Circuit breaker Part 5: System and Switching Conditions

First-pole-to-clear-factor: 

The first-pole-to-clear-factor (Kpp) is depending on the earthing of the network. The first-pole-to-clear-factor is used for calculating the transient recovery voltage for three-phase faults. (1.3) 

In general, the following cases apply: 

• Kpp = 1.3 corresponds to three-phase faults in systems with an effectively earthed neutral. 
• Kpp = 1.5 corresponds to three-phase faults in isolated systems or resonant earthed systems. 
• Kpp = 1.0 corresponds to special cases, e.g. two-phase railway systems, short-line fault. 

A special case is when there is a three-phase fault without involving earth. This case corresponds to Kpp = 1.5. This case is covered by the IEEE standards.

Rated Transient Recovery Voltage:

Voltage that appears between the contacts of the circuit breaker after arc extinction during opening process while clearing a fault.

The rated transient recovery voltage (TRV) is the peak transient voltage (expressed in kV) that corresponds to the first pole-to-clear when interrupting a three-phase fault at rated short-circuit current.

Rated Surge Impedance and other Short-Line Faults Characteristics:

When a short-circuit occurs on an overhead line not far from a circuit breaker, travelling waves will generate a very steep first part of the transient recovery voltage. 

The Rate of Rise of Recovery Voltage, RRRV is depending on the short-circuit current and the surge impedance.

The surge impedance may vary depending on e.g. type of conductors. 

In standards IEC and IEEE, the surge impedance has been standardized to a value of 450 Ω.

Level of TRV and the RRRV are key factors in determining whether the fault can be cleared successfully.

Capacitive Switching Class:

There are two different capacitive switching classes:

• Class C1: Circuit breaker with a low probability of restrike during capacitive switching.
• Class C2: Circuit breaker with very low probability of restrike during capacitive switching.

A circuit breaker intended for Class C2 can of course also be used for Class C1.

Time constant:

The time constant of the system is equal to the ratio between inductance and resistance in the network (L/R) and is expressed in ms. The standard value is 45 ms. 

The time constant will affect the required DC component.

If a required X/R-ratio has been given, the time constant in ms can easily be calculated by dividing the X/R-ratio with (2 x π x f), where f is the rated frequency.

X/R = 14 corresponds to a time constant of 45 ms at 50 Hz