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Switch disconnector for use in DC circuits

Richard Henderson

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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    When paralleling up electronics, to ensure good sharing, we would use 'ballasting' resistors that are a few times the circuit impedance, so that the variations in wiring layout are less serious. If this cannot be done then a traveling wave layout is used, where the input and output are located on diagonals of the group of devices sharing the load, such that the one nearest the input with lowest input resistance, is furthest from the  output, so that the total path lengths of all devices are more or less matched lengths. Such things work on a PCB but are not so easy to arrange in a contactor - so instead  some de-rating is needed

    Mike

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    When paralleling up electronics, to ensure good sharing, we would use 'ballasting' resistors that are a few times the circuit impedance, so that the variations in wiring layout are less serious. If this cannot be done then a traveling wave layout is used, where the input and output are located on diagonals of the group of devices sharing the load, such that the one nearest the input with lowest input resistance, is furthest from the  output, so that the total path lengths of all devices are more or less matched lengths. Such things work on a PCB but are not so easy to arrange in a contactor - so instead  some de-rating is needed

    Mike

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