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

Table of content

• 0:0 min

• 26 Aug 2024

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

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.

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.

### 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

### 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

### 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.

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.

Power frequency withstand voltage Rating of a circuit breaker shows the ability to manage the sudden increase in the voltage which is very high than the higher system voltage. It is due to sudden changes in the load or disconnecting a large portion of load at once.

This voltage due to power frequency is for a very short time generally 60 seconds but the circuit breaker must capable to handle the power frequency over voltage.

The following chart shows the different rating of voltage levels of circuit breaker i.e. Nominal System Voltage, Highest System Voltage, Power Frequency Withstand Voltage and Impulse Voltage Levels.

### Short Time Operation Capacity

The short time capacity of a circuit breaker is the specified short period in which the circuit breaker carry the fault current while remaining closed.

To reduce the unwanted operation of a circuit breaker like the fault current for a very short time or sudden change or reduce in loads, the circuit breaker should not trip and disconnect the circuit if the fault disappear automatically and handle the electromagnetic force and temperature rise. If it exceeds the specified time in seconds or milliseconds, the breaker then will open the contacts to make sure the max possible protection to the connected portion of loads and equipment.

There are different classes like B, C, D and Class 1, Class 2 and Class 3 with associated curves are used. Class 3 is best which allows maximum 1.5L joule/second tested as per IS 60898. For example, oil circuit breaker has the time capacity of 3 seconds and it should not exceed the exact 3s while carrying the short current. The rated short time current capacity should equal to the rated breaking capacity of circuit breaker. Therefore, care must be taken for sensitive device while considering the time capacity rating of breakers.

### Normal Current Rating

The normal current rating of a circuit breaker is the RMS value of current it is capable to carry continuously at its rated voltage and frequency without changes in operation due to the rise in temperature during the normal operation.

The normal current should be 125% of the rated current of the circuit. For example, if the load current is 24A, the circuit breaker rating should be as follow.

= 24A x 125%

= 24A x 1.25

Circuit Breaker Current Size = 30 A

Another way, the breaker current size can be device by 0.8 to find the load current. i.e a 25A breaker can be used for 20A lighting load etc.

Load current = Breaker Current rating x 0.8

Load current = 25A x 0.8 = 20A.

## FAQ

### Why is the circuit breaker rated in MVA?

That is because a circuit breaker has nothing to do with power factor but should just interrupt the current at the rated voltage and hence the rating in MVA

### Why is circuit breaker rating given in KVA?

Breaking capacity of a circuit breaker
The rated breaking capacity is defined as the maximum fault current a circuit breaker can successfully interrupt without being damaged. It is always expressed in KVA and the current corresponding to breaking capacity is known as the breaking current

### How the capacity of circuit breaker is rated?

Every circuit breaker has a specified amperage (amount of current). This rating is labeled on the breaker itself. The standard for most household circuits are rated either 15 amps or 20 amps. An important note to remember is that circuit breakers can only handle about 80% of their overall amperage

### What is the rating of a MV circuit breaker?

Some standard current ratings as defined in Indian Standards are: Low voltage circuit breakers (up to 1000V): 16A, 25A, 32A, 40A, 63A, 100A, 200A, 400A. Medium voltage circuit breakers (1kV to 52kV): 250A, 400A, 630A, 800A, 1250A, 1600A, 2000A, 2500A, 4000A

### What are the two current ratings for circuit breakers?

Two important ratings to consider are; short-circuit current rating (this is commonly referred to as the maximum interrupting rating) and the short-time current rating. In this paper, we will discuss these circuit breaker ratings and how they can affect the protection and selective coordination of the system

### What is the Ka rating of a circuit breaker?

The KA rating is a measure of the circuit breaker’s ability to handle excessive current safely. It represents the maximum fault current that a circuit breaker can interrupt without causing damage to the breaker itself or the electrical system

### Why is kVA rated?

Hint: Iron loss on voltage and copper loss of a transformer depends on current. Hence, total transformer heat loss depends on volt-ampere (VA) and independent of phase angle among voltage and current i.e., it is independent of power factor. That is why the rating of the transformer is in kVA and not in kW

### Why are breakers 80% rated?

the nec recognizes that the operation of overcurrent protective devices may be affected by the heat in a system. Because of this, it defines the concept of continuous loads and the 80% limitation in order to try and offset the effects of heat in the system when sizing a circuit breaker

## What is the Suitable Wire Size for 20A Breaker and Outlet?

How to Find the the Right Wire Size for a 20-Amp Breaker and Outlet Based on NEC? Choosing the the…

## Why Do We Need to Install a Starter with a Motor?

Why Do We Need to Connect a Starter with Electrical Motors? Essential and Need of Starter with Motor A starter is a device used…

## Why are Generators and Alternators Rated in kVA, Not in kW?

Why is an Alternator or Generator Rated in kVA, Not in kW? As we already know that why is a transformer…

## Why Does AC Need More Insulation than DC at the Same Voltage Level?

Why Does AC Voltage Require More Insulation than DC Voltage for the Same Working Voltage Level? For the same working…

## Why is a Power Plant Capacity Rated in MW and not in MVA?

Why is the Rating of a Power Plant Capacity Expressed in MW instead of MVA? For the following reasons, a…

## Why are Overhead Power Lines Loose on Electric Poles & Towers?

Why are Overhead Power Lines Loose on Utility Poles and Transmission Towers? Transportation of people, goods, and services today is…

## Why Radio Waves Are Chosen For Close Range Transmission?

Why Radio Waves Are Chosen For Close Range Transmission? INTRODUCTION Back then in the stone ages, information were disseminated either…