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VA rating for switches?

R

redbelly

Jan 1, 1970
0
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
 
B

Ban

Jan 1, 1970
0
redbelly said:
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?
It's because the load is characterized in power units. Since we have fixed
mains voltages, this is at the end equivalent to current.
 
R

redbelly

Jan 1, 1970
0
Ban said:
It's because the load is characterized in power units. Since we have fixed
mains voltages, this is at the end equivalent to current.

Not really. Switches are also used in DC circuits, which can have a
variety of supply voltages (typically between 1.5V and 24V).

Mark
 
D

Dan Mills

Jan 1, 1970
0
redbelly said:
Not really. Switches are also used in DC circuits, which can have a
variety of supply voltages (typically between 1.5V and 24V).

Mark

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.
 
B

Ban

Jan 1, 1970
0
redbelly said:
Not really. Switches are also used in DC circuits, which can have a
variety of supply voltages (typically between 1.5V and 24V).

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.
 
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