Inverter trips: overcurrent, overvoltage and earth fault

It's 4am. The induct belt on the sorter has stopped. The drive display shows an overcurrent trip. Someone resets it, the belt runs for ten minutes and trips again. Reset. Trip. By the third go someone is on the phone asking whether there's a spare drive in stores.

There probably is. It probably won't help.

Most drive trips are the drive telling you about something else. The motor, the cable, the load, the ramp times or the supply. The drive is the messenger. Change it and the new one sends the same message.

Read the trip before you reset it

Before anyone touches reset, write down the trip code. Then look at the drive's fault log if it has one. Many drives store the last few trips with the output current, DC bus voltage and frequency at the moment they tripped. That's your best evidence, and some drives lose it on a power cycle.

Take a photo of the display and the log. Then ask the operator what the machine was doing. Starting? Running steady? Slowing down? Stopping?

If the code means nothing to you and the manual's gone, our guide to reading drive fault codes without the manual will help.

Overcurrent: when did it go?

Overcurrent means the output current went over the drive's limit. The timing is the clue.

• Straight away, the instant it starts? Suspect a short. That could be phase to phase, phase to earth, in the motor or in the cable. A failed output stage in the drive can do the same.
• While speeding up? The acceleration ramp may be too short for the load. Or the load is stuck: a jam, a seized bearing, a motor brake that hasn't released.
• At steady speed? Something has loaded the motor suddenly. A jam, a snatch, a product pile-up. Look at the mechanics.

For the "straight away" case, don't keep resetting. Every reset into a short puts full fault current through the drive's output. Isolate, then test the motor and cable as below.

For the ramp case, check whether anyone's changed the parameters. A ramp that's been shortened to get more speed out of the line will trip on a heavy load. Note the old value before you change anything.

Overvoltage: look at the stop

Overvoltage on a drive almost always means the DC bus voltage has gone too high. The usual cause is the motor feeding energy back into the drive while it slows down. A heavy flywheel, a big fan, a loaded conveyor on a decline or a hoist lowering will all do it.

So the question is, when does it trip? If it's on the stop or the slow-down, look at:

1. The deceleration ramp. Too short, and the motor turns into a generator faster than the drive can cope.
2. The braking resistor, if there is one. Isolate and check its resistance against the rating on it. Look for burn marks or a cracked element.
3. The braking chopper and its wiring. A loose connection to the resistor means the drive can't dump the energy.
4. The load. Has anything changed that adds inertia or makes the load run on?

If it trips on overvoltage with the motor stopped, look at the incoming supply. A high supply voltage or spikes on the supply can push the bus up. That's one for a power quality check.

Earth fault: start with the motor and cable

An earth fault trip means the drive has seen current leaking to earth. On a breakdown the usual suspects are the motor windings and the motor cable.

Look for the obvious first. Water in the terminal box after a washdown. A cable crushed by a forklift or rubbed through on a frame. A gland that's let go. Motors in wet areas and on washdown lines are the usual ones.

Then test. Our guide to insulation resistance testing on motors covers the readings. One rule matters more than any other here. Disconnect the motor cable from the drive before you put an insulation tester on it. Test voltage into the drive's output terminals can damage the output stage. Test the cable and motor together from the drive end, then split them if the reading is low.

If the motor and cable test clean, look at the cable run. Long screened cables have more leakage, and some drives are set close to the limit. Check what's changed. A new longer cable, an added filter, a motor moved further away.

The drive can bite after it's off

This is where drives catch people out. Isolating the supply doesn't make a drive safe straight away. The DC bus capacitors hold their charge after the power is off.

HSE's guidance on safe working practices says that on equipment that can store charge, such as capacitors, any stored charge should be safely discharged before work starts (HSG85, paragraph 48)*. The guidance on the Electricity at Work Regulations makes the same point. Isolation alone may not prevent charging, because energy may be stored in capacitors within equipment (HSR25, paragraph 195).

So isolate, lock off, then wait. Most drives have a warning label with the wait time on the front. Respect it. Then measure the DC bus with a suitable voltage detector before you touch anything. Our guide to safe isolation has the full sequence.

A drive that's stopped or disabled on its control input is still connected to the supply. The PUWER code of practice says emergency stops and interlocked guards are not suitable as isolation (L22, paragraph 195). The same logic applies to a drive's stop input.

Heat and dirt make every trip worse

Drives hate heat. A drive in a hot, dusty panel with its fan clogged will trip more on everything, and fail sooner. If several drives in the same panel are tripping, look at the panel before the drives. Filters, fans, air conditioning and the amount of fluff on the heatsinks.

We cover this in electrical panel faults: heat, dust and loose terminals. In summer, read hot weather and breakdowns too.

The cost of the wrong fix

The sorter at 4am is a common story. Someone fits a new drive, it trips the same way, and now you've lost an hour and spent a drive. Then they find a cracked braking resistor or a wet motor terminal box.

Drive faults need someone who knows drives. A fitter who can swap one is not the same as an engineer who can read the fault log, check the ramps and find the leak to earth. On a site where one drive can stop the whole sorter, that skill is worth having on call. Every hour spent guessing is an hour of missed trailers.

Here's our view. Nobody swaps a drive until the motor and cable have been tested and the trip log has been read. And nobody changes a ramp or a current limit without writing down the old value.

What to do on Monday

1. Back up the parameters on your critical drives, and keep a copy off the drive.
2. Put a note in every drive panel: photo the trip code and fault log before reset.
3. Check the braking resistors on your high-inertia loads for damage and correct resistance.
4. Check the panel fans and filters on any panel with more than one drive in it.

* Paragraph numbers correct as of 7 September 2026 and refer to HSG85, third edition (2013), HSR25, third edition (2015), and L22, fourth edition (2014, amended 2018). Check HSE's HSG85 page for the latest.