Adequate insulation between tram rails and the buried stray current mat on the Nottingham Express Transit tramway was confirmed using a fault finding technique developed by Radiodetection and Carillion.

A Radiodection PCM transmitter was used to inject a 4Hz electrical signal into the rail under test, with a magnet used to clamp the signal wire to the rail head. A modified RD4000 receiver mounted on an A-frame was used to measure signal loss. The A-frame is usually used as a voltage pick-up device to detect voltages caused by ground currents from damaged cables and pipes, but for the tram application it was converted into a single-turn inductive pick-up.

Measurements were taken at regular points along the track, generally at tram stops for ease of access. The signal flow in the bonded rails was summed and plotted, so that areas of significant loss could be identified. These were then revisited for a more detailed survey with measurements at closer intervals. The RD4000’s current direction facility allowed the point of loss to be pinpointed to within 0·1m.

The faults were traced to alignment shims which had been put under pressure during the rail alignment process, damaged cross-bonding cables, and mechanical damage to insulation caused during installation.

After repairs the area was resurveyed using a transmitter operating as a constant current generator, which forced current into the less significant faults to make them more visible as the other faults were repaired.

Four full surveys found eight significant faults in 16 km of rail. The final survey showed a linear decay of the signal from dampness and natural debris, demonstrating the integrity of the track.

Radiodetection Ltd, UK