Locating a blow-out or open conductor on a buried electrical power cable …
Open conductor or blow-out occurs due to a high fault current, a dig-in or a failed splice. An insulation resistance test may indicate a high resistance value or high withstand voltage if the fault and ground is dry. If the ground is moist the resistance measurement may show a moderate to low resistance value at the fault. If the phase conductor or concentric burns back the fault gap distance may be excessive and may not arc or “thump”, refer to fault position ‘A’. Burn backs are common when viewing the cable from the Source side of the circuit since it is the Source side that sees the highest magnitude of fault current during a cable failure. Testing from the Load side of the cable may render better results as shown in position ‘B’.
An open conductor or blow-out will appear as a positive reflection on a Time Domain Reflectometer (TDR) as shown by the solid pulse echo signature. If the conductor or concentric is burned back and the gap value is excessive, the fault may not arc (thump). This will result in little or no change in the pulse echo signature when the Arc Reflection test is performed. The solid signature shown below represents the “before surge” pulse echo signature; the dotted line represents the “after surge” signature.
As stated previously, an open conductor or blow-out will appear as a positive reflection on a TDR. If the gap distance between the conductor and concentric has not burned back the fault image will invert or reflect negative when the Arc Reflection test is performed as seen in the dotted pulse echo signature. This also indicates good arc or ‘thump’ condition.
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