How is the value of C.B first pole to clear factor derived 1.3 for solidly earthed system?

 

Responding to Krishna’s 20-Aug (12:53) query… is your question related to the circuit breaker’s asymmetrical-capability factor?

Regards, Phil Corso ([email protected])

 

I believe first pole to clear factor is required to be ascertained for three phase fault only.

 

Actually, it comes from ANSI C37.04! But please note that it is the asymmetrical (rms)-to-symetrical (rms) current. It is not, repeat, not the first-cycle peak.

 

Why it is called "First pole to clear factor"? What is the significance?

 

A transient recovery voltage (or TRV) for high-voltage circuit breakers is the voltage that appears across the terminals after current interruption. It is a critical parameter for fault interruption by a high-voltage circuit breaker, its characteristics (amplitude, rate of rise) can lead either to a successful current interruption or to a failure (called reignition or restrike).
The TRV is dependent on the characteristics of the system connected on both terminals of the circuit-breaker, and on the type of fault that this circuit breaker has to interrupt (single, double or three-phase faults, grounded or ungrounded fault).

Characteristics of the system include:
- Type of neutral (effectively grounded, ungrounded, solidly grounded)
- Type of load (capacitive, inductive, resistive)
- Type of connection: cable connected, line connected

The most severe TRV is applied on the first pole of a circuit breaker that interrupts current (called the first-pole-to-clear in a three-phase system).

 

First pole to clear factor is the ratio of distance between faulty phase and healthy phase to the faulty phase. The value should not come more than 1.5.

 

> How is the value of C.B first pole to clear factor derived 1.3 for solidly earthed system?

Formula for the first-pole-to-clear factor=Kpp=
=3Xo/(2X1+X0)
where X0 is the zero sequence, and X1 the positive sequence reactance of the system