Function and rated characteristics of circuit breaker

The circuit breaker is a device that ensures the control and protection on a network. It is capable of making, withstanding and interrupting operating currents as well as short-circuit currents.

The circuit breaker have to sustain and withstand the following currents- Normal current, Over load current or thermal current & Short circuit current.

So a circuit breaker must carry current in normal condition and should be capable to break current, make current under both normal and fault condition. Plus it must have withstand capability of sustaining short circuit current for minimum 1-3 sec. Short circuit current might vary 1KA (1000amp) to higher value as per design.

Compulsory rated characteristics of a circuit breaker 

      1.      Rated voltage

      2.      Rated insulation level.

      3.      Rated normal current.

      4.      Rated short-time withstand current.

      5.      Rated peak withstand current.

      6.      Rated short-circuit duration.

      7.      Rated supply voltage for opening and closing devices and auxiliary circuits   

      8.      Rated frequency

      9.      Rated short-circuit breaking current

     10.  Rated transient recovery voltage

     11.  Rated short-circuit making current

     12.  Rated operating sequence

     13.  Rated time quantities

                                                        Fig: A typical Air Circuit Breaker (ACB)

Special rated characteristics of circuit breaker

These characteristics are not compulsory but can be requested for specific applications:

1.      rated out-of-phase breaking current

2.      rated cable-charging breaking current

3.      rated line-charging breaking current

4.      rated capacitor bank breaking current

5.      rated back-to-back capacitor bank breaking current

6.      rated capacitor bank inrush making current

7.      rated small inductive breaking current

Definition - General characteristics of circuit breaker

Rated voltage of circuit breaker

The rated voltage is the maximum rms. value of the voltage that the equipment can withstand in normal service. It is always greater than the operating voltage.

Rated insulation level

The insulation level is characterized by two values- the impulse wave withstand (1.2/50 µs), the power frequency withstand voltage for 1 minute.

Rated normal current

With the circuit breaker always closed, the load current must pass through it in compliance with a maximum temperature value as a function of the materials and the type of connections. IEC sets the maximum permissible temperature rise of various materials used for an ambient air temperature of no greater than 40°C.

  

Rated short-time withstand current Isc

This is the standardized rms value of the maximum permissible short-circuit current on a network for 1 or 3 seconds.

Ssc : short-circuit power (in MVA)

U : operating voltage (in kV)

Isc : short-circuit current (in kA)

Rated peak withstand current & and making current

The making current is the maximum value that a circuit breaker is capable of making and maintaining on an installation in short-circuit. It must be greater than or equal to the rated short time withstand peak current. Isc is the maximum value of the rated short-circuit current for the circuit breakers’ rated voltage. The peak value of the short time withstand current is equal to:

2.5 • Isc for 50 Hz

2.6 • Isc for 60 Hz

2.7 • Isc for special applications.

Rated short-circuit breaking current of circuit breaker

The rated short-circuit breaking current is the highest value of current that the circuit breaker must be capable of breaking at its rated voltage.

It is characterized by two values:

1. The rms. value of rated short-circuit breaking current; 2.the percentage of the aperiodic component corresponding to the circuit breaker’s opening duration, to which we add a half-period of the rated frequency.

2. The half-period corresponds to the minimum activation time of an over current protection device, this being 10 ms at 50 Hz.

Rated Transient Recovery Voltage (TRV) of circuit breaker

This is the voltage that appears across the terminals of a circuit breaker pole after the current has been interrupted. The recovery voltage wave form varies according to the real circuit configuration. A circuit breaker must be able to break a given current for all recovery voltages whose value remains less than the rated TRV.

Rated out-of-phase breaking current of circuit breaker

When a circuit breaker is open and the conductors are not synchronous, the voltage across the terminals can increase up the sum of voltages in the conductors (phase opposition). In practice, standards require the circuit breaker to break a current equal to 25% of the fault current across the terminals, at a voltage equal to twice the voltage relative to earth.