> > > If a switch contact is rated 0.5A @ 125VAC, what is the correct way to
> > > calculate its rating @ 5VDC?

> >Can't help with a specific calculation (but I'm sure it has a lot to do
> >with the resistance of the contacts and joules / energy)

> >Looking through the C&K and MEC catalogues, a typical example
> >for a small toggle switch is 0.25A 125VAC / 9W AC / 6W DC which
> >I infer from that  you de-rate by 1/3rd from AC W to DC W

That MAY be a good starting point but I'd be careful drawing conclusions
from a single data point.

There are a number of factors involved (and I certainly don't understand
them well). Dissipated energy is important BUT things like arc quenching can
make a vast difference to performance. An AC switch opening at the sort of
rate you would get with finger operation is liable to quench an arc in a
maximum of one half cycle of the switching frequency. When switching DC,
slow opening can lead to an arc over an extended period. For this reason
switches which are expressly intended for high energy per size DC switching
will often be designed to snap open to minimise arc time. Most common main
switches (at least in NZ equipment) are not provided with snap open
operation.



        RM

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