LV Sub-Main Fault


EME Power Systems received a call from a customer asking us if we could help to investigate an ever increasing intermittent tripping problem on a parallel feeder circuit from an LV MCCB.

Upon arrival, the load already removed and no other suspect causes, our engineers were able to confirm isolation and prove the circuit enabling us to carry out insulation resistance checks to each cable individually. It was quickly identified that one of the legs was reading very low insulation resistance and, following discussion with site, we were able to reduce the load, adjust the protection settings on the MCCB and get the circuit back on supply utilising the healthy leg of cable.

We concentrated on the faulty leg, aware that these types of fault can be very difficult to locate, and utilised our in house fault location and cable tracing equipment to soon pinpoint a likely spot and had our civils team standing by to start excavating. With the cable exposed and identified, our craft staff commenced with a strip and test to start sectionalising the cable. However it soon became apparent that, although not directly the problem, water had not only got into the outer sheath of the cable but, causing more concern, into two of the phase cores resulting in the copper conductor showing signs of deterioration.

With differing insulation resistance in either direction, we were able to further sectionalise the cable until we found healthy sections from either end of the circuit. Liaising with the customer, we ensured that minimum disruption was caused to their operations, and overall a 100 metre section of cable was overlaid and jointed out.

Once the cable was tested and the out of hours shutdown organised, we reconnected the parallel feeders, verified and reset the protection settings and reenergised the circuit, returning the customers network back to normal operation. Having reinstated the trenching as we ‘went’, we finalised the reinstatement of the joint holes and left site fully operational.

Contracting engineer James Grigg commented:

As EME Power Systems have both the in house equipment, and craft staff with the required knowledge and experience of locating and repairing LV and 11kV cable faults, we are able to carry them out swiftly and safely.