IEC 60898-1 applies to a.c. air-break circuit-breakers for operation at 50 Hz or 60 Hz, having a rated voltage not exceeding 440 V (between phases), a rated current not exceeding 125 A and a rated short-circuit capacity not exceeding 25 000 A.
Circuit breakers are manufactured in standard sizes, using a system of preferred numbers to cover a range of ratings. Miniature circuit breakers have a fixed trip setting; changing the operating current value requires changing the whole circuit breaker. Larger circuit breakers can have adjustable trip settings, allowing standardized elements to be applied but with a setting intended to improve protection.
Preferred Ratings
For low voltage distribution circuit breakers, International Standards, IEC 60898-1 defines the rated current as the maximum current that the breaker is designed to carry continuously. The commonly available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A,and 125 A however, manufacturer have provide current ratings of 1 A, 2 A, and 4 A.
Instantaneous Tripping Currents
The circuit breaker is labeled with the rated current in amperes prefixed by a letter, which indicates the instantaneous tripping current that causes the circuit breaker to trip without intentional time delay expressed in multiples of the rated current:
| Type | Instantaneous tripping current | Notes |
|---|---|---|
| B | 3-5 times rated current In | For example a 10 A device will trip at 30–50 A. These MCBs are usually used for lighting and smal power loads |
| C | 5 to 10 times In | These MCBs are typically used for any types of loads except for motors. |
| D | 10-20 times In | These MCBs are usually used for motors and primary protection of transformers due to their in-rush currents. |
| K | 8 to 12 times In |
For the protection of loads that cause frequent short duration (approximately 400 ms to 2 s) current peaks in normal operation. This type of circuit breaker is not covered in IEC 60898-1. |
| Z | 2 to 3 times In for periods in the order of tens of seconds. | For the protection of loads such as semiconductor devices or measuring circuits using current transformers. This type of circuit breaker is not covered in IEC 60898-1. |
