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 agree the problem with paralleling low impedance devices like switches is ensuring equal current division - whatever you use to link the poles together is likely to have a resistance in the same order of magnitude as the switch itself - so it would be easy to have one pole having proportionately significantly more resistance overall than another - even if it 0.0002Ω vs 0.0001Ω the current will still divide 1:2 rather than 1:1. Probably the least worst approach is to link each terminal to its neighbour daisy-chain fashion and then connect the main conductors to opposite ends (e.g. extreme left a the top, extreme right at the bottom) so the link resistances tend to add up the same overall. But still there'd be some imbalance due to variation in the resistances of individual links and indeed of the switching elements themselves.

    In most other situations a tiny traction of an Ohm makes far less difference - e.g. when connecting long cables in parallel small differences in link/termination resistance would be swamped by the overall resistance of each cable core, or when paralleling the supply side of a 3P DB to be used single phase, the linking is only done on the supply side of the incomer - the current division is controlled by the connected loads on each of the three bus-bars.

        - Andy.

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  • I agree the problem with paralleling low impedance devices like switches is ensuring equal current division - whatever you use to link the poles together is likely to have a resistance in the same order of magnitude as the switch itself - so it would be easy to have one pole having proportionately significantly more resistance overall than another - even if it 0.0002Ω vs 0.0001Ω the current will still divide 1:2 rather than 1:1. Probably the least worst approach is to link each terminal to its neighbour daisy-chain fashion and then connect the main conductors to opposite ends (e.g. extreme left a the top, extreme right at the bottom) so the link resistances tend to add up the same overall. But still there'd be some imbalance due to variation in the resistances of individual links and indeed of the switching elements themselves.

    In most other situations a tiny traction of an Ohm makes far less difference - e.g. when connecting long cables in parallel small differences in link/termination resistance would be swamped by the overall resistance of each cable core, or when paralleling the supply side of a 3P DB to be used single phase, the linking is only done on the supply side of the incomer - the current division is controlled by the connected loads on each of the three bus-bars.

        - Andy.

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