Motor Cable Megohmmeter: Testing Windings and Cables After a Trip

When the motor trips and the multimeter says nothing is wrong

Downtime pressure is the whole problem. A motor is tripping or running hot, production is stopped, and a standard multimeter shows continuity that looks perfectly normal, because a multimeter cannot apply the high voltage that provokes a weak insulation path. A motor cable megohmmeter stresses the insulation with 500V or 1000V DC and measures the leakage, which is what turns an intermittent, invisible fault into a number you can act on.

The goal on a call-out is a quick, defensible diagnosis: enough evidence to either return the machine to service with confidence or to condemn it without second-guessing. That means going beyond one spot reading.

A safe, methodical test after a trip

1. Isolate and prove dead. Follow safe isolation under the Electricity at Work Regulations 1989 and prove the circuit dead with a voltage indicator to HSE GS38. Most insulation testers include an AC/DC voltage function for exactly this step.
2. Discharge. After any DC insulation test, leave the leads connected so the circuit's stored charge bleeds away before you touch anything.
3. Split the circuit. Disconnect the motor from its supply cable so you can test each part on its own, rather than reading them as one.
4. Test the cable run conductor-to-conductor and conductor-to-earth.
5. Test each winding to earth at the appropriate voltage, and note the winding temperature, since resistance falls as temperature rises.

Reading the numbers without fooling yourself

The BS 7671 minimums (1.0 MΩ for circuits up to 500V, tested at 500V DC) are a floor for fixed wiring, not a health target for a motor. A healthy small motor usually reads far higher. What matters is context: the reading compared with last time, and how the resistance behaves over the length of the test. A one-second glance can miss a winding that is drying out or absorbing moisture.

That is why a timed method is worth the extra minute. The PI DAR insulation tester method shows whether resistance climbs the way sound, dry insulation should, and it is the single best way to separate simple damp from genuine degradation. For the basics of test-voltage selection, see the electrical insulation meter guide.

Making a restart decision you can stand behind

The two failure modes are equally expensive: returning an unreliable motor to service, and replacing a sound motor on weak evidence. A megohmmeter, used in sections and with PI/DAR, lets you say which winding or cable is at fault and how confident you are, so the restart-or-replace call is backed by figures, not a hunch.

The MegaOhmTes 1000V Digital Megohmmeter Insulation Resistance Tester suits this work: 500V and 1000V tests, a 0.01 MΩ to 100 GΩ range, PI/DAR, and AC/DC voltage plus frequency on one unit so the safe-isolation check and the insulation test use the same tool, at £119.41 with a 12-month UK warranty. For very long supply runs where readings climb into the gigohms, see the 100G ohm megger meter guide.

Honest limits

Insulation testing points you at the fault; it does not fix it or replace competent, safe working. Always discharge after testing, correct for temperature when comparing readings, and remember that a spot value alone is weak evidence. Large HV motors and generators need specialist procedures and, often, formal condition monitoring beyond a handheld tester.

Frequently asked questions

Why can't I just use a multimeter to check a motor?

A multimeter reads low resistance and continuity, so a damp or degrading winding can still look fine at a few ohms while failing under running voltage. A motor cable megohmmeter applies 500V or 1000V DC and measures leakage in megohms, which is what exposes moisture and insulation breakdown before the motor trips again.

How do I separate a cable fault from a winding fault?

Test in sections. With everything isolated and discharged, disconnect the motor from its supply cable and test each on its own to earth. If the cable run reads healthy but the winding is low, the fault is in the motor; if the cable is low with the motor disconnected, the supply cable is suspect.

What insulation reading means a motor is bad?

There is no single magic number: it depends on machine size, temperature and history. Treat readings near the BS 7671 floor, a sharp drop from a previous test, or a low polarisation index as reasons to investigate rather than restart. Trend and PI/DAR matter more than one spot value.