Motor overload relays — trip class 10A/10/20/30, and why the wrong class causes nuisance trips or motor damage

The motor starting methods guide covers how the inrush current of a three-phase motor differs by starting method (direct-on-line, star-delta, soft starter, VFD). This article covers an additional, independent aspect: how long a motor overload relay may allow an elevated current before tripping — defined by the trip class of IEC 60947-4-1.

What the trip class actually tests

IEC 60947-4-1 defines the trip class through one standardised test: a cold relay (not recently loaded, at ambient temperature) is loaded with 7.2x the set current (Ie), and the time to trip is measured. The standard classes are:

Class 10A: trips within 2-10 s.
Class 10: trips within 4-10 s.
Class 20: trips within 6-20 s.
Class 30: trips within 9-30 s.

The higher the class number, the longer the relay may allow an elevated current to persist before tripping — intended so that a motor with a long, legitimate starting time is not tripped needlessly on every start.

Note: Class 10A is not simply "a shorter version of Class 10" in name only — its lower bound is faster (2s instead of 4s), specifically intended for applications where a hot restart (a motor started again shortly after stopping, so already partly warmed up) requires faster protection than standard Class 10/20 relays provide.

Why the class choice differs per application

Small, fast-starting loads (small fans, pumps without significant inertia) typically have a starting time of only a few seconds and are usually protected with Class 10 or 10A — a long trip time here would only unnecessarily let a genuine overload persist longer.
Loads with high inertia (large fans, centrifuges, conveyors, crushers) have a starting time that can exceed Class 10's 10-second limit — with such a motor, a Class 10 relay trips on every normal start, which in practice is "solved" by raising the trip class to Class 20 or 30, so the legitimate starting current is no longer mistaken for an overload.

The two risks of choosing the wrong class

Class too low for the actual starting time: the relay trips repeatedly during a normal, legitimate start — nuisance tripping, often "solved" by raising the class without assessing the underlying cause (starting time exceeding what the class allows).
Class too high for a motor with a short, normal starting time: the relay allows a genuine overload (for example a locked rotor or a mechanical problem in the driven load) to persist far longer than necessary, increasing the risk of thermal damage to the motor windings — the trip class then protects the motor less quickly than the correct, lower class would have.

Distinction from circuit-breaker settings (LSI)

The circuit-breaker LSI settings guide covers the Ir/Isd/Ii/Ig settings of a circuit breaker, which protect the cable and circuit against overload and short circuit according to a generic curve. A motor overload relay with a trip class specifically protects the motor winding according to its own thermal characteristic, and is an additional, separate protection on top of (not instead of) circuit protection.

Practical relevance

When selecting or checking a motor overload relay, the trip class must be matched to the actual starting time of the driven load (found on the motor nameplate, in the machine builder's specification data, or via a starting-current measurement) — not to the factory default or to "whatever hasn't tripped so far".

Common mistakes

Raising the trip class as soon as the relay trips during starting, without checking whether the actual starting time of the load is the cause — this can mask a symptom instead of resolving it (for example a mechanical problem unnecessarily extending the starting time).
Applying Class 20 or 30 by default "to be safe", even for motors with a short, normal starting time — this unnecessarily delays protection during a genuine overload.
Confusing trip class with the Ir/tr setting of a circuit breaker — both protect against overload, but the trip class is specifically matched to the thermal characteristic of the motor winding, not to a generic circuit curve.