power factor arcs

[urbanxer]

I heard some linemen talking that they experinced larger than normal arcing when opperating an air-break switch near a capacitor bank. Does the power factor have anything to do with arcing? Does being closer to a capacitor bank have an effect?

 

[mc5w]

Inductive power factors tend to keep an arc going past voltage zeroes which increases the chance of restriking during the next half cycle. However, a sustained arc needs something like a 50% to 60% power factor.

Capacitors can be very hard to switch on and switch off as well. If a capacitor is fully discharged the inrush current can be very high when closed near the supply voltage peak. Inrush current can be even higher if trapped charge is opposite the supply voltage. Trapped charge when turning off can also cause an arc restrike when the power supply reaches opposite polarity and the ions are not all back to ground state.

Resistance heaters generally have the easiest breaking problem. Tungsten light bulbs are bad news on making because inrush current is 6 to 10 times full load beacuse of the change of resistance with temperature. Generally tungsten rating will be 2/3 the code G motor starting rating.

Mike Cole. mc5w@earthlink.net

 

[buzzp]

The caps will supply power for a bit after the switch is open creating a longer arc time (more energy available from caps).

 

[rconnett]

Another factor is that the capacitor can have a trapped voltage after the current is interrupted. Since the switch operates slowly, the gap across the switch might not be adequate to keep the arc from re-striking across the air gap when the voltage waveform goes thru zero & has the opposite polarity. (In other words, theoretically the trapped voltage on the capacitor might be -1 per unit when the voltage on the switch might be +1 per unit). On an instantaneous / peak basis, this can be quite a voltage across the air gap!