primary and null wire, I have 20 kΩ.

Again, a short as far as mains is concerned.

The resistance increases with 0.1 Ω per second.

Electrochemical processes do that.

You need to replace the entire length of the buried wire. It has been compromised, for whatever reason. Investigating the reason will also be important.

And in some areas, the line goes under concrete.

A directional boring company will get the conduit installed for you in an hour or two. Then you won't have to worry.

When installing new cable, it must either be in continuous watertight conduit when underground, or you must be using cable rated for direct burial.

The line is not very deep, sometimes only 20 cm.

Perhaps it should go deeper then? Did you nick it with a space while doing yard work?

Anyway, it's very likely that the brouhaha stems from cable unsuitable for the application and/or incorrect burial method.

Answer 2

Question's Third Revision

A circuit breaker would trigger on a short circuit but the resistance measurements indicate no short circuit. A Residual Current Device (RCD which can also be known as an RCCD) does not require a short circuit to be triggered. The resistance measurements (e.g. 5 kOhm; Active - Earth) indicates no short circuit therefore the protection device is likely to be an RCD. The remote end of the cable should be accessed and ensure any/all loads are disconnected. If the remote load(s) had not been removed prior to testing then the author should engage a professional to remove the load(s) then test for an earth leakage problem.

Was the wiring updated to add an RCD to a power board before the old wiring had power applied? RCDs can be retrofitted to existing installations as a safety upgrade, new solar system installation, new extension works, etc. The 5 kOhm load is unlikely to trigger a circuit breaker but the additional protection provided by an RCD would trigger on the 48mA (240V / 5000 Ohm) earth leakage current. If the remote load has not been removed then it is highly likely that the measurements are due to defects or incompatibility of the remote load.

Regarding the attempt to trace. Is this actually an RF, fixed frequency audio or swept frequency audio test set? If an audio test set then your multimeter may be able to measure the open circuit, AC output voltage. Then measure the AC output voltage when connecting to each of the three power leads. It may be as simple as the set is unable to provide a high enough voltage to be detected on the active lead when underground. If this is the case a wide bandwidth amplifier with an additional, non-inductive, series resistor on the ouput (low value suitable for amplifier protection) should provide enough to drive the active line. Just be aware that the test set output into the amplifier may need an attenuator to reduce the input voltage to the amplifier. If this is a simple squarewave source using an RF detector an audio amplifier with low pass filtering may reduce the higher order harmonics too much. As you can already perform tracing on the Neutral wire it should be possible to confirm proper amplifier operation before tracing the Active wire. If there is a capacitor filtering the Active wire it may bypass the higher frequency harmonics and prevent tracing.

If the active wire in the power cable has been cut with insulation damaged then tracing to a break may also provide the location of the 5 kOhm resistance.

If tracing does not work then adaption of a simple TDR test may provide an answer. Alternatively a Murray Loop Bridge circuit or related variants might determine the location of a cable fault. Another overview is Loop Tests For Locating faults in Underground Cables Commercial equipment suppliers such as Megger provide their own overviews Fault Finding Solutions

Question Original... In your situation my first attempt would be using a Time-domain Reflectometer (TDR) to measure the distance to the fault. Given the likely type of power cable you need a sample (say a few metres) of the buried cable to determine its characteristics as an underground twisted pair transmission line. Refer to Wikipedia TDR entry for an introduction to this method.

There are many methods to determine cable fault location and this has been listed for Murray Loop Bridges, and other methods of cable fault location. Refer to the Methods of Fault Location in Cable System

The problem will most likely be the access to suitable test equipment. If the TDR method failed then I would attempt to construct a Murray Loop Bridge from parts.

Answer 3

A circuit toner/tracer that injects Radio Frequency (RF) into a cable may be able to trace the wire. They are meant to track cabling inside walls. You'd have to make sure the circuit is not live and trace a combination of wires and from both ends, using two that are not shorted.

More info on how they work can be found on the nets such as How does a cable tone-and-probe kit work?

Based on the measurement where resistance rapidly increases (more than feasible due to heating from the resistance measurement itself), there is essentially the equivalent of a capacitor (such as an electrochemical battery) in the line. Capacitors in DC will charge up, reading greater resistance as they do. In AC, they will barely impede power transmission, acting more like a filter (depending on resistance). Capacitors can be made from conductors (with insulation between them) placed in close enough proximity for charges in one to significantly affect the other. In this case, that proximity was likely achieved by crushing the wires together.

I suggest treating your garden as an alley or other thoroughfare when it comes to determining the needs; an extra 6" or foot of depth may add greater versatility to the area. Because it is high voltage, wire should be buried inside a (PVC these days) conduit (conduit takes loads that might otherwise crush) and deeper than 2 feet (deeper means loads, such as vehicles, above soil are less likely to affect). Code exists to help by providing info on what might be needed to prevent this sort of failure.

Answer 4

I got two cheap (china-grade) cable detectors from Amazon. None of them worked. Sent them back and got a sophisticated one, sold by Amazon itself (Amazon Basics Black Cable Detector, 133 euros). I would post the link here, but I don't know if it is considered spam - but I am not affiliated to Amazon in any ways. That worked, and I figured out where was the short. I replaced that junction. All works fine now.