This article discusses the key parameters of a circuit breaker in detail. We all have a good knowledge of circuit breakers. They work in electrical systems to protect our electrical equipment in case of faults in the form of short circuits or over-voltages. Let’s discuss the parameters in detail.
Circuit Breaker Parameters
Circuit breakers are normally in CLOSED conditions in sub-stations. It means they allow to flow of electricity in normal conditions. After a fault, they do trip and come in OPEN condition. It means when they are in normally open condition, no faulty current will pass through them and this way they help us to protect the electrical equipment at our houses or substations. They use arc extinguishing media to remove the fire arc when they do tripping.
Here are 5 important parameters of circuit breaker:
- M2 & C2 class
- Power Frequency and Lightening impulse voltage
- Rated Current & STC
- Rated Operating sequence
- First Pole to Clear Factor
Parameters Detail
Let’s discuss all the parameters in detail:
M2 & C2 class:
M stands for mechanical endurance class. According to international standards (IEC), this M class into two further types. First is the M1 class which means the circuit breaker will perform 2000 no-load operations. M1 class indicates that the circuit breaker gas performed 2000 no-load operations without any major maintenance.
Second is the M2 class, called the “extended mechanical endurance class”. M2 class which means the circuit breaker needs to perform 10,000 no-load operations without any major maintenance.
Both M1 and M2 types come under the type test that a Circuit breaker has to perform without any major issue of maintenance. So the M2 class is better than the M1 class.
C class stands for capacitive breaking of the circuit breaker. It is divided into two types according to IEC. First is the C1 type and this class indicates that it has a low probability of restrike during the capacitive current breaking. C2 class indicates that it has a very low probability of restrike during the capacitive current breaking. So, the C2 class is more strict or rigid to achieve.
Power Frequency and Lightening Impulse Voltage:
These two factors are called the basic insulation level (BIL) of circuit breakers. Power frequency voltages can occur due to the following reasons:
- Phase to earth faults
- Load rejection
- Ferranti effect
- Ferro resonance
A circuit breaker must be able to withstand d with these power frequency voltages. For the 145kV circuit breaker, IEC defines 275kV RMS voltage value. The circuit breaker of 145kV rating needs to withstand 275kV RMS voltage in case of power frequency faults. 275kv will be given to this circuit breaker for one minute to undergo the test.
Similarly, lightning strokes may generate lightning impulse voltages. For a 145kV circuit breaker, IEC defines 650kV-peak as the value that a 145kV circuit breaker must withstand in case of lightning impulse voltages. These values are valid for 1000MSL (meters from sea level) altitude level. For 2000 MSL from level these values will not be applied anymore. The altitude factor needs to be applied here in this case.
Rated Current & STC:
Rated current is the value of current that flows normally from the circuit breaker continually without any issue. It can vary from 400-Amps to 4000-Amps in circuit breakers used in our power systems in normal state. This value can be affected by ambient temperature. With higher ambient temperature the current rating will go down.
STC stands for short-time current in an abnormal state. It can go up to 63kAmps due to short-circuiting. It may vary from 25kAmps to 63kAmps depending on the system and voltage rating.
Rated Operating sequence:
The rated operating sequence is also called as duty cycle. Major faults in power systems are transient which means they remain in the power system for a very short time. There are auto-re-closer relays to reclose the system from a fault.
This is the sequence of the auto-re-closing system:
O-0.3s-CO-3min-CO
O = In case of fault the breaker will be open for 0.3 seconds.
C = It means after opening a circuit breaker, it will reclose automatically to see if the fault has been removed. If the fault is removed, the breaker will be in a closed state.
O = If the faults remain in the systems the breaker will again open. But now this time it will wait for 3 minutes.
C = Then it again will be closed after the close command.
If it is mentioned on the nameplate of the circuit breaker it means this circuit breaker is capable of performing this duty cycle or rated operating sequence.
First Pole to Clear Factor:
In a three-phase system, all the phases are 120 degrees away from each other. And these things will impact the opening of the circuit breaker. First, the “Red” phase will open, and then after some time, the “Blue” phase will open and the “Yellow” phase will open last. There is a distance when the two poles are opening. But it is not mandatory which pole will open first and then the other will open. It could be any pole (R, Y, and B). It depends on at what instant the fault has occurred.
But the phase that will open first will experience the highest transient recovery voltage than the transient recovery voltage when all phases are opened. This is what we call the first pole to clear factor. It defines the difference between the first pole transient recovery voltages to transient recovery voltages when all poles are opened.
It is mentioned as 1.2 or 1.5 depending on the fault or voltage rating as per IEC standards. The value of 1.3 (first pole to clear factor) indicates circuit breaker poles are capable of carrying 1.3 times of transient recovery voltage that all poles will see when faults are cleared or all poles are opened. It can be any phase.
Conclusion:
We have discussed the main parameters of a circuit breaker that is used to prevent electrical faults. These include the M2, and C2 class, rated current, power frequency, and the first pole to clear factor. Circuit breaker poles separate at zero current level.
