Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A technician is performing insulation resistance testing on a new 600V motor feeder. According to generally accepted industry practices and standards like the NETA MTS, what is the minimum acceptable insulation resistance value for this feeder?
- A100.0 Megohms
- B1.0 Megohm
- C10.0 Megohms
- D0.5 Megohms
Show answer & explanationAnswer & explanation
Correct answer: B. 1.0 Megohm
For circuits rated 600V or less, a common industry standard for minimum acceptable insulation resistance is 1.0 Megohm. This is often referenced in NETA (International Electrical Testing Association) standards.
Why the other options are wrong
- A. Excessively high for a minimum standard; this level indicates excellent insulation but is not the minimum requirement.
- C. While higher is better, 10.0 Megohms is not the minimum standard; it might be a target for excellent insulation.
- D. Too low for a 600V circuit; 0.5 Megohms is typically for lower voltage systems or as a very minimum threshold.
Minimum Insulation Resistance
The lowest acceptable resistance value of an electrical insulator, measured in Megohms, to ensure safe and reliable operation of equipment and wiring.
- Varies with voltage rating of the equipment.
- Often specified by standards like NETA or manufacturer guidelines.
- Typically 1.0 Megohm for systems up to 600V.
Memory trick: MEASURE the MEGOHMS for MOTOR safety.