What is a Circuit Breaker?

A circuit breaker (CB) is an essential protection device used in electrical installations. It is used to prevent potential fires or other disasters as a result of faulty wiring or equipment failures. A CB detects when too much current is flowing through it and cuts the power until the problem can be resolved.

How does a Circuit Breaker Work?

A CB consists of a simple switch which makes a connection in the electrical circuit. When the load current in the circuit rises above a certain safety level, the switch in the circuit breaker opens, thus preventing any further current from flowing through.

Flush Distribution Board fitted with Miniature and Moulded Case Circuit Breakers. [Switchboard Manufacturers, JHB]

What are the Fundamental Characteristics of Circuit Breakers?

The fundamental characteristics of circuit breakers include:

  1. Rated Voltage (Ue)
  2. Rated Current (In)
  3. Tripping Current Range for Overload Protection (Ir) & Short Circuit Protection (Im)
  4. Short Circuit Current Breaking Rating (Icu or Icn)

Rated Operational Voltage (Ue)

The rated voltage of a circuit breaker is the voltage at which a circuit breaker has been designed to operate in normal (undisturbed) conditions (i.e 230V/380V/415V). The circuit breaker has also been assigned other voltage levels with which to operate during disturbed conditions.

Rated Current (In)

The rated current of a circuit breaker 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 (stated by the manufacturer) (i.e. 60A/80A/100A). An increase in ambient temperature will result in the circuit breaker being de-rated (see Hydraulic Circuit Breakers).

De-rating a circuit breaker is achieved by reducing the trip-current setting of its overload relay and marking the circuit breaker accordingly.

Overload & Short Circuit Relay Trip-current Setting (Ir & Im)

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


Performance curve of a circuit breaker thermal-magnetic protective scheme (left); Performance curve of a circuit breaker electronic protective scheme (right) [engineers-practical-knowledge]


  • Ir: Overload (thermal or long-delay) relay trip-current setting.
  • Im: Short circuit (magnetic or short-delay) relay trip-current setting.
  • Icu: Breaking capacity.
  • Ii: Short circuit instantaneous relay trip-current setting.

Rated Short Circuit Breaking Capacity (Icu or Icn)

The short circuit current-breaking rating of a circuit breaker is the highest value of current that the circuit breaker is capable of breaking, over a short period of time, without being damaged (i.e. 6kA/10kA/25kA). The current value found in the standards is the rms value of the AC component of the fault current; this is usually given in kA rms (see the meaning of kA ratings here).


These are the basic characteristics of Circuit Breakers. For more info on finding the right breaker visit our Interesting Articles Page.

Author: Terry Berman

Biomedical & Electrical Engineer at FNB.