- Cable Faults Definition: Cable faults are issues in electric cables that disrupt the flow of electricity, including short circuits, earth faults, and open circuits.
- Causes of Cable Faults: Faults can be caused by damaged insulation due to water, moisture, ageing, or improper handling.
- Detection Methods: Faults are detected using tests like the megger test and multimeter to identify the type and location of the fault.
- Fault Burning: This method reduces resistance in the faulted cable, making it easier to locate and repair.
- Localisation Techniques: Methods such as the Murray Loop Test and Voltage Drop Test are used to find the exact location of faults in cables.
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.
Multiple types of faults can occur simultaneously in a cable.
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
To detect a cable fault, use a megger test to identify the type of fault. Measure the fault resistance with a multimeter if needed. First, examine the terminal box, as faults often occur there. Check the outdoor box before the indoor box if both are present. If no fault is found, inspect the cable joints.
If high fault resistance is detected, perform fault burning to reduce it before conducting a Murray Loop Test. Use a high voltage pressure testing set for fault burning. Burn the core with the least resistance if multiple cores are faulty. The burning duration depends on the fault and cable condition, usually taking 15 to 20 minutes.
Use a megger to detect cable faults by measuring the earth resistance of each core. A short circuit between a core and earth will show zero or very low resistance. Lack of continuity between two ends indicates an open circuit in that core. If all three cores lack continuity, all three 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.
