I've tried Googling to answer a question, but had no luck.
Mechanical switches often have a "VA" rating and I am trying to understand why. Shouldn't there simply be a current rating? With the switch closed, the voltage drop across the contacts is practically nil; I would think there would be some maximum current it can handle, regardless of the open circuit voltage. Could someone explain why a VA rating is appropriate?
Thanks,
Mark
It's because the load is characterized in power units. Since we have fixed mains voltages, this is at the end equivalent to current.
-- ciao Ban Bordighera, Italy
Not really. Switches are also used in DC circuits, which can have a variety of supply voltages (typically between 1.5V and 24V).
Mark
No problem:
The specs say "Contact rating: 0.4 VA max. @ 20 V AC or DC max."
Regards,
Mark
Makes sense to me. Thanks Dan.
Mark
They all give Ampere ratings for low voltages, like 2A@30V; 3A@12V; 4A@5V . I looked around at a few switch manufacturers and didn't actually find a single one specifying the switches by power rating. This is only done for motor switches and -relays, but then those are operated at a fixed mains voltage. So the OP should tell us where he saw these strange specification.
-- ciao Ban Bordighera, Italy
The real reason is that arc damage on opening is dependent on both current and voltage. As the switch opens an arc forms between the contacts, and if there is too much power being handled then this can destroy the switch.
Incidentally, this is why the DC voltage rating of a switch is often lower (sometimes much lower) then the AC rating. With AC the voltage drops to zero twice per cycle allowing the arc the extinguish. With DC the contact gap has to open far enough for the arc to extinguish.
HTH.
Regards, Dan.
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