Switch disconnector for use in DC circuits

I'm working on a project with 48VDC motors and I have specified a 4 pole 80A switch disconnect. It is to be used for isolation purposes not on-load switching.

At the time when I specified it, I found some information on-line about derating of parallel contacts in this sort of application, but foolishly, I didn't keep a record of it!

Does anyone know of any general rules or guidelines in this area? From memory, there was a derating factor of something like 0.7 to 0.9, although I can't see any reason why each pole wouldn't be able to carry its nominal current whether in parallel or not.

The circuit carries a maximum of 120A for very brief periods and will usually be around half that, so I'm not worried about it actually being overloaded.

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  • I recall, but can not substantiate, reading that three pole switches and contactors used with all 3 poles parralled  can be loaded to TWICE the single pole loading, and not to three times as might be supposed.

    There is no reason why each pole can not carry the full load current, the problem is that the load current may not divide equally between all 3 poles. 

    Make certain that the switch selected is rated for DC, or alternatively use a switch rated for AC, but at TEN TIMES the voltage, that is a 480 volt AC switch.

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  • I recall, but can not substantiate, reading that three pole switches and contactors used with all 3 poles parralled  can be loaded to TWICE the single pole loading, and not to three times as might be supposed.

    There is no reason why each pole can not carry the full load current, the problem is that the load current may not divide equally between all 3 poles. 

    Make certain that the switch selected is rated for DC, or alternatively use a switch rated for AC, but at TEN TIMES the voltage, that is a 480 volt AC switch.

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