Cable fault location is required anywhere where the fault cannot be seen. This is a multi-step process that must be performed as safely and as quickly as possible to prevent customers going without power.
Step 1 - Cable isolation and safety procedures: A cable fault is nearly always a permanent fault. This means that the cable in question will be in a condition where the protection devices at one or both ends of the cable will have tripped, leaving the cable isolated but NOT earthed (grounded).
The first task is for the authorised person on the site to make the cable safe by isolating and then earthing (grounding) one or both ends. Only after the appropriate procedures have been carried out can any testing personnel be allowed to approach the cable and prepare for testing.
Step 2 - Cable identification: Where multiple cables exist, cable identification testing will identify the correct cable to work on. Clear identification before a cable is cut is intrinsic to safe maintenance work. Any mistakes here can be fatal and may cause much longer outages for the connected customers.
Step 3 - Cable tracing: When an underground cable is first laid, it rarely runs in a straight line, but rather meanders in depth and direction. Cable tracing is done to determine that the route of the cable is following the expected path.
Step 4 - Fault identification: The first major procedure is to determine the phase on which the fault has occurred and if it is of low or high resistance. This test determines the correct technique, and therefore equipment, needed to diagnose the fault. Typically, if the fault is found to be below 100 Ohms, a low voltage pulse (eg: 40 V) from a TDR (time domain reflectometer) can be used. If the fault is a higher resistance (> 100 Ohm), a low voltage pulse will likely not see it. For these types of faults, an impulse generator (shock discharge) or bridge, will be necessary.
Step 5 – Fault prelocation: A reliable and precise pre-location method is necessary to locate a cable fault quickly and efficiently. Good pre-location can determine the fault position to within a few percent of the cable length and will reduce pinpointing time to a few minutes.
a) If it is a low resistance fault, pre-location is likely to be the only means necessary for location.
b) For high resistance faults, ARM (arc reflection) or ICE (impulse current) techniques on an SWG (surge wave generator) should be used. Alternatively, the decay method with an HV DC tester (bridge) can be used for pre-location.
Step 6 - Pinpointing: The aforementioned test methods will get the operator within 5% distance of the fault. Acoustic pinpointing techniques must be employed at this stage in order to narrow the margin of error to 0.1%. In most cases, shock discharge generators are used for pinpointing in conjunction with acoustic methods. The discharge creates a loud noise, which is pinpointed precisely using an acoustic pinpointing device. This device evaluates the time difference between the acoustic signal (speed of sound) and the electromagnetic (nearly the speed of light) impulse of the shock discharge. When the shortest time difference is indicated, the exact fault location is revealed.
Step 7 - Re-energisation of the cable: Once all testing and repairs are completed, the Safety/Testing documentation is cancelled and the cable is handed back to the appropriate operators so they can reinstate it and re-energise the loads on the newly repaired cable.
Important advice when using the ARM/ICE Equipment:
It is important to find the lowest voltage that will cause the fault to appear during the above-applied testing procedure. The idea of 'thumping the cable with the maximum available voltage (Joules)' should not be condoned. For example, if a cable fault has left damage in the faulted phase that, when applying a gradually increasing ARM voltage, appears at 6 kV, then once this is established, only 10% more voltage, say 7 kV, need be applied for the fault location position show up clearly. What is fundamentally important is that the energy used is proportional to a square of the voltage (V2). If the cable is repeatedly ‘thumped with very high over-working voltages’, other lesser points of damage could be initiated into insulation failure, thus creating more jointing/repairs needed on the cable.