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# 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.

Parents
• Thanks all. That makes a lot of sense. The minor difference in impedances would clearly have a significant effect I've subsequently found the de-rating factors from various manufacturers which seem range from around 0.7 to 0.9 for 2 poles in parallel.

The following is for Siemens contactors, but I'd assume follows the same principles. I know it says AC-1, but the same document states that they are suitable for DC-!

- parallel connection of 2 main contacts: Ie AC-1 x 1,8
- parallel connection of 3 main contacts: Ie AC-1 x 2,5
- parallel connection of 4 main contacts: Ie AC-1 x 3,0

Also, similar derating information from Schneider shows different factors need to be applied when considering durability, breaking current, voltage etc.

In summary, I'd say that the principles pointed out about not assuming that all poles will carry the same current are sound, but trying to get 'universal' derating factors is a forlorn hope as they really need to be taken from the manufacturer.

Anyway, in my case, I've now found those and gained a little understanding as to why they need to be applied in the first place.

thanks all

• Thanks all. That makes a lot of sense. The minor difference in impedances would clearly have a significant effect I've subsequently found the de-rating factors from various manufacturers which seem range from around 0.7 to 0.9 for 2 poles in parallel.

The following is for Siemens contactors, but I'd assume follows the same principles. I know it says AC-1, but the same document states that they are suitable for DC-!

- parallel connection of 2 main contacts: Ie AC-1 x 1,8
- parallel connection of 3 main contacts: Ie AC-1 x 2,5
- parallel connection of 4 main contacts: Ie AC-1 x 3,0

Also, similar derating information from Schneider shows different factors need to be applied when considering durability, breaking current, voltage etc.

In summary, I'd say that the principles pointed out about not assuming that all poles will carry the same current are sound, but trying to get 'universal' derating factors is a forlorn hope as they really need to be taken from the manufacturer.

Anyway, in my case, I've now found those and gained a little understanding as to why they need to be applied in the first place.

thanks all

Children
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