Cable fault location on low-voltage (LV) cables throughout Austria and southern Germany

Key facts at a glance

For you as a business

A low-voltage supply cable rarely serves just one customer. For utility companies, municipal utilities and local authorities what counts is therefore how quickly the position is established – and that the road opening can later be justified to the highway authority and to internal audit. We record the method, the measured distance and the confirmation obtained by pinpointing, so that the excavation instruction does not rest on a verbal statement. On operational sites there is a further point: a supply cable often feeds a single installation which stands still for as long as the measurement takes, so the appointment belongs in a production break.

How the job runs

1. Report and coordination. You report the outgoing feeder, cable type, length and fault pattern, and name who will carry out the switching operations. We clarify which cabinets and substations have to be accessible.

2. Isolation and earthing. The cable is completely isolated from both ends, secured against re-energisation and earthed – by the network operator or by your own electrician, in accordance with your operating instructions. CFT does not carry out switching operations.

3. Establishing the condition. Insulation and resistance are recorded core by core. This establishes which cores are affected, which type of fault is present, and whether measurement can continue directly.

4. Prelocation and line tracing. Time domain reflectometry, bridge measurement or sheath fault location gives the distance; in parallel the route is marked at the surface. Only distance and route together give a position.

5. Pinpointing and handover. The position is narrowed down, marked and photographed. From the road opening onwards, your cable jointer takes over.

6. Check measurement after jointing. The cable is cut for the repair; both resulting sections then belong to be measured for further fault positions before the supply is restored.

Equipment and methods

Insulation and resistance measurement

The first reading does not say where the fault is, but what one is dealing with. The insulation resistance of every core is measured against earth and against its neighbours, together with the conductor resistance. That is the point at which everything else is decided – and at the same time the documented condition of the feeder at the moment of failure, which you can take into your own operating records.

Time domain reflectometry (TDR)

A pulse travels through the cable, every discontinuity reflects it differently, and the instrument calculates the distance from the transit time. On a supply cable the trace is usually easy to read, because there are fewer joints between two cabinets than on a medium-voltage ring that has grown over decades. What it gives is a length along the cable – not yet a position on the ground.

Bridge measurement (Murray loop)

Bridge measurement needs no echo: it compares resistances instead of reflections and gives a distance even where reflectometry yields nothing. Where there is an earth fault on one core, the ratio of the lengths follows from the comparison with a sound core in the same cable. What is required is a reachable far end, a consistent cable construction throughout, and a fault of sufficiently low resistance.

Ley effect

Where the TDR trace remains ambiguous, the Ley effect helps to narrow things down. What is evaluated is how the applied signal behaves along the route: consistent up to the fault position, no longer beyond it. That is a narrowing down, not a pinpointing – it says on which section the ground is to be opened.

Sheath fault location on plastic-sheathed cables

A sheath fault is damage to the outer covering through which moisture reaches the screen. Location works via the voltage gradient: a pulsed DC voltage drives current through the damaged point into the soil, two earth spikes pick up the gradient at the surface, and directly above the fault it reverses. On sealed surfaces and over reinforcement the gradient is blurred.

Surge and burn-down to make the fault earth-sensitive

If the insulation value stays too high, the fault position yields nothing to measurement; it is then subjected to surge voltage and burned down until it becomes earth-sensitive. On low voltage this is not the normal case, but it does occur – and when it does, the feeder is completely out of service, because the neighbouring phases are frequently damaged as well. What that means for your supply, and what has to be in place beforehand, is set out in full on the page for medium voltage, where this type of fault predominates.

Cable identification in trench and cable tray

In the open trench several systems lie side by side, some of them live. Identification establishes which cable in the bundle is the one being sought before anybody cuts; if all the cables are of identical construction, your operations team has to be involved.

What you receive

You receive the fault position marked and a damage report as a PDF with the method, readings, distance, route and photographs – ready to be attached to the excavation instruction.

Costs and effort

There is no flat rate, because cable type, length of run, type of fault and accessibility determine the effort; we agree the cost range by telephone beforehand. In the network sector an insurer will not normally cover the location work.

The check measurement is included. If our measurement was wrong, we bear it. If it was correct and the excavation was simply too narrow or too shallow, we charge for the call-out – the measurement was not the reason.

Frequently asked questions