Multi-Pole Switch Ratings

[dknguyen]

I was wondering something about XPST switches, where X > 1. THe current ratings on these things, is it for the overall current running through the switch? Or is it for the current in each pole? And is it limited by the individual pole or the heat dissipation of the entire switch?

The reason I ask is I am planning to get a 3PST 30A switch that is to control two 15A motor lines and one low current logic power line. If the current is limited by the pole, this effectively means I lose 1/3 of the current carrying capacity because the unused current capacity from the low current logic pole cannot be "given" to the two motor poles. If it is limited by the entire switch then the current on the two power poles can be increased since they can "tale" the unused current capacity from the logic pole.

Basically, if I take a 3PST switch, and only use two of the poles, is the current capacity reduced?

 

[hjames]

I'd swear that I usually hear the rating described as "contact rating", but I can't find any obvious mention of this anywhere.

I'm looking at some high capacity switches with 10mOhm max contact resistances, so power dissipated in each contact would max out at ~2W per contact? Considering the size of the wires needed to carry this load, it doesn't seem like they'd overheat that easily.

 

[dknguyen]

Contact rating is for relays (possibly also including multi-pole relays) isnt it?

Yeah, the resistance is really low, so I am not sure what is limiting the current rating of the switches. I assumed it was continuous but the datasheet has inductive, resistive AC and DC ratings, so it might be more to do with the amount of current the switch can break rather than how much it can carry.
https://www.electro-tech-online.com/custompdfs/2006/08/stoggleshighcap.pdf

 

[hjames]

I'd guess that breaking (as well as the contact bouncing when 'making') is what defines the switch rating. Isn't the material that the switch is made of good up to 300-400 deg. F? I don't think overheating is even a concern...

 

[dknguyen]

I guess I will have to stick a soft switch (only low current needed) on the logic to stop the motor drivers before turning on and off the main power to the motors so that the MOSFETs can deal with the voltage spikes, rather than the switch.