Circuit Breaker Ratings and Specifications - Making & Breaking Capacity

Importance of Ratings and Specifications of Circuit Breaker :

Rating and specifications play a very important role in the operation and control of a circuit breaker. It gives an idea of the type of material used and the quality of work it can perform. In general, it carries all the information regarding manufacturing and installation point of view. Usually, every electrical equipment is provided by specification details printed on a plate which is placed at the backside of the equipment.


If specifications are provided then the user will have a clear view of how to operate the circuit breaker within the provided ratings. It helps in proper maintenance and control of devices with protective measures.


With the knowledge of ratings, the circuit breaker can be protected from damage due to which the cost of the circuit breaker can be reduced. Depending upon the specifications, the type of circuit breaker i.e., ac or dc, and application of the circuit breaker can be determined.


Circuit Breaker Ratings and Specifications :

The circuit breaker needs to work under severe electrodynamic forces, thermal stress, etc. These are very high under short circuit fault conditions. Hence, its rating will be different when compared with other equipment. Its rating specifies all the characteristics values that define the working conditions for which it has been designed.


A few of the important specifications that every circuit breaker must have are explained as follows,
  • Rated voltage
  • Rated current
  • Rated frequency
  • Making capacity
  • Breaking capacity
  • Rated Operating Sequence or Duty Cycle

Rated Voltage :

The rated voltage of a circuit breaker is defined as the highest rms value of voltage for which it is designed. It is the upper limit for the operation of the circuit breaker. Usually, the rated voltage will be greater than the rated nominal system voltage.


Rated Current :

The Rated current of a circuit breaker is defined as the highest rms value of current that it can carry continuously while maintaining the temperature (in various parts) within the prescribed limits.


Rated Frequency :

The rated frequency of a circuit breaker is the frequency at which it has been designed to operate. If the circuit breaker is intended to operate at some other frequency, then its effects like temperature rise, breaking time, etc., must be taken into consideration. The standard rated frequency is 50 Hz.

Rated Short Circuit Breaking Capacity :

The breaking capacities are divided into two types symmetrical breaking capacity and asymmetrical breaking capacity.


The symmetrical breaking capacity of a circuit breaker is defined as the product of symmetrical breaking current and recovery voltage VR. The waveform of a line current during a three-phase short-circuit fault is shown below.

Making & Breaking Capacity of Circuit Breaker

The variation in the shape of short-circuit current is due to the presence of the DC component of the current which exponentially decreases with time. Symmetrical breaking current is the highest RMS value of the ac component of the short-circuit current (during contact separation) that a circuit breaker can break under specified conditions of recovery voltage. From the above figure, the symmetrical breaking capacity is,

Making & Breaking Capacity

The asymmetrical breaking capacity of a circuit breaker is defined as the product of asymmetrical breaking current and recovery voltage VR. Asymmetrical breaking current is the highest RMS value of the total short-circuit current (both ac and dc component) that a circuit breaker can break under specified conditions of recovery voltage. From the above figure, the asymmetrical breaking capacity is,

Making & Breaking Capacity

Rated Short Circuit Making Capacity :

The making capacity of a circuit breaker depends on its ability to withstand the effects of electromagnetic forces (EMF) when it makes the circuit during short-circuit faults. In terms of current, it is defined as the maximum value of sub-transient current (including dc component) during the first cycle at which a circuit breaker can be closed onto a short circuit.


The making current or making capacity of a circuit breaker is given by,
= 1.8 x √2 x Symmetrical breaking current or breaking capacity
Factor √2 converts rms value to maximum value and factor 1.8 takes the doubling effect of short-circuit current (due to dc component) into account.

From the waveform shown above, it can be seen that the current (i.e., major field operating loop current) will be very high near the instant of short-circuiting (since this current corresponds to the sub-transient period). It can also be seen from the same figure, that the current near the instant of contact separation will be less when compared with the former current. Since this current corresponds to the transient period.


As soon as the circuit breaker makes the circuit on the existing short-circuit, the current starts flowing from its sub transient period. So, the circuit breaker has to withstand the electrodynamic forces corresponding to the sub-transient period.


Usually, the breaking operation of a circuit breaker occurs in the transient period. So, the circuit breaker has to withstand the electrodynamic forces corresponding to the transient period. Hence, the making capacity of a circuit breaker is more than its breaking capacity.

Rated Operating Sequence or Duty Cycle :

Rated operating duty of circuit breaker prescribes the sequence of opening and closing operations which can be performed at specified time intervals. For circuit breakers without auto-reclosing features, either of the following below rated operating duties are specified.

O - t - CO - t' - CO
O - t'' - CO

Where,
  • O = Opening operation
  • CO = Closing operation followed by opening operation without any intentional time lag
  • t = Time lag in minutes i.e., 3 min for CB not to be used for rapid auto-reclosure
  • t' = Time lag in minutes i.e., 3 minutes
  • t'' = Time lag in seconds.
Similarly, the operating duty of a circuit breaker with auto-reclosing is as follows,
O - Δt - CO
Where, Δt is the dead time of the circuit breaker, having the units as cycles. The operating duty of a circuit breaker with auto-closing is,
B - Δt - MB
Where,
  • B = Breaking operations
  • MB = Making operation followed by breaking operation without any intentional time lag.

Rated Insulation Level :

The circuit breakers connected in the power system must sustain various effects like regulation, Ferranti effect, etc., and the same can be tested by performing various tests. However, during a single-phase to ground faults, the voltage between the healthy line to ground increases. In order to predict this higher value of insulation is required further which can be provided for each pole (internal and external), between live parts.


Rated Duration of Short Circuit :

The circuit breaker's short time current is an RMS current that can carry in a closed position during a particular time under given conditions and it is expressed in kA for a span of one second which is the rated duration of the short circuit. For the duration, the circuit breaker should carry the current equal to its breaking capacity.


Rated Peak Withstand Current :

The capacity of a circuit breaker to withstand the instantaneous value of short circuit current under closed position is defined as rated peak withstand current. Usually, this value is rated in kA instantaneous and the value suggested for such current is equal to rated short circuit making current.


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