These relays protect motors against overload but must sustain the temporary overload of starting and only trip when starting lasts too long.

Hence the necessity for relays adapted to the starting time. To meet this need, the IEC 60947-4-1 standard has several categories of overload relay each defined by its tripping time:

The relay size should be chosen on the basis of the motor’s rated current and the estimated starting time.
Limits of use are characterized by curves based on the time and value of the current setting (in multiples of Ir) as shown below: