contact rating for switchgear coil 2

[ribbert]

I am adding a new pressure switch to operate a close coil for a 4 kV breaker. The coil is rated 6 amps at 125 VDC. One switch that I am considering is rated at 10 amps resistive, 5 amps inductive at 125 VDC. The switch will pick up the close coil typically 3-4 times per month. A seperate breaker interlock typically opens the close coil circuit. Is above described switch acceptable?

 

[rhatcher]

I believe that the basis of the different ratings of your relay for resistive and inductive loads is not current carrying capacity but current interrupting capacity. An inductive load will resist a change in current flow so it is much harder to interrupt an inductive load than a resistive load.

The closing coil of a circuit breaker is an inductive load, so the rating of your relay is inadequate for this use.

 

[peterb]

As you say, a breaker "b" auxiliary contact interrupts the closing coil current when the breaker has closed, so there is normally no breaking duty on the pressure switch contact. The problem arises if the breaker does not close, say due to a mechanical hangup. The closing coil is normally not continuously rated and will burn to a crisp if it is kept energized for any extended period by the closed pressure switch contact.
"Normal" breaker control circuits would have momentary contacts doing the closing, and this is what I suggest that you consider - use the pressure switch to operate a one-shot auxiliary relay circuit, with a closed time of say 0.25 sec. The final control contact should be rated to interrupt the closing coil current in case the breaker hangs up (there are small control relays available to switch 10A @ 125 VDC).

 

[jbartos]

Suggestion: Doublecheck with the switch manufacturer since the coil inductive current (6A) is in excess of your switch 5A inductive current rating. Usually, the contact inductive ratings are noticeable smaller then the resistive current ratings. It is not unusual when a manufacturer of 10A resistive contact rating permits approximately 1A of inductive current, e.g. what used to be the Honeywell manufacturer. You could possibly end up with much higher resistive contact rating, e.g. 30A.