Insulation resistance testing on motors

It's the start of nights. The extract fan motor on line 2 has tripped twice. The shift manager wants the spare fitted. The spare is in the stores, still boxed, and it took months to come.

Before anyone undoes a gland, get the insulation tester out. Ten minutes of testing tells you where the fault lives. It could be the windings. It could be the cable. It could be neither.

Swap the motor on a hunch and you might fit your only spare to a cable that's chafed through on the tray. Then it trips again. Now you've got no spare and the same fault.

A motor swap without a test is a guess. An expensive one.

Dead first, then test

An insulation test is a dead test. Isolate at the starter or drive, fit your lock, and prove dead at the motor terminals. HSE's guidance on the Electricity at Work Regulations says the tester you prove with should itself be proved, "preferably immediately before and immediately after" you test the conductors (HSR25, paragraph 196). If your team is hazy on the full routine, start with safe isolation, lock off and prove dead.

Check the leads too. GS38 wants probes with finger barriers and an exposed tip no more than 4mm* across (GS38). It also lists using kit rated below the installation's CAT rating as a cause of accidents. A cheap meter with cracked leads has no place in a motor control centre.

The regulations don't stop you putting a test voltage on kit, as long as that doesn't cause danger (HSR25, paragraph 197). Remember what that means. While you hold the button, the cable is charged. If someone's at the terminal box while you test from the starter, they're touching your test voltage. Tell them before you press.

Get the drive out of the circuit

If the motor runs off an inverter, disconnect the motor cable at the drive output before you test. An insulation tester pushes a high DC voltage down the cable. Do that through a drive and you can kill the output stage. Then you've got two faults.

The same goes for soft starters, filters and anything with electronics in it. Thermistor leads in the terminal box need lifting too. Use the test voltage the motor maker gives. If the plate and the manual don't say, ask before you guess.

Split it at the terminal box

Start at the starter or the drive end. Test each phase to earth with the motor and cable together. A good reading here and the fault is probably somewhere else. Go back to the control side, or look at why the motor trips on overload.

A low reading means split it. Open the terminal box, lift the cable off the motor, and test the two halves on their own. Cable first, then motor. This is half-split fault finding in its simplest form, and it saves a lot of guessing.

On the motor, test each phase to earth. Take the star or delta links off and test phase to phase as well. That shows you a winding that's broken down to its neighbour, which a phase to earth test can miss.

Reading the number

The number on its own tells you less than people think. Write down the winding temperature next to it. A motor that's sat cold in a damp corner all weekend can read low, then come good once it's warm and dry. One that's run hot all shift will read lower than it does cold.

Hold the test and watch the reading. On good insulation it climbs as the winding charges, then settles. A reading that drops away while you hold it is worth a closer look. Some engineers take a longer timed reading and compare it with the one-minute figure. It's a good habit on big motors.

We won't give you a pass mark here. The right figure depends on the motor's voltage, size and age, and on the maker's data. Take it from the maker or your own site standard, and use the same one every time.

The real value is the trend. Test your important motors at PPM and log it. A reading that falls a bit every year is a motor telling you it's on the way out. That's condition monitoring on a small budget, using a tester you already own.

Discharge it before you touch it

A long cable and a big winding store charge. Most modern testers discharge the circuit at the end of the test, as long as you keep the leads on until they finish. Pull the clips early and that charge stays where it is.

HSE's guidance says stored electrical energy may need guarding against, for example with earthing connections for the time the work takes (HSR25, paragraph 195). The law also says you must take adequate precautions so equipment made dead doesn't become charged while you work on it (regulation 13). A test you've just done counts.

When the reading's low

Low in the cable? Look where it's been disturbed. Cable trays, glands, the bit that flexes when the motor moves on its base. Water in a trunking low point.

Low in the motor? Work out if it's damp or burnt. Smell the terminal box. Check the drain holes aren't blocked. Look for water in the box itself. A damp motor can often be dried out and saved. A burnt one goes to a rewinder or the skip, and before you order a new one, read find out why first. If washdown water got in once, it'll get into the new one.

Who's on shift with the tester

The test takes ten minutes if someone on shift owns a tester, knows how to take a drive out of circuit, and knows what the reading means. The regulations expect anyone doing this work to have the knowledge or experience to prevent danger, or proper supervision (regulation 16).

If nobody on nights has those skills, the choice at 2am is grim. Fit the spare and hope, or leave the line stood until days. Both cost money. One burns a spare that took months to arrive. The other burns hours of output. Put your cost of an hour of downtime next to the price of having an electrician on call, and the sums are usually clear.

What to do on Monday

1. Check each shift has an insulation tester with GS38-style leads, and someone trained to use it.
2. Take a reading, with the temperature, on your ten most important motors at the next PPM. Log it.
3. Label every motor that runs off a drive, so nobody tests through an inverter.
4. Add "test before you swap" to your motor-trip routine, before anyone orders a spare.

* GS38 probe tip figure correct as of 14 September 2026. Check GS38 on HSE's website for the latest. Paragraph numbers refer to HSR25, third edition (2015).