Cable fault location on medium-voltage (MV) cables throughout Austria and southern Germany

Key facts at a glance

For you as a business

For utility companies, municipal utilities, local authorities and industrial businesses it is not only the position found that counts but its traceability: we document the method, the prelocation distance and the confirmation obtained by pinpointing, so that the road opening is justified to the highway authority and to internal audit. Isolation including earthing, and the repair, remain on your side; we carry out the measurements. Because the measuring equipment and the evaluation for these fault types are kept specifically for this purpose, we travel for it throughout Austria and to southern Germany, not only within the area around the two sites.

Burning down means a shutdown – and of more than one core

Before anything is located, the condition is established. Insulation and resistance measurement say which cores are damaged and how high-resistance the fault is. That value decides how the job continues – and whether your installation stands still while it does.

If the value is low enough, measurement and pinpointing follow directly and the route is recorded in the same pass. If it is too high, the fault position yields nothing: it first has to be surged and burned down until it becomes earth-sensitive. During that process the feeder is not partly but completely out of service. Burning down regularly damages the neighbouring phases as well; what was still running in a limited way beforehand may not run at all afterwards.

For utility companies, municipal utilities and industrial businesses this means: before burning down, it has to be settled how the time without supply will be bridged. Either a standby supply is available – a switchover in the network or a temporary arrangement, whichever your installation allows. Or the excavator is on site and opens up as soon as the position is established, so that jointing can follow immediately and the feeder is missing only for the duration of the repair.

That decision belongs in the job coordination, not at the measuring point. Tell us when you report the fault whether a switchover is possible and who arranges it.

How the job runs

1. Report and coordination. You report the outgoing feeder, cable type, length and fault pattern, and name the contact for the switching operations. We clarify which substations have to be accessible and whether a switchover is possible if it comes to that.

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

3. Establishing the condition. Insulation and resistance are recorded core by core on the isolated feeder. This establishes what is damaged and how high-resistance it is – and with it, whether measurement can continue directly or whether burning down has to come first.

4. Prelocation and line tracing. Time domain reflectometry, bridge measurement or sheath fault location gives the distance to the fault position, in the case of high-resistance faults after burning down. In parallel the route is marked at the surface, and the positions of joints and branches are recorded as well.

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

6. Check measurement after jointing. The feeder is cut for the repair; both resulting sections then have to be measured for further fault positions before the supply is restored. Whether your own operations team does that or we do is settled beforehand.

Equipment and methods

Insulation and resistance measurement

The first reading does not answer where the fault is, but what one is dealing with. Core by core, the insulation is recorded against earth and against the remaining cores, together with the resistance. The result sets the course for the entire job: a low-resistance fault can be followed up immediately, a high-resistance one cannot. For your operating records the same finding is at the same time the survey of the feeder’s condition at the moment of failure.

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

High-resistance faults show no echo in the TDR trace, and it is precisely this type of fault that predominates on medium-voltage cables. If the insulation value stays too high, the fault position is invisible to every measuring method. It is therefore subjected to surge voltage and burned down until the transition resistance falls far enough for it to become earth-sensitive – only then does fault location work at all. This is the step with consequences for your operation: the feeder is completely dead while it happens, and the neighbouring phases are frequently damaged as well. It therefore has to be settled beforehand how the loss of supply will be bridged – by a switchover or temporary arrangement, or by having the excavator standing ready to open up immediately after the location work.

Sheath fault location on plastic-sheathed cables

A sheath fault is damage to the outer plastic sheath through which moisture reaches the cable screen; the installation often keeps running while corrosion is already at work. For location, a pulsed DC voltage is applied to the screen, the current leaves the cable at the damaged point and creates a voltage gradient which is picked up with two earth spikes and reverses direction above the fault position. On sealed surfaces, over reinforcement, or where the screen is earthed at several points, the gradient is blurred.

Ley effect

On distribution runs that have grown over time, the TDR trace often gives no unambiguous picture, because every joint and every branch split sets a marker of its own. The Ley effect does not work with echoes but with the behaviour of the applied signal along the route: consistent up to the fault position, no longer beyond it. That allows the section in which the ground is to be opened to be narrowed down – a narrowing which, together with the measured distance and the route, becomes the excavation instruction.

Time domain reflectometry (TDR) in network operation

In the distribution network, time domain reflectometry also serves as a survey of the existing assets: the trace shows joints, branch splits and terminations as recurring markers. If the current trace is compared with an earlier measurement of the same run, the new discontinuity stands out.

Bridge measurement (Murray loop)

Unlike time domain reflectometry, bridge measurement does not depend on an echo: it evaluates resistances instead of reflections and gives a distance even where the trace shows nothing but joints and branches. With an earth fault on one core, the ratio of the lengths follows from the comparison with a sound core of the same cable. What is needed is access to the far end, a consistent cable construction and a sufficiently low-resistance fault.

Cable identification in trench and cable tray

In distribution networks several systems lie side by side, some of them live. Cable identification identifies the cable being sought within the bundle beyond doubt before it is cut; if all the cables are of identical construction, the involvement of your operations team is required.

What you receive

You receive the fault position marked and a damage report as a PDF with the method, readings, distance, route and photographs, which can 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 before the job. Cover by an insurer is not the normal case in the network sector.

The check measurement is part of the job. If there is no fault at the marked position because our measurement was wrong, we measure again at no charge. If the measurement was correct and the excavation was simply not searched carefully enough or not dug deep enough, we charge for the call-out, because there was no measuring error on CFT’s part. The same rule applies to cable location and pipe location.

Frequently asked questions