CSLB C-10 Electrical ContractorSafety and TestingHard
A technician is performing an insulation resistance test on a newly installed 480V motor feeder circuit using a megohmmeter (Megger). The test results show an insulation resistance of 2 megohms. According to industry best practices and common standards, what does this reading indicate for a 480V system?
- APoor insulation, indicating a fault and requiring immediate repair.
- BExcellent insulation, indicating a new and healthy circuit.
- CMarginal insulation, requiring further investigation or corrective action.
- DAcceptable insulation, but warrants monitoring for degradation.
Show answer & explanationAnswer & explanation
Correct answer: C. Marginal insulation, requiring further investigation or corrective action.
For a 480V system, a common rule of thumb (e.g., 1 megohm per 1000V plus 1 megohm) suggests a minimum of about 1.5-2 megohms. A reading of 2 megohms is at the lower end of acceptable for new installations and is often considered marginal, especially for 480V, warranting further investigation or retesting. It's not excellent, but not yet 'poor' to the point of immediate fault.
Why the other options are wrong
- A. Poor insulation would typically be much lower, often below 1 megohm, potentially in the kilohm range, indicating a clear fault.
- B. Excellent insulation for a new 480V system would typically be much higher, often in the tens or hundreds of megohms.
- D. While acceptable for some older systems, for a new 480V circuit, 2 megohms is borderline and indicates potential issues or future problems.
Insulation Resistance Interpretation
Evaluating megohmmeter readings to determine the health and integrity of electrical insulation, typically using industry rules of thumb or specific standards.
- Higher resistance is better
- Readings depend on voltage and equipment type
- Trends over time are important
Memory trick: Resistance Rule: 1 Megohm per 1000V + 1 MΩ is a good start!