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Cable Fault Profiles and Troubleshooting … Part III

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Locating a fault within a splice cavity on a buried electrical power cable …

A failed splice can be a difficult fault to identify and locate. Since the physical distance between the phase conductor and concentric is increased, the gap distance is typically excessive. This creates a fault that may not arc or ‘thump’.

If you are using a surge generator (thumper) as your fault locating tool make sure to pay close attention to the thumper’s voltmeter and ammeter during discharge.  This will reveal a great deal of information about the condition of the fault.  If you have a fault within a splice cavity, during a surge discharge (thump) the voltmeter will give a weak drop and will never approach zero volts. The fault may not arc or break-down on every discharge cycle.  During a proof test or high potential test the cable may hold voltage for a brief period before flashing over.  During a surge discharge (thump) the ammeter will give a weak kick forward.

A splice failure is very similar to an open conductor (blow-out) in the sense that the resistance and gap value at the fault will typically be very high.  A high energy surge generator may be required to successfully break down the fault and applying burn current may be required to improve break-down performance.

The pulse echo pattern for a splice is a small positive reflection that is immediately followed by a small negative reflection. Refer to the solid signature shown above. If water is present within the splice the negative reflection will typically be more pronounced. When the Arc Reflection test is performed the signature of the splice may change slightly, becoming more pronounced on the negative as shown in the dotted signature above. This indicates that during the surge or ‘thump’ the resistance of the splice is becoming lower and that a ‘thump’ is occurring. If the gap between the conductor and concentric is too excessive the Arc Reflection test will show no change in the pulse echo signature. This indicates that no ‘thump’ is occurring and burn-down may be required.

Protec Equipment Resources carries a complete range of advanced cable fault locating systems and high energy thumpers.  High energy systems include the powerful Megger PFL20M and PFL40A arc reflection systems.  Outfitted with the Aquatronics SDAD directional acoustic pinpointing unit, cable faults are located quickly and with minimum stress to service aged cable.  Let Protec’s highly trained technical support staff make you a successful cable fault technician!