Why was Circuit Breaker Capacity Rated in MVA and Now in kA and kV?

Circuit Breaker Rating – Breaking Capacity, Making Capacity, Voltage & Current Rating, Duty Cycle & Short Time Operation Rating of Breaker

Please don’t kill me to mention the surprise MVA rating of a circuit breaker as we all have heard about the 500 or 1000 MVA circuit breakers. These ratings won’t appear on recent models as it was the old logic and things have been changed now. To clear the basic concept and know what exactly happened to the rules, lets see the following explanation. It is actually the breaking capacity (not breaking current) of circuit breaker which is now expressed in kA instead of MVA (as it was before).

Before we go in details, let’s know what exactly a circuit breaker do and what are the different types of rating of CBs.

A circuit breaker is a control and protective device used for switching mechanism and protection of the system which:

• Make and break the circuit manually or automatically in normal and fault conditions.
• Break the circuit automatically and close the path to the short circuit and over currents to flow through it.
• Carry the fault current for a very short time while other series connected circuit breaker clearing the fault happening on the connected circuit.

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Based on the three duties of a circuit breaker mentioned above, there are six ratings of a circuit breaker as follow:

• Breaking Capacity
• Making Capacity
• Duty Cycle of Circuit Breaker (Rated Operating Sequence)
• Voltage Rating
• Short Time Operation Capacity
• Normal Current Rating

Breaking Capacity (Earlier MVA, Now kA)

Breaking capacity is the maximum fault or short circuit current (RMS) a circuit breaker can withstand or interrupt by opening its closed contacts at rated recovery voltage without damaging the circuit breaker and connected appliances.

The breaking capacity of a circuit breaker is expressed in RMS value because of symmetrical and asymmetrical factors due to the presence of ripples and DC components during the fault for a very short time.

The breaking capacity of a circuit breaker was rated in MVA earlier considering the rated breaking current and rated operation voltage of a CB. It can be calculated as follow:

Breaking Capacity = √3 x V x I x 10^-6 … MVA

or

Interrupting or Breaking Capacity = √3 x Rated Line Voltage x Rated Line Current x 10^-6 … MVA

Example:

What is the Interrupting or Breaking Current of a circuit breaker having rating of 100MVA breaking capacity and 11kV rated service voltage.

Solution:

Breaking Current = 100 x 10^-6 / ( √3 x 11kV) = 52.48 kA

Why the Breaking Capacity is Expressed in kW instead of MVA?

It is clearly illogical to express the rating of a circuit breaker in MVA because there is a very low voltage and highest current during the short circuit circuit faults. When the breaker open the contacts to eliminate the fault currents, the rated voltage appears across the circuit breaker contacts. In short, the same rated quantities do not appears continuously during the fault currents. That’s why the breaking capacity rating of a circuit breaker can’t be expressed in MVA.

For these reasons, the manufacturers follow the recent and revised international standards to express the circuit breaker breaking capacity rating in breaking symmetrical current in kA at rated voltage instead of MVA. The breaking capacity rating of circuit breaker in amperes or kA are followed by the breaking current and Transient Recovery Voltage (TRV) as it can be both symmetrical and asymmetrical during the short circuit faults.

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Making Capacity

The making capacity of a circuit breaker is the peak value of current including the short term ripple factors and DC components during the first cycle of fault current wave after closure of circuit breaker contacts.

Keep in mind that the circuit breaker making capacity rated in kA expressed in peak value instead of RMS value (the breaking capacity is rated in RMS value). This is because the possibilities of making the breaker contacts successfully during fault currents while handling the electromagnetic forces and arc making and quenching without damaging the breaker and circuit.

These harmful forces are directly proportional to the square of max instantaneous value of current when closing. This is why the making capacity is stated in Peak value as compared to the breaking capacity which is expressed in RMS value.

The value of short circuit currents are maximum at first phase or waves in case of max asymmetry in an phase connected to the breaker. In simple words, The making current is equal to the max value of asymmetrical current i.e. Breaker Making Capacity is always greater than the Breaker Breaking Capacity.

The rated short circuit making current is taken as 2.5 x RMS value of of AC components of rated breaking current as theoretically, the fault current can rise to twice of its symmetrical fault level at the initial stage.

The breaker making capacity can be calculated as follow.

To convert the Symmetrical Breaking Current from RMS to the Peak Value:

Breaker Making Capacity = Symmetrical Breaking Current x √2

Multiply the above expression by 1.8 to include the doubling effect of maximum asymmetry. i.e. effect of short circuit current with consideration to slight drop in current during the first quarter cycle.

Breaker Making Capacity = √2 x 1.8 x Symmetrical Breaking Current = 2.55 x Symmetrical Breaking Current

Breaker Making Capacity =  2.55 x Symmetrical Breaking Current

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Duty Cycle of Circuit Breaker or Rated Operating Sequence

It is shows the mechanical duty requirement of circuit breaker switching mechanism.

Duty cycle or rated operating sequence of a circuit breaker can be expressed as follow:

O – t – CO – t’ – CO

Where:

• O = Opening operation of circuit breaker
• t = 0.3 sec for first auto re closing duty if not specified
• t’ = Time between two operations (restore the initial condition and prevent inappropriate heating of CB contacts
• CO = Closing operation immediately after opening operation without time delay

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Rated Voltage

The value of safe maximum voltage limit that the breaker can be operated without any damage is known as the rated voltage of circuit breaker.

The value of rated voltage of breaker depends on the insulation thickness and insulation material used in construction of a circuit breaker. The breaker rated voltage related to the highest system voltage due to the rise in voltage because of no load or sudden change in load to the lower value. This way, it can handle the rise in the system voltage to the highest rated capacity. For instance, the circuit breaker should withstand the 10% of the rated system voltage in case of 40kV system where the limit is 5% above the 400kV system voltage. This way. a circuit breaker to be used on 6.6 kV line should be rated about 7.2 kV and so on due to the corresponding highest system voltage

On the other side, a 400V AC rated voltage circuit breaker should not be operated at higher voltage i.e. 1000V or more whereas, a 1000V AC rated voltage breaker can be used on 400V of system voltages. If we use the breaker at the rated voltage level, it will capable to quench the arc produced in the breaker contacts. If we use the the breaker on higher voltage levels instead of rated voltage, the transient recovery voltage (TVR) as compared to the dielectric strength of the arc quenching medium. In that case, the arc may still exist as the arc quenching medium is unable to distinguish it successfully which leads to the damage to the circuit breaker or insulation of breaker.

Generally, the circuit breaker rated voltage is higher than the bus and load rating in power system. Typically, there are two types of circuit breaker related to voltage levels i.e. Low Voltage Breakers and High Voltage Breakers having the following features.

• Low voltage breakers can be used for 1kV AC and 1.2kV DC while the high voltage level is more than the low voltage breakers.
• High voltage circuit breakers are used in both indoor and outdoor controls in high voltage system while low voltage circuit breakers are used in indoor application.
• Low voltage breakers are more complex and operate frequently than the high voltage breakers because of less phase to phase and phase to ground clearances. The testing methods are difference for both types of voltage levels breakers.

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Expect of the above voltage rating, two additional voltage rating may be taken in the account while considering the level of voltage for circuit breakers for different operation.

1. Impulse Voltage Rating
2. Power Frequency Withstand Voltage Rating

Impulse voltage rating of a circuit breaker shows the capability to handle the transient impulse by lightning or switching impulses.  The duration of impulse or transient voltage rating of a circuit breaker is in microseconds. For this reason, its contacts with respect to the insulation are designed to withstand the transient peak voltage for a very short time or period.