Motor with low insulation resistance to earth
An insulation resistance test on the motor reads low to earth — a sign the winding insulation is degraded, damp, or contaminated, risking trips and failure.
Safety first
Insulation testing applies a high test voltage — follow your test procedure, isolate, discharge, and ensure no one is in contact. A motor with low insulation can cause earth-leakage trips and shock risk.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Moisture / condensation in the windings
Most likelyDamp from standing idle, washdown, or a humid environment lowers insulation resistance, often recoverable by drying.
- 2
Contamination (dust, oil, conductive debris)
#2Build-up on the windings provides leakage paths to earth.
- 3
Aged / heat-damaged insulation
#3Years of heat cycling break down the insulation permanently.
- 4
Physical damage to winding insulation
Least likelyA nick, abrasion, or burnt spot creates a direct low-resistance path to the frame.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeIsolate, disconnect the motor, and carry out an insulation resistance test winding-to-earth per your procedure.
Insulation resistance well above the minimum your procedure allows.
Insulation is adequate — the low reading may have been cabling; re-check.
Low reading confirmed — determine whether it's moisture, contamination, or damage.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Low insulation to earth
→ step 2 - 2decision
With the motor disconnected, is IR still low?
Yes→ step 3No→ step 4 - 3decision
Does it recover after drying/cleaning?
Yes→ step 5No→ step 6 - 4result
Motor is fine — the low reading was in the cabling. Re-check that.
- 5result
Moisture/contamination — restored; fix why it got in.
- 6result
Aged/damaged winding — plan repair or replacement.
Common mistakes apprentices make
- Testing with the motor still connected and blaming the motor for cable leakage.
- Writing off a motor that just needs drying out.
- Not addressing why moisture or contamination is getting in.
- Ignoring a steadily falling reading over time until it fails.
When to stop & escalate
A winding that won't recover after drying needs a motor repair/rewind assessment. Recurrent moisture/contamination points to an environmental or sealing problem to fix at source.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
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Related faults
Motor running hot / overheating
The motor runs but gets excessively hot — too hot to touch, smell of hot insulation, or thermal protection cutting in after a while.
RCD / RCBO keeps tripping
An RCD or RCBO trips repeatedly — immediately on reset, randomly during the day, or only when certain equipment runs. The earth-leakage protection is doing its job; something is leaking.
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.