The reason the sheath deserves its own test programme is that a sheath fault can exist for years without affecting any electrical measurement taken on the conductor. The cable performs normally while moisture migrates slowly through the breach, and the first indication is often a failure at a point far from the original damage.

What the sheath test proves about the cable system

The sheath, screen or armour of an underground cable performs two functions. It provides a continuous metallic barrier that keeps water away from the insulation, and it provides the return path for fault current and for the capacitive charging current of the system. Both functions depend on the sheath being continuous and on its bonding and earthing arrangements being intact.

A sheath test establishes the integrity of that metallic path and the condition of the outer insulation between it and earth. On a cable where the jacket is intact, the sheath-to-earth resistance measured with a direct voltage is high and stable. A breach reduces that resistance, and the size of the reduction indicates how severe the breach is.

The test also covers the bonding and earthing arrangements, which are part of the sheath system rather than accessories to it. Cross-bonding links, sheath voltage limiters and earthing connections all affect how the sheath behaves under fault and under normal load, and a defect in any of them produces an electrical condition that the test can detect.

Test methods and their reach

The first method is a direct voltage insulation resistance test on the sheath, applied with the far end isolated and the bonding arrangement set appropriately. It establishes whether a fault exists and gives a value for the sheath-to-earth resistance, which is what determines whether the fault is severe enough to act on and what method will be needed to locate it.

For location, two families of method are used. Methods that track a signal along the route use an applied signal and a receiver moved along the surface above the cable, detecting the position where the signal path changes. Methods that measure from the ends derive a distance from the resistance or from the time behaviour of the sheath circuit.

The reach of each method is limited by the physics of the fault rather than by the instrument. A low-resistance fault with a clean metallic path to earth produces a strong, locatable signal. A high-resistance fault produces a weak one, and a fault that is only conductive when wet may produce nothing at all. The result of the test is therefore a statement about the sheath condition at the time of the test, not about the sheath condition in general.

Fault resistance and detection limits

Fault resistance is the single most important input to the choice of method. A sheath fault with a resistance of a few kilo-ohms behaves like a solid connection for practical purposes and can be located with any of the available methods. A fault with a resistance of hundreds of kilo-ohms produces a signal that may be at the limit of detection, and the location method has to be chosen accordingly.

The practical consequence is that the initial insulation resistance measurement determines the strategy. A low value means the fault can be located directly, and the work can be planned with confidence. A high value means the test may have to be repeated under different conditions, or that a method that measures the sheath circuit from both ends will be more productive than a method that tracks a signal along the route.

Wet conditions are the second variable. Many jacket faults are more conductive when the surrounding soil is wet, which means a test performed in dry weather may find nothing where a test after rain finds a clear fault. Where a fault is suspected but not found, repeating the measurement under wetter conditions is a reasonable step before concluding that no fault exists.

Access to sheath and bonding points

The test requires access to the sheath at defined points. On a straight-through cable the sheath is accessible at the terminations and at any joint where it has been brought out for bonding. On a cross-bonded system the bonding links at each sectionalising position have to be opened or reconfigured for the test, and the configuration used determines what the measurement represents.

The bonding arrangement is therefore part of the test setup rather than part of the background. A measurement taken with the links in one configuration and a second taken with them in another are measuring different circuits, and comparing them produces a conclusion that has no physical meaning. The configuration should be recorded for every measurement and reproduced for the next one.

Access also determines how much of the route can be tested in a single session. A long route with few accessible bonding points may have to be tested in sections, with the results combined afterwards. Where sections are tested separately, each measurement should be recorded with the section it covers so that the combined picture can be reconstructed.

Interference from parallel routes

Buried cables rarely run alone. A cable in a shared trench with other cables, a gas main, a water main or a metallic pipeline has neighbours that affect both the measurement and the location method.

Parallel cables carrying load current induce voltages in the sheath, which appear as a background signal during the test. Metallic pipelines and other services provide alternative paths for currents in the ground, which can carry part of the test signal away from the cable and make the signal appear to travel along a different alignment.

The practical approach is to establish the interference conditions before the search rather than after a confusing result. Where a pipeline runs alongside the route, the location method should be one whose signal is confined to the cable rather than one that relies on a ground-return path. Where induced voltages are significant, the measurement should be taken with the loading conditions recorded, because a second measurement taken under different loading will not be comparable.

Interpreting results along the route

A sheath measurement taken at the ends gives a total value for the section tested. Interpreting it along the route requires either a location method or a series of measurements taken from different access points.

Where the route has accessible bonding points at intervals, measuring the sheath resistance between adjacent points localises the fault to the section between them. That approach is slower than a tracking method and it produces a section rather than a position, but it is more robust where the signal environment is poor.

Where a tracking method is used, the position is marked in the same way as a conductor fault position, with a survey reference and the distance from the nearest known feature. The record should state the method, the signal arrangement, the fault resistance measured, and the bonding configuration used, because those are the conditions under which the position was established.

When a sheath fault implies corrosion or mechanical damage

The position and pattern of sheath faults point to their cause. A fault at a joint, at a crossing under a road, or at a point where the route changes direction is usually associated with mechanical damage during installation or with movement afterwards. A fault in a straight section of route, with no obvious feature at that point, is more consistent with external corrosion or with damage caused by later excavation in the area.

A single fault is a localised problem. A cluster of faults along a section suggests a soil environment that is attacking the jacket, or a section of cable that was damaged before installation and has been degrading since. The distinction changes the response: a single fault is repaired, while a pattern may justify replacing a length of cable or applying additional protection along the section.

The only way to establish the cause with certainty is to expose and inspect the point, but the position should be recorded even where no repair is carried out immediately. A sheath fault that is left in place becomes a monitoring point, and a repeat measurement at the same position later establishes whether it is deteriorating.

Recording the located position

The record should identify the circuit and the section, state the bonding configuration used for the measurement, give the sheath resistance measured and whether it was taken in dry or wet conditions, and describe the method used to locate the position.

For the located position, the record should carry the distance from the nearest known feature, the depth if established, the surrounding services, and the soil condition observed. Those details allow the next investigation to begin at the right place, and they allow the pattern of faults on the route to be reviewed.

Over time, the value of this record is in the pattern. Sheath faults tend to recur at the same kinds of locations, and a route whose fault record is plotted against its features shows where the vulnerabilities are. The construction and installation documentation published by cable manufacturers such as Prysmian and Nexans describes the sheath and jacket arrangements that the measurements depend on, the cable classes are defined in IEC 60502-2 for medium voltage extruded cables and in IEC 60840 for higher voltage classes, and the accumulated field experience with cable system diagnosis is coordinated through CIGRE study committees. The sheath and cable testing range is grouped on the cable fault testing hub.