CSLB C-20 HVAC ContractorControls, Electrical and SafetyMedium
A technician uses a megohmmeter to test the insulation resistance of a compressor motor winding to ground and reads 0.3 megohms. According to common industry standards, what does this reading indicate?
- AThe motor is properly grounded and safe to energize
- BThe winding insulation is in acceptable condition for continued service
- CThe reading indicates a normal capacitor discharge value
- DThe motor should be shut down; insulation has degraded and poses a ground-fault/shock hazard
Show answer & explanationAnswer & explanation
Correct answer: D. The motor should be shut down; insulation has degraded and poses a ground-fault/shock hazard
Industry standards generally consider insulation resistance readings below 1 megohm (often 2 megohms minimum for larger motors) unacceptable, indicating degraded winding insulation that could allow current to leak to the motor casing, creating a shock or ground-fault hazard. A 0.3 megohm reading is well below this threshold and calls for motor replacement or rewinding.
Why the other options are wrong
- A. A low insulation resistance reading indicates a potential fault path to ground, not proper grounding.
- B. 0.3 megohm is far below the accepted minimum (typically 1-2 megohms), so it is not acceptable.
- C. Megohmmeter readings measure insulation resistance, not capacitor discharge values.
Megohmmeter Insulation Test
A megohmmeter (megger) measures insulation resistance between motor windings and ground; readings below 1 megohm typically indicate insulation breakdown and unsafe operating conditions.
- Minimum acceptable: usually 1-2 megohms depending on motor size/voltage
- Low readings indicate moisture intrusion or winding insulation failure
- Test performed with motor de-energized and disconnected
Memory trick: Low megohms = leaky insulation = 'mega' problem.