Idea for a new tester...........

It will only work for dead short faults, but low impedance ohm meters are a thing.

Two wires are attahed tto each wire to be measured - one to inject current where we do not care about voltage drop, and one to measure voltage, ideally unaffected by the current passing.

This is often done with a thin coaxial lead,  rather as used for audio phono plugs with the current on the outer and the voltage sense or the inner, the inner and outer go to opposite jaws of a croc-clip, and only meet at the very point you measure from. This eliminates the test leads.

An accurately  known current of 0.1mA 1mA, 10mA or similar is passed ,and the voltage measured. With lots of averaging pulsing the current and subtracting the reading with no current from the reading with current, a few microvolts can be determined, so the reading is in miliohms. Something about the complexity of the arduino nano has a 10 bit ADC, is quite capable of the sums, driving a basic  LCD display and reading a few buttons. The rest is software. 

Mike.

BT and the Teleco's already used such an instrument for finding faults in multi hundred meter long lines.

It uses electronic pulses sent down the line and then the reflections from the fault which bounce back to the kit from the line are then analysed by the software in the kit to produce a graph.  When you look at the graph of the line's characteristics from start to finish on the screen you can then determine sort of where the fault is likely to be by the dips and bumps in the trace.  I gather does get difficult where the BT line is comprises of different types and conductor thickness along the route to determine which is the fault and which is the reflection from the discontinuity at the joins.

I guess they are not cheap!

I've forgotten what the kit is called - something like a Time Delay Reflectometer

Yes, time domain reflectometer (TDR).

You will need to know the characteristic impedance of the cable for it to work … this is of course known for conductive cables used in telecomms, but not always available for power cables unless you test a sample yourself.

The other thing TDR testing relies upon, is the characteristic impedance being pretty consistent along the length of the cable.

Whilst things may well be different for new power cables, I'm sure manufacturing tolerances for insulation and spacing of older power cables would lead to variances in characteristic impedance along the length of the cable that are too great to make the technique as productive in locating the fault. Joints in the cable also alter the characteristic impedance, so having someone experienced at what they're looking at when the “blips” come back on the traces is paramount.

I think easier to use a low resistance ohmmeter and an app for your phone, or even an Excel spreadsheet.

Given that the kind of thing David Savery is describing in his video is just measuring R1+R2 (where the short is the link between L + E) then dividing it by by the (R1+R2)/m value for copper looked up in table L1 of the OSG, having that function as a dedicated meter seems a bit of an overkill. Even if you don't have the OSG to hand, the value from the table is just (18.1 / 2.5 + 18.1 / 1.5)

Of course as Mike points out, using a dedicated low-impedance meter will give you a more accurate result.

Actually, the TDR doesn't care about the cable impedance, because it measures this continuously across the trace. It does need to have a good idea of the cable “velocity factor” but this is easily found with a known length of similar cable. It will find any kind of fault where the cable impedance changes suddenly, and often can see joints or whatever because the impedance changes. They are often used on underground cables by the DNO as one can see all the joins and branches, very handy looking for problems. They are not incredibly expensive for fairly simple instruments, prices increase with range, accuracy, etc. as usual. They work by generating a very fast risetime pulse, usually less than 100ps, and recording the voltage across the test terminals as the pulse is reflected from changes in cable impedance. 100ps gives a resolution of about an inch, which should be good enough for electrics. Ones for high-frequency electronic tests may use pulses with a risetime of 1ps or less and are expensive!

I can vouch for the basic technique of using resistance measurements to locate a short - I did just that once many years ago to trace a problem with a friend's kitchen socket which appeared to be unearthed yet showed continuity between its two c.p.c.s … turned out some muppet had tried to extend the ring using a single 4-terminal joint box and jointed the c.p.c.s outside .. but had picked the wrong pairs of c.p.c.s. to join. ?

I've only needed to use that technique on a very few occasions however, so would doubt the value of having a dedicated meter to do it. I could be built into something like an MFT I suppose, but selecting the right cable size (of out of very many) would be tedious using the simple button human interfaces they usually use. A windows like drop-list would be easier.  So I guess stick with an ordinary Ohm meter (there's not much need for 4-wire probes if you can zero out the normal leads) and perhaps an App for a phone or a simple bit of calculation built into a web page somewhere.

   - Andy.