Testing the insulation resistance of electrical cables using a guard circuit with time-resistance method involves a specialized approach to ensure accurate measurements and to account for surface leakage currents. The time-resistance method is also known as the "time-resistance technique" or "time-resistance ratio method." Below is a step-by-step guide on how to perform this test:
Note: Insulation resistance testing involves working with electrical equipment, which can be hazardous. It is essential to follow all safety precautions and use appropriate personal protective equipment (PPE) during the testing process. If you are not familiar with electrical testing procedures, it is advisable to seek assistance from a qualified electrician or engineer.
1. Gather Equipment:
Insulation resistance tester (also known as a megohmmeter)
Guard circuit or guard terminal on the insulation resistance tester
Test leads (shielded cables are preferable)
Cleaning materials (clean cloth, alcohol wipes, etc.)
Personal Protective Equipment (PPE) such as safety gloves, safety glasses, and insulated shoes
2. Prepare the Cable:
Ensure that the cable to be tested is completely disconnected from any electrical source or system.
Clean the surface of the cable's insulation to remove any contaminants that might affect the test results.
3. Set Up the Insulation Resistance Tester:
Connect the test leads to the insulation resistance tester.
Check that the guard circuit or guard terminal feature is available and operational on your megohmmeter.
4. Connection Configuration:
Connect one end of the test lead to the guard terminal of the megohmmeter. The guard terminal is responsible for maintaining a constant potential around the cable during the test.
Connect the other end of the test lead to one of the cable's conductor or terminal.
Connect another test lead from the cable's conductor (the same one you connected the first test lead to) to the common (COM) terminal of the megohmmeter.
Connect the test lead from the guard terminal to the guard (G) terminal of the megohmmeter.
Verify that all connections are secure.
5. Perform the Test:
Set the insulation resistance tester to the appropriate test voltage, typically in the range of 500V to 1000V for low-voltage cables.
Initiate the test on the insulation resistance tester and allow it to perform the measurement.
6. Interpret the Results:
The megohmmeter will display the insulation resistance value in ohms (Ω).
The guard circuit technique helps minimize surface leakage currents and provides more accurate results for cables with contaminated or wet surfaces.
A higher insulation resistance value indicates better cable insulation condition.
Compare the measured insulation resistance value with the recommended minimum values for the type and rating of the cable being tested. Consult relevant standards or manufacturer's documentation for reference values.
7. Post-Test Procedures:
After the test is complete, discharge any residual voltage on the cable by short-circuiting the cable's terminals with a safe discharging tool or using the discharge function on the megohmmeter.
Clean the cable's insulation surface to remove any residue from the test.
Remember to document the test results for future reference and to monitor any trends in insulation resistance values over time. If the measured insulation resistance is significantly lower than expected or decreases over time, it may indicate a potential issue with the cable's insulation that requires further investigation and corrective action. In such cases, it is advisable to consult with a qualified electrician or engineer to diagnose and address the problem.