It can be of three (3) types of electric cable faults found. They are
- There may be a short circuit between two conductors,
- There may be earth fault, i.e., fault between conductor and ground,
- There may be open circuit caused due to disconnection of the conductor.
There can be more than one of the faults at a time.
The primary cause of the 1st and 2nd fault is due to damage of insulation caused by water, moisture or other reason. Due to the defect in the armour, plumbing or the lubricant compound gets out due to excess heat, insulation of the cable can get damaged.
Moreover, due to ageing, the insulation can get damaged. Normally the lifespan of a cable is about 40 to 50 years. PVC cable gets damaged due to wrong handling. If the compound elements decrease in the terminal box, the fault in cable occurs. If we do not join or terminate the cable properly, there may be an open circuit fault. Due to depression in the soil, there can be a stretch in the joint which may lead to an open circuit fault. Moreover, if we do not fit the clamps properly below the terminal box, an open circuit fault may occur. Apart from these, all reasons for the short circuit may also lead to an open circuit fault.
Detection of Fault in Cable
If there is any fault in the cable, through megger test, it is to be detected that, what types of the fault have occurred. If required the fault resistance should get measured by multimeter. After the fault is detected, one should first examine the entire terminal box. In most of the occasion, you will see that the fault is in the terminal box. If there is indoor and outdoor box in the cable, then we check the outdoor box first, then the indoor box. If we do not find any fault in the terminal box, then we have to locate the place where the cable got a fault. If there is joint in the cable one should inspect the same, too.
If fault resistance is found more then where the fault occurs, one should burn the insulation “Fault Burning” and can decrease the resistance and after that Murray Loop Test can be done. Normally V.C. high voltage pressure testing set is used in fault burning work. If there is faults in more than one core then the core which is having less resistance that is to be burnt. The burning depends upon the fault and conditions of the cable. Normally the rate of resistance comes down within 15 to 20 minute.
If there is a fault in the cable, we should detect the type of fault with a megger. We normally measure earth resistance of each core. If there is a short between the core and the earth, the I.R. of such core will show ‘ZERO” or very less in megger meter. If we do not find continuity in any core between two ends, there is an open circuit on such core. If there is no continuity in all the three cores, we can understand that the entire three cores are open circuited.
After detecting the fault, we have to repair the cable.
Localisation of Cable Fault
There are different methods to detect the location of the fault in the cable. We adopt different methods in different situations. Some of the methods are herein below:
- Murray Loop Test
- Voltage Drop Test.