GOOD CABLE INSULATION
When voltage is impressed across any insulation system, some current leaks into, through, and around the insulation. When testing with dc highvoltage, capacitive charging current, insulation absorption current, insulation leakage current, and by-pass current are all present to some degree. For the purposes of this document on cable fault locating, only leakage current through the insulation will be considered.
For shielded cable, insulation is used to limit current leakage between the phase conductor and ground or between two conductors of differing potential. As long as the leakage current does not exceed a specific design limit, the cable is judged good and is able to deliver electrical energy to a load efficiently.
Cable insulation may be considered good when leakage current is negligible but since there is no perfect insulator even good insulation allows some small amount of leakage current measured in microamperes. See Figure 1.
The electrical equivalent circuit of a good run of cable is shown in Figure 2. If the insulation were perfect, the parallel resistance RP would not exist and the insulation would appear as strictly capacitance. Since no insulation is perfect, the parallel or insulation resistance exists. This is the resistance measured during a test using a Megger® Insulation Tester. Current flowing through this resistance is measured when performing a dc hipot test as shown in Figure 1. The combined inductance (L),series resistance (RS), capacitance (C) and parallel resistance (RP) as shown in Figure 2 is defined as the characteristic impedance (Z0) of the cable.
WHEN CABLE INSULATION IS BAD
When the magnitude of the leakage current
exceeds the design limit, the cable will no longer deliver energy efficiently. See Figure 3.
Why A Cable Becomes Bad
All insulation deteriorates naturally with age,
especially when exposed to elevated temperature due to high loading and even when it is not physically damaged. In this case, there is a distributed flow of leakage current during a test or while energized. Many substances such as water, oil and
chemicals can contaminate and shorten the life of insulation and cause serious problems. Cross-linked polyethylene (XLPE) insulation is subject to a condition termed treeing. It has been found that the presence of moisture containing contaminants, irregular surfaces or protrusions into the insulation plus electrical stress provides the proper environment for inception and growth of these trees
within the polyethylene material. Testing indicates that the ac breakdown strength of these treed cables is dramatically reduced. Damage caused by lightning, fire, or overheating may require replacement of the cable to restore service
Fault Finding Solutions: Megger
Megger is a globally recognized brand that manufactures electrical test and measurement equipment, commonly used in diagnosing faults in electrical systems. Megger instruments are widely used for fault finding, troubleshooting, and maintenance of power systems, electrical installations, and components. Below are key aspects of fault finding solutions using Megger equipment:
1. Insulation Testing
- Purpose: To identify faults in cable insulation, electrical motors, and transformers that may lead to short circuits or breakdowns.
- Process:
- A Megger insulation tester applies a high voltage (typically 500V, 1000V, or more) to measure the resistance between conductors and the ground.
- Low resistance readings indicate potential insulation faults or degradation.
- Applications:
- Cables, switchgear, transformers, and motors.
- Periodic maintenance checks to prevent unexpected failures.
2. Continuity Testing
- Purpose: To detect open circuits or high-resistance connections in wiring systems.
- Process:
- A Megger continuity tester applies a low-voltage DC current to the circuit and measures resistance.
- High resistance indicates poor connections or broken circuits.
- Applications: Wiring systems, ground conductors, and small electrical devices.
3. Earth/Ground Resistance Testing
- Purpose: To ensure the effectiveness of the earthing system in dissipating fault currents safely into the ground.
- Process:
- Megger ground testers use methods like the fall-of-potential method or clamp-on testing to measure ground resistance.
- Excessively high resistance suggests inadequate grounding, which could compromise safety.
- Applications: Substations, industrial plants, and residential buildings.
4. Cable Fault Location
- Purpose: To pinpoint the exact location of faults in underground or overhead cables.
- Techniques:
- Time Domain Reflectometry (TDR): Sends a signal through the cable and analyzes reflections caused by faults or impedance changes.
- Bridge Methods: Used to locate resistance-related faults in cables.
- Applications: Utilities, telecommunications, and industrial systems.
5. Circuit Breaker Testing
- Purpose: To verify the performance of circuit breakers, ensuring they operate correctly during faults.
- Process:
- A Megger circuit breaker analyzer checks the breaker’s timing, resistance, and tripping mechanism.
- Faulty operation may indicate mechanical wear or electrical issues.
- Applications: Power stations, substations, and industrial power systems.
6. Transformer Testing
- Purpose: To diagnose issues in transformer windings, core, and insulation.
- Key Tests:
- Turns Ratio Test (TTR): Measures the turns ratio between primary and secondary windings to detect faults.
- Winding Resistance Test: Identifies open circuits or high resistance in windings.
- Dielectric Tests: Assess the condition of insulation systems.
- Applications: Distribution and power transformers.
7. Motor and Generator Testing
- Purpose: To identify winding and insulation faults in rotating machines.
- Process:
- Insulation Resistance Test: Checks for degradation in insulation.
- Polarization Index (PI): Evaluates insulation quality over time.
- Surge Testing: Detects turn-to-turn winding faults.
- Applications: Electric motors, alternators, and generators.
8. Low Resistance Testing
- Purpose: To measure the resistance of joints, connections, and low-resistance components.
- Process:
- A Megger low-resistance ohmmeter (DLRO) uses a high current to measure resistance accurately.
- Abnormal readings suggest loose connections, corrosion, or defective components.
- Applications: Switchgear, busbars, and welded joints.
9. Leakage Current Testing
- Purpose: To detect unintended current leakage in circuits that may lead to safety hazards or energy loss.
- Process:
- A Megger leakage current tester measures small currents leaking to the ground.
- High leakage current indicates insulation failure or defective equipment.
- Applications: Electrical appliances, control panels, and industrial equipment.
10. Portable Appliance Testing (PAT)
- Purpose: To ensure the safety of portable appliances by checking for electrical faults.
- Process:
- Megger PAT testers perform insulation, leakage current, and functional tests.
- Defective appliances are identified and flagged for repair or replacement.
- Applications: Offices, hospitals, schools, and other facilities.
Advantages of Using Megger Solutions
- Accuracy: High precision in fault diagnosis.
- Versatility: Covers a wide range of electrical systems and components.
- Ease of Use: User-friendly interfaces and portable designs.
- Reliability: Robust equipment suitable for challenging environments.
- Safety: Enables fault detection before catastrophic failures.
Conclusion
Megger instruments are indispensable tools for electrical engineers, technicians, and maintenance personnel. They ensure system reliability, improve operational efficiency, and enhance safety by enabling early fault detection and resolution in electrical systems.
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