CIRCUIT BREAKER CAPACITIES
CB capacities are quoted under specified conditions of severity e.g., power factor, recovery voltage and rate rise of recovery voltage.
1. VOLTAGE RATING
Standard voltage ratings of power circuit breakers are in terms of three phase line-to-line voltage. These ratings are based on operation at an altitude of 1,000 meters or less. For higher altitudes, the rating should be de-rated.
2. NORMAL CURRENT RATING
It is the rms value of current which the CB is capable of carrying current continuously as its rated frequency under specified conditions.
3. SHORT TIME RATING
It is the period for which the CB is able to carry fault current while remaining closed.
4. ARC VOLTAGE
It may be defined as the voltage that appears across the contact during the arcing period.
5. RECOVERY VOLTAGE
It is approximately equal to the system voltage. When contacts of the circuit breaker are opened, current drops to zero after every half cycle.
6. RESTRIKING VOLTAGE
It may be defined as the voltage that appears across the breaking contact at the instant of arc extinction.
7. RATE OF RAISE OF RE-STRIKING VOLTAGE (R.R.R.V)
It is defined as the ratio of the peak value of the restriking voltage to the time taken to reach to peak value. It is usually in kV and time in microseconds so that R.R.R.V is in kV/micro-sec.
It is one of the most important parameters as if the rate at which the dielectric strength developed between the contacts is greater than RRRV, and then the arc will be extinguished.
8. DC OFFSET
A periodic waveform has a DC offset if the average value of the waveform over one period is not zero.
9. BREAKING CURRENT
It is the current (rms) that a CB is capable of breaking at given recovery voltage and under specified conditions. (p.f, RRRV)
10. MAKING CURRENT
The making current of a CB is the peak value of the maximum current wave (including the d.c. component) in the first cycle of the current after the circuit is closed by the CB.
11. SYMMETRICAL BREAKING CURRENT
Symmetric breaking current is the rms value of the a.c component of the short circuit current at the instant of contact separation.
In India breaking current is equal to the rms value of ac component.
12. ASYMMETRICAL BREAKING CURRENT
Asymmetrical breaking current is the rms value of the total current comprising a.c and d.c components of the current, at the instant of contact separation. It is equal to the rms value of total current.
In America breaking current is equal to the asymmetrical breaking current.
13. BREAKING CAPACITY
The breaking capacity of CB is generally expressed in terms of MVA. It is the product of rated breaking current expressed in kA and the corresponding rated voltage expressed in kV.
The factor which is depending on the type of circuit for which the apparatus is intended.
1 for single phase circuit and 1.732 for three phase circuit.
[OR]
It is current (r.m.s) that
a circuit breaker is capable of breaking at given recovery voltage and under
specified conditions (e.g. power factor and RRRV)
14. SYMMETRICAL BREAKING CAPACITY
It is equal to 1.732 x V x
I x 10^-6 MVA (Three-phase)
I = rated breaking current,
V = rated service line voltage
15. MAKING CAPACITY
The peak value of current
(including d.c component) during the first cycle of current wave after the
closure of CB is known as making capacity. (Doubling effect value = 1.8)
Making capacity = 1.414 x
1.8 x symmetrical breaking capacity
Making capacity = 2.55 x
symmetrical breaking capacity
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