Overload relay trip class 10, 20 or 30?

An overload relay is set to the motor's full-load current and then asked to ignore the starting current. How long it ignores it is the trip class, and it is the one setting on the relay that people leave at the factory default.

The default is usually Class 10. For a pump or a fan that is correct. For a screw compressor or a loaded conveyor it is the reason the relay trips every morning.

What the class number means

IEC 60947-4-1 defines the class by the trip time at 7.2 times the set current, starting from cold:

The number is not a delay timer. It describes the shape of the thermal curve: all classes trip at the same current eventually, they differ in how much starting time they tolerate before deciding it is a fault.

Choosing from the starting time

The rule that works in practice: the trip class must be longer than the starting time, with margin, and as short as possible otherwise.

1. Read the motor's starting time from the machine builder, or measure it - a clamp meter with a min/max function on the starting current gives you the duration directly.
2. Add 50-100 % margin. A motor that accelerates in 4 s gets Class 20, not Class 10.
3. Check what the relay can physically do. Thermal relays are usually fixed at one class or have a small selector; electronic relays often cover 10, 20 and 30 in one device.

Margin matters because starting time is not constant. A conveyor that starts in 3 s empty takes 8 s when it is fully loaded, and the relay only sees the current, not the reason.

Why not always use Class 30

Because the class also decides how long the motor runs while overloaded. A Class 30 relay will let a blocked conveyor draw 3 x Ie for half a minute before it trips. On a motor with a short thermal margin, that is enough to damage the winding before the relay acts.

The protection is coordinated with the motor's thermal limit: the relay's I²t curve must sit below the motor's damage curve for the whole range. Moving to a longer class moves the relay curve up, and on a small motor with little thermal mass it can cross the damage curve. That is what "the relay was set too high" looks like after a burnt winding.

So: choose the shortest class that survives a normal start.

Setting the current

The relay dial is set to the motor's rated current at the actual supply voltage, from the nameplate - not to the contactor rating and not to the cable size.

Two corrections are often needed:

• Ambient temperature. Thermal relays are calibrated at 20 °C (some at 40 °C). In a cabinet at 55 °C the bimetal starts hotter and trips early; most manufacturers publish a derating chart, and electronic relays compensate internally.
• Ganged mounting. Relays mounted side by side heat each other. A row of five is not five independent relays.

Electronic relays with a current transformer are less sensitive to both, which is one reason they appear on anything that matters.

Thermal versus electronic

For a single pump in a clean cabinet, a thermal relay is the right answer. For a machine where a blocked load must be distinguished from a start, the electronic version pays for itself the first time it avoids a burnt motor.

What the relay does not do

An overload relay protects against overload - sustained current above the set value. It does not protect against:

• Short circuit. That is the job of the motor circuit breaker or fuses upstream, and the two devices have to be coordinated so the breaker clears a fault without the relay being destroyed.
• Stalled rotor at start. A motor that never accelerates draws locked-rotor current indefinitely; a Class 10 relay will trip, but not before the winding has taken the heat.
• Phase imbalance on a thermal relay. It responds to the highest phase current, so a 20 % imbalance may trip late.

For a motor starter assembly, the standard combination is a manual motor protective breaker, a contactor, and an overload relay, matched by the manufacturer's coordination table. The table exists because the three devices have to be tested together - mixing brands in that stack is where selective protection quietly stops working.

The five-minute field check

• Relay class matches the measured starting time with margin.
• Dial set to nameplate current, adjusted for cabinet temperature.
• Trip class selector actually moved off the default, if the load needs it.
• Reset set to manual on anything a remote start could re-energise unexpectedly.
• Auxiliary contact wired to the PLC, so a trip is visible instead of being a mystery stoppage.

The last one is the difference between a two-minute reset and an hour of fault finding.