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The fault code on a drive tells you why the drive protected itself, not where the problem is. Resetting an over-current trip and restarting usually brings the same fault back within minutes. Below are the eight codes we meet most often, the typical causes behind each, and how we tell them apart on site.
| Code (common) | Meaning | Where to look first |
|---|---|---|
| OC / OCx | Over-current | Acceleration time, motor cable, mechanical jam |
| OV / OU | DC bus over-voltage | Deceleration time, braking resistor |
| LV / UV | Under-voltage | Supply, loose terminal, input fuse |
| OH / OHx | Over-temperature | Fan, filter, panel climate control |
| OL / OLx | Overload | Motor current parameter, mechanical load |
| SC | Output short circuit | Motor cable, motor winding |
| EF / GF | Earth fault | Cable insulation, motor insulation |
| PHL | Phase loss | Input fuse, contactor, terminal |
OC — over-current: the most misdiagnosed code
An over-current fault appears at three different moments, each pointing to a different cause. During acceleration it means the ramp time is short for the load's inertia — lengthening it usually solves it. At constant speed it points to a mechanical bind, a bearing problem or an increase in load. During deceleration it means the ramp-down time is too short.
There is a fourth possibility: motor cable length. On long cables the cable capacitance makes current ripple, and the drive can read that as over-current. Where the distance between drive and motor reaches tens of metres, an output reactor (du/dt filter) is needed.
OV — DC bus over-voltage: the load is feeding the drive
This fault almost always appears during deceleration. The reason: as the motor slows, the inertial energy flows back into the drive and pushes up the DC bus. When bus voltage exceeds the threshold, the drive protects itself. It is common on high-inertia loads such as hoists, conveyors, centrifuges and fans.
- Lengthen the deceleration ramp — the simplest fix and usually enough.
- Add a braking resistor — the route that turns the energy into heat when the ramp cannot be lengthened.
- A braking unit or regenerative drive — for applications that brake continuously.
OH — over-temperature: the panel is at fault, not the drive
This is the most frequent summer call-out. The drive measures its own heatsink temperature; past the threshold it first derates and then trips. The cause is usually not the drive but the panel around it: a blocked filter, a fan running backwards, no clearance left between drives, or a panel sitting in the sun.
LV — under-voltage: usually a loose terminal
If the supply really is low, the whole plant shows symptoms. When only one drive reports LV, the cause is almost always local to it: a loosened input terminal, an aged fuse, an oxidised contactor contact. Measure while the drive is under load; at no load the voltage can look normal.
OL — overload: parameter or real load?
If the motor's rated current parameter was not entered at commissioning, or entered wrongly, the drive reads a normal load as excessive. The first check is to compare the motor nameplate with the parameter. If the parameter is right, the load really has increased: material building up on a conveyor, thickening in a mixer, a blockage in a pump.
EF — earth fault: do not intervene before measuring
An earth fault points to insulation breakdown in the cable or the motor winding. Resetting this code and carrying on is dangerous. The correct route is to isolate the drive and megger the motor and cable. Never measure with the drive connected — the megger voltage damages the drive's output stage.
If the fault repeats, a parameter backup is essential
A parameter backup taken at commissioning turns every later fault from hours into minutes. Without one, replacing a drive means entering every setting from scratch, with no record of which values were corrected on site. On every drive project we put the parameter backup in the handover file.
Frequently asked questions
Is it harmful to reset a drive fault and restart?
Resetting after a one-off supply disturbance is normal. But if the same code repeats, resetting hides the problem and wears the equipment. With earth fault (EF) and output short circuit (SC) in particular, restarting without measuring damages both the drive and the motor.
Does every drive need a braking resistor?
No. If the load's inertia is low and the ramp can be lengthened, it is not needed. A braking resistor is required on loads that feed energy back: hoists, lifts, centrifuges, large fans and conveyors that must stop quickly. Adding one unnecessarily creates a significant heat load inside the panel.
How long can the cable between drive and motor be?
The limit depends on the drive model and cable type; manuals give separate figures for shielded and unshielded. Long runs bring two problems: current ripple from cable capacitance, and voltage overshoot at the motor terminals (du/dt). Both are controlled with an output reactor. Where the distance reaches tens of metres, plan for the reactor from the start.
Why should the motor cable be shielded?
A drive output produces high-frequency switching that couples noise into neighbouring signal cables. Shielded cable drains that noise to earth. The shield must be bonded 360 degrees at both ends — a pigtail at one end does almost nothing. Sensor and encoder cables should also run on a separate route from the motor cable.
Who should back up the drive parameters?
Whoever commissions it. The backup can be taken through the operator panel, with the manufacturer's software, or at minimum as screenshots. Without one, replacing a drive means repeating the entire original commissioning. On the projects we run, the backup is handed over together with the test record.