Question

Cable Fault Testing Practical

Answer 1

Practical Significance

Cable faults can be occur with insulation damage due to ageing, excess heat, wrong handling, improper termination of cable, and moisture in the soil. To restore the supply fault finding and repairing of cable fault is necessary.

Competency and Practical Skills

This practical is expected to develop the following skills for the industry identified competency: ‘Plan wiring/cabling activities using relevant materials following safe practices’

i. Use Megger for fault finding

ii. Interpret the megger results.

Practical Outcome

1. Investigate (and record observations) a cable failure by insulation breakdown (1St case)

2. Investigate (and record observations) a cable failure by insulation breakdown (2nd case)

Relevant Affective domain related Outcome(s)

i. Practice good housekeeping with safety measures.

ii. Demonstrate working as a leader/a team member.

iii. Maintain tools and equipment.

iv. Follow ethical practices.

Minimum Theoretical Background

There are three types of cable fault.

1. Short circuit between two conductor,

2. Earth fault, i.e., fault between conductor and ground,

3. Open circuit caused due to disconnection of conductor.

At a time one or more faults may occur. The primary cause of the short circuit and earth fault is due to damage of insulation caused by water, moisture or other reason. In an electrical machine due to the defect in the armature, plumbing or the lubricant compound gets out due to excess heat; insulation of the cable can get damaged. The open circuit fault can occur due to wrong handling of conductor, improper termination of cable, loose clamping in terminal box and depression in soil which stretch the joint.

Apart from these, all reasons of short circuit may lead to open circuit fault.

Apart from the various sophisticated testing equipment, megger test is a basic test for finding cable fault. Before doing megger test, it is necessary to check outdoor and indoor terminal box respectively

Type of fault in the cable, can be detect using megger by measuring earth resistance of each core. If there is a short between the core and the earth, the insulation resistance of such core will show ‘ZERO” or very less in megger. 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, means entire three cores are open circuited. After deducting the fault, repair the cable.

Practical set-up / Circuit diagram / Work Situation

Resources Required

Precautions to be Followed

1. The megger should not be used on live system.

2. The handle of the megger should be rotated in clockwise direction.

3. Don’t touch the terminals of megger while conducting a test; otherwise it will give an electric shock.

4. Rotate the handle of megger at its rated speed.

5. Keep the megger in horizontal position while operation.

Procedure

1. Open the cable ends from both side as shown in Fig. 1

2. Connect the megger between the faulted conductor and ground as shown in Fig. 2.

3. Measure and record this resistance reading.

4. Connect the instrument between each of the other phase conductors, and ground and record the readings.

5. Connect a short between the phase and neutral as shown in Fig.3, do a loop test for continuity and record the reading.

6. Repeat all tests from end B and record all readings.