Generally, operating procedures associated with faults in the power system can be classified as follows:

1-Pre-fault stage :

Operations in this stage aimed to preventing of  faults and so interruptions in the power system. In this way, power system’s reliability will increase. So this operations is called reliability centered maintenance (RCM). This type of operation itself  is classified into three categories :  preventive maintenance (PM), predictive  and preventive maintenance (PPM ) and condition monitoring (CM).

2-Concurrent with fault stage:

At this stage, the fault must be detected and isolated from the power system. This operation is done automatically by the  protection system.

3- Before fault locating :

At this stage, the approximate range of  fault locations will be  determined.  This stage is called power system step restoration. In this stage, operators by re-closing of some circuit breakers in power system and monitoring that they trip again or not,  can estimate a more accurate range of fault locations.

Even in some cases, because of transient faults, the whole of system may be restorated (fault natural clearance). Sometimes auto-reclosers and sectionalizers is used to do this step automatically.

4-Test, Analyze, Localizing and Locating  (Pin-pointing) of  fault (TALL):

This stage is one of the most  important stages which is associate to faults and needs expertise.  In the “testing phase”, operators use equipments such as ohm meter , Meager and Hi-Pot  for testing of lines and cables. Ohm meter is used for measuring the loop resistance between phases, phase and neutral and phase and shield (in the cables).  Meager is used to measure the insulation resistance and also short circuit fault resistance. And finally, Hi-Pot is used to measure the leakage current of any part of power system.

In “analyzing phase”, by using the test results, operators must gain a proper perspective of fault type, fault resistance and the suitable pack of methods which is needed  in the next stages.

In “localizing of the fault phase”, different methods is used. The first method is “sectioning” (opening the jumpers in overhead lines or cutting the cable in the middle point (in underground lines). Then, operators test again each section of line to determine which is faulty. This operation is repeated in the faulty sections until the approximate location of  fault is identified. The other methods which can be used in this phase are “Time domain reflectometry (TDR)” and “Murray bridge”. TDR method itself is categorized to “low voltage TDR”, “differential TDR” and “high voltage TDR”. The last one, itself is categorized to “surge reflection” and “surge decay”.

In “fault locating phase”, depending on the circumstances and results, three possible methods can be used:

In the first method, which is called “acoustic detection”, operator by  arc thumping with a proper receiver, listens to the audible or ultrasonic waves which is transmitted by arc.  Where receiver shows the maximum value, is the exact fault location, unless the inappropriate reflections by pipes, trees and …. leads to mistake. This method is used when the fault resistance is in a proper range which arc can be generated in fault resistance by a Megger, Hi-Pot or surge wave generator. When the fault resistance is very high, “Burn/Proof” method is used for decreasing fault resistance.

In the second method, which is called “electromagnetic surge detection”, detecting of magnetic waves (50 Hz frequency) is considered. This method is used in low resistance faults and in this way, operators trace for the maximum value of magnetic waves to find the exact fault location.

In the third method, which is called “voltage (or earth) gradient method”, voltage drop in the earth surface is measured by a proper two blade probe. Where the voltage drop will be zero, is the fault location.

5- After fault locating  stage :

In this stage, fault will be repaired and network will be restorated.

Note : A hidden stage, which is necessary for fault locating stage, is tracing the cable path. In all of the fault locating methods, operator must know the exact path of the line.