Definition: Breaking capacity or interrupting capacity of the circuit breaker is a crucial parameter for selecting the breaker for the electrical network.
The short circuit fault current of the electrical installation must be first known to select the suitable rating of the breaker for electrical installation.
At the time of a short circuit fault in the electrical network, a very high current of magnitude 30 KA to 50 KA, depending on the size of the transformers in the electrical network, flows in the circuit.
The electrical circuit interrupting device must be able to interrupt high short circuit current without damage.
The technical term to show the fault interrupting the current capacity of the breaker is known as the breaking capacity of the circuit breaker.
Breaking Capacity of Circuit Breaker Formula
Importance of Breaking Capacity in Circuit Breakers
When a fault occurs in the electrical network, high current flows through the breaker poles. After tripping the breaker at fault, if the breaker is again switched on under fault conditions, a large current flows through the breaker.
If the breaker does not interrupt that fault current, then it may cause severe damage to the breaker. It also creates heavy stress on the electrical network.
For interrupting the fault current when the breaker is switched on under fault conditions, the other important parameter, termed making current capacity, must be checked.
It is crucial to understand why the making current of the breaker is more than the breaking current.
Under normal conditions, the power system operates under balanced conditions, with all equipment carrying normal current.
At the time of the fault, the power system gets unbalanced, and the system voltage and the current get disrupted. The fault current has sub-transients, transients, and DC components.
The making current of a circuit breaker is the peak value of the maximum current loop during sub-transient conditions, including DC components, when the breaker is closed under fault conditions.
The circuit breaker should meet all the requirements of:
- Nominal breaker capacity
- Breaking capacity
- Making capacity
Making Current vs Breaking Current in Circuit Breaker
Let symmetrical breaking current = I
- Peak Value of symmetrical current = 1.414 I
- The DC component current is almost equal to the peak value of the current during the sub-transient period.
- Making current = 1.414 I × 1.8 = 2.55 I
The making current of the breaker is 2.55 times the breaking current.
Example:
If the system fault current is 35 KA:
- The breaking capacity of the breaker should be 40 KA for 3 Sec
- The making current capacity of the breaker shall be 100 KA
Breaking Capacity of Circuit Breaker Expressed In
The breaking capacity of a circuit breaker is usually expressed in kiloamperes (kA) RMS at a specified system voltage. For example, a breaker rated at 25 kA at 11 kV means it can safely interrupt a short-circuit current of up to 25,000 amperes at that voltage level.
In high-voltage networks, breaking capacity may also be expressed in MVA (megavolt-amperes). This represents the total apparent power the breaker can interrupt, combining both voltage and current. For example, a breaker rated for 500 MVA at 33 kV is designed to interrupt large fault energy safely.
These ratings ensure that the circuit breaker matches the fault levels of the electrical system it protects, preventing damage during short circuits.
How to calculate the breaking capacity of an MCCB?
The breaking capacity of an MCCB is usually provided by the manufacturer and expressed in kA RMS at a particular voltage level. However, if you want to estimate or verify the required breaking capacity based on system parameters, use the following formula:
Where:
- V = Line voltage (in volts)
- I = Short-circuit current (in amperes)
- √3 = 1.732 (for 3-phase systems)
- Result is in kVA
To get the breaking current (in kA):
Example:
If your system voltage is 415 V and the prospective short-circuit current is 25,000 A:
So, the MCCB selected must have a breaking capacity of at least 25 kA at 415 V.
FAQs on Breaking Capacity of Circuit Breakers
The circuit breakers are generally not operated at their maximum breaking current (Icu). However, they are tested for maximum interrupting capacity to ensure their reliable operation.
To determine the interrupting capacity, the IEC has introduced a rated short-circuit interrupting capacity (Ics), expressed as a percentage of Icu. This standard is denoted as IEC 60947-2.
Specifically, the Ics are expressed as 25%, 50%, 75%, or 100% of Icu.
The rated breaking capacity, also known as Icu or Icn, refers to the maximum fault current level a circuit breaker can safely interrupt without damage.
These fault-currents are high and occur with very low probability. In most cases, the fault-currents are significantly lower than the Icu rating of the circuit breaker.
High currents of low probability, also known as (Ics), must be appropriately interrupted to ensure that the circuit breaker is immediately ready for reclosure after the faulty circuit has been repaired.
This is important as it helps to ensure that the circuit breaker functions correctly and prevents any further damage or accidents.
Breaking capacity, also known as interrupting rating, is the maximum amount of current that a fuse, circuit breaker, mcb, mccb, or other electrical apparatus can interrupt without causing damage or creating an unacceptable electric arc.
The formula for the calculation of breaking capacity is as follows.
The breaking capacity of a circuit breaker is expressed in RMS value.
The rupturing capacity (also called breaking or interrupting capacity) of a circuit breaker is rated in kiloamperes (kA) RMS at a specific system voltage and power factor. It represents the maximum short-circuit current the breaker can safely interrupt without damage.
For example, a breaker may be rated as 40 kA at 415 V, 0.2 power factor. This means it can safely break a short-circuit current of 40,000 amperes at 415 volts under specified conditions.
This rating ensures that the breaker is suitable for the worst-case fault level expected in the system.
Conclusion: Why Breaking Capacity Matters in CBs
In conclusion, the breaking or interrupting capacity of the circuit breaker is the maximum current that a circuit breaker can interrupt without being damaged. It is a short-time rating. If the fault current flows for a longer time, it can damage the breaker.
Related Articles:
- Advantages and Disadvantages of Oil Circuit Breakers
- Making Capacity of Circuit Breaker
- Motor Protection Circuit Breaker or MPCB
- What is an Electric ARC?- Arc in Circuit Breaker
- What is an Oil Circuit Breaker?- Construction, Working, Types
- Sulphur Hexafluoride(SF6) Circuit Breaker
- Inverse Time Circuit Breaker and Instantaneous Trip Circuit Breaker
- Difference between Isolator and Circuit Breaker
- Difference Between Relay and Circuit Breaker
- Difference between Fuse and Circuit Breaker
- Types of Circuit Breakers on the Basis of Arc Quenching Medium
- Circuit Breaker Test Equipment
- Routine Tests of Circuit Breaker
- Why Making Current of Circuit Breaker is More than Breaking Current?