Fundamental Characteristics of Circuit Breaker.

Fundamental characteristics of a circuit breaker are:

1. Its rated voltage Ue
2. Its rated current In
3. Tripping current level adjustment ranges for overload protection (Ir or Irth) and for short circuit protection (Im)
4. Its short circuit current breaking rating (Icu for industrial CBs; Icn for domestictype CBs).

Rated operational voltage (Ue)

This is the voltage at which the circuit breaker has been designed to operate.

Rated current (In)

This is the maximum value of current that a circuit breaker, fitted with a specified over current tripping relay, can carry indefinitely at an ambient temperature .If ambient temperature increase circuit breaker will be derated.

Overload & Short circuit relay trip-current setting (Ir & Im)

Short circuit tripping relays (instantaneous or slightly time-delayed) are intended to trip the circuit breaker rapidly on the occurrence of high values of fault current.

Figure (left) - Performance curve of a circuit breaker thermal-magnetic protective scheme; Figure (right) - Performance curve of a circuit breaker electronic protective scheme

Where

• Ir: Overload (thermal or long-delay) relay trip-current setting

• Im: Short circuit (magnetic or short-delay) relay trip-current setting

• Ii: Short circuit instantaneous relay trip-current setting.

• Icu: Breaking capacity

Rated short circuit breaking capacity (Icu or Icn)

The short circuit current-breaking rating of a CB is the highest value of current that the CB is capable of breaking without being damaged. The value of current quoted in the standards is the rms value of the AC component of the fault current, normally given in kA rms