Locating cable sheath faults with a high-voltage Murray bridge

On high-voltage XLPE cables the outer sheath does more than protect the cable mechanically. It keeps water away from the metallic screen and, on single-core circuits with special bonding, it insulates the screen from earth so that circulating currents and screen voltages stay under control. A damaged sheath lets moisture in and can upset the bonding scheme, so sheath faults are worth finding even when the cable is still in service.

How sheath faults are detected

The usual check is a DC voltage test between the metallic screen and earth. After installation, the HV cable standards IEC 60840 and IEC 62067 call for a DC test on the oversheath, commonly 10 kV for one minute, with the test method described in IEC 60229. Many utilities repeat a sheath test during routine maintenance, especially after civil works near the route. When the test fails, or the sheath insulation resistance is low, the fault has to be located.

Why pulse methods struggle

A TDR looks for impedance changes between conductors. A sheath fault is a leak from the screen to the surrounding soil, often with a resistance of kilohms or more, and it barely changes the screen-to-earth impedance seen by a travelling pulse. Bridge methods do not rely on reflections, which is why they are the standard approach for sheath faults.

The Murray loop principle

The Murray loop is a Wheatstone bridge built from the cable itself.

1. At the far end, connect the faulty screen to a healthy conductor of the same length and cross-section, usually the screen of another phase.
2. At the near end, the instrument connects to both ends of this loop and drives current from its source, through the fault, to earth.
3. The bridge is balanced when the resistance ratio of its two arms equals the ratio of the loop resistance on either side of the fault.

At balance the instrument reads the fault position as a fraction of the total loop. When the faulty and healthy conductors have equal length and resistance per metre, the distance from the near end is:

Distance to fault = bridge ratio × loop length (twice the cable length)

Because the result is a ratio of resistances, it does not depend on the fault resistance itself — as long as enough current flows through the fault for the bridge to balance with good resolution.

Why a high-voltage source matters

A sheath fault may be hundreds of kilohms or intermittent. A low-voltage bridge cannot push enough current through it to give a stable balance. A high-voltage bridge uses a regulated constant-current source, typically up to several kV, so the fault conducts steadily and the bridge reading settles. Our high-voltage bridge cable fault locator combines the bridge and a 15 kV constant-current source in one case for this reason.

Site details that decide accuracy

• Open the bonding. On cross-bonded circuits, remove the links in every link box so the section under test is a single continuous screen, isolated from earth except at the fault.
• Use a matching return conductor. The formula assumes equal resistance per metre. If the return conductor is different, correct for its resistance or use a two-ended measurement.
• Keep connections low-resistance. Clean the terminations and use short, heavy jumpers at the far end; jumper resistance adds directly to the loop.
• Know the true length. Use as-laid lengths including joint bays, not the route-map distance.
• Measure from both ends on long sections. Two independent results that agree are the best protection against a wrong excavation.

From distance to location: pinpointing

The bridge gives a distance along the cable. To find the spot on the ground, apply DC pulses between the screen and earth. Current leaking at the fault creates a voltage gradient in the soil. Walking the route with an A-frame — two earth spikes and a sensitive voltmeter — the needle points towards the fault and reverses as you pass over it. Start a little before the prelocated distance and work towards it.

Checklist

1. Isolate, discharge and earth the circuit; open link boxes.
2. Measure sheath insulation resistance on all three phases to identify the faulty one and a healthy return.
3. Install the far-end jumper between the faulty and healthy screens.
4. Connect the bridge, raise the test current until the reading is stable, and record the ratio.
5. Repeat from the other end if possible and compare.
6. Pinpoint with step voltage, then repair and retest the sheath.

Need help with a sheath fault on a specific circuit? Send us the cable type, section lengths and bonding arrangement.