What is Trip Class?
Overload relays are rated by a trip class that defines the length of time it will take for the overload relay to trip in an overload condition. The most common trip classes are Class 10, Class 20, and Class 30. In Europe, tripping curves of overload relays are defined by IEC Standard. In North America, the NEMA Standard defines trip classes.
IEC components are typically application-rated. This means the controller is sized very close to its operational limit for a given application. IEC motors are also generally more application-rated. For these reasons, the Class 10 trip is most common on IEC applications. Because NEMA products are applied with more built-in excess capacity, the Class 20 trip is the most common.
Trip classes according to NEMA
NEMA Standard MG-1 defines 4 types of Classes. The most common classes are 5, 10, 20 & 30.
Class 5, 10, 20 & 30 overload relays will trip within 5, 10, 20 & 30 seconds respectively at 600% of motor full load amps.
A Class 10 overload relay, for example, has to trip the motor offline in 10 seconds or less at 600% of the full load amps (which is usually sufficient time for the motor to reach full speed). Many industrial loads, particularly high inertia loads, require Class 30.
Class 5 is usually used for motors requiring fast tripping.
Class 10 is commonly used to protect artificially cooled motors such as submersible pump motors of low thermal capacity.
Class 20 is usually sufficient for general-purpose applications.
Class 30 is usually required for high inertial loads to help prevent nuisance tripping.
Trip classes according to IEC
A classification of the starting typology is linked to the characteristics required by the load and by the consequent behavior of the thermal relay. Compensated thermal relays have an operating principle for which their behavior remains unchanged when the working temperature varies.
The standard establishes the tripping times corresponding to 7.2 x Ir (Ir setting current of the thermal protection), based on which the concept of trip class or starting class is introduced, as shown below.
The meaning of the different terms in this table can be better explained by referring to the following considerations.
The parameter 7.2 x Ir is the multiple of the current set on the protection relay and the multiplying factor 7.2 is fixed by the product standard.
“Ir” usually coincides with the rated current of the motor “Ie”, the value 7.2 x Ir may be considered the current that the motor absorbs during the starting phase.
The trip classes usually considered and most commonly used are 10A, 10, 20 and 30 referred to as the time “Ti” of the middle column.
It is quite common to speak of normal starting and associate it the trip classes 10A and 10, or heavy starting making reference to trip classes 20 and 30. The other trip classes and the tripping time indicated with band “E” have been recently introduced in an amendment of the Standard IEC 60947-4-1 and are characterized by a restricted trip range due to the increase of the minimum non-tripping time.
The limits fixed for “Ti”, which is the generic tripping time of the thermal protection, have the following meaning:
– The lower limit represents the minimum time under which the relay mustn’t trip in order not to interfere during starting;
– The upper limit is the time within which the relay shall surely trip. Such limit is fixed regarding the standard characteristics of the machine allowing the stator windings, or however the motor in general, to withstand the starting current and the thermal effects generated by the current for quite short times.
Using an easy numerical example, the meaning of the information reported in the table results is clearer. By assuming to have a motor for a specific application that needs a starting time of 5s, the choice of a thermal protection device classified as trip class 10A and 10 would not be correct because, from a theoretical point of view, it could trip already at 2s or 4s; therefore it is necessary to choose a relay in class 20, which up to 6s does not trip, thus allowing complete starting of the motor. The figure below shows a typical example of the protection curves of a relay for motor starting; from their analysis, it is evident the correspondence between the trip time value which “Ti” assumes and the division into the different classes 10A – 10 –20 – 30 which characterizes the relay. It is possible to see how in correspondence with 7.2 x Ir (the value established by the Standard), the relay declared in class 30 has a tripping time of about 23s (item A), thus in compliance with the indications of the table above.
The characteristics of the load to be driven by the motor, the motor type, and the starting modality are elements that influence the starting time and consequently the choice of the thermal protection device. With the only purpose of offering an indication linked to real applications, it is possible to state that ship propellers, compressors, and centrifugal pumps may be included in the category of normal starting, therefore with thermal protection class 10 or 10A, whereas – for example – centrifugal fans, mixers, and mills can be considered part of heavy starting, therefore with thermal protection class 30. It is evident how it is important to define the operating conditions to ensure a correct choice of the motor, and also of the protection device to obtain optimum working and protection conditions.