Locating cable faults — TDR versus insulation resistance testing

The insulation resistance testing guide covers how a megger establishes that there is an insulation fault in a cable. This article covers the follow-up question that arises once the cable is too long or inaccessible to simply replace or expose in its entirety: where exactly is that fault?

Two different measurements, two different purposes

Insulation resistance testing (megger): applies a test voltage across the insulation and measures the resistance — confirms that there is an insulation fault and how severe it is (broadly), but says nothing about the fault's location along the cable.
TDR (time domain reflectometer): sends a short, low-voltage pulse through the cable and measures how long it takes for a reflection of that pulse to return from the fault. From that travel time and the known propagation velocity of the signal in that cable type, the TDR calculates the distance to the fault — without the cable having to be exposed along its entire length.

Both measurements are complementary: the megger shows that there is a fault, the TDR shows where.

The limit around high-resistance faults

A TDR works reliably for a low-resistance fault — a short circuit or a break, where the pulse gives a clear reflection. For a high-resistance fault (in practice often above the order of 200 Ω), the low-voltage test pulse of a simple TDR is often too weak to produce a usable reflection — the fault then remains largely "invisible" to a standard TDR, even though insulation resistance testing does show a problem.

Note: specialised, high-voltage-based fault-locating techniques exist for that kind of high-resistance fault, which temporarily lower the fault's resistance so it can still be located with a TDR or an acoustic method. That is specialised, potentially hazardous work for experienced cable-fault crews — this article describes the principle, not an instruction to carry this out yourself.

Order of testing

The usual sequence for a suspected cable fault:

First, continuity and a low-voltage resistance check to confirm the fault, before a higher test voltage is applied.
Only then, if needed, the full insulation resistance test with the megger.
Caution with repeated high test-voltage measurements on an already suspected fault: a high-voltage test can change the character of the fault itself (for example temporarily "sealing" a marginal fault), which makes subsequent TDR location harder rather than easier.
TDR measurement to determine the distance to the fault, once the fault is confirmed and (as far as possible) low-resistance enough for a usable reflection.

Practical relevance

For a cable that shows a fault on insulation resistance testing, but whose physical route is long, underground or otherwise hard to access (see also the underground cables — digging depth guide), a TDR measurement prevents the entire cable route from being needlessly dug up or exposed — the excavation location can be limited to the distance indicated by the TDR, with a margin for the accuracy of the equipment used and the cable-type setting.

Common mistakes

Relying only on insulation resistance testing to physically locate the fault — that measurement confirms a fault, but gives no distance information; a TDR is needed for that.
Trying to locate a high-resistance fault with a standard TDR without accounting for the limit around high-resistance faults — the pulse may then give no usable reflection.
Repeatedly running high-voltage tests on an already suspected fault before the TDR measurement — this can change the fault characteristic and make subsequent location harder.