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| Ev Archive for October 2002 |
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| 1331 messages, last added Tue Oct 22 14:03:22 2002 |
[Date Index][Thread Index]
Re: AC Vs DC ratings...
>I have seen a couple of posts concerning AC ratings on switches Vs using DC
>on them and the switches not lasting as long. Can someone tell me the
>difference between the effect a DC current has on a switch and an
>equivalent RMS AC current. I guess what I am asking is : what effect does
DC
>have over AC in switch contact life?
In an AC circuit the voltage (and ideally the current) goes through zero 120
times per second (at least here in the USA, 100 times per second most places
overseas).
When you open a contact on a high voltage circuit it usually creates an arc
from one contact to the other. In an AC circuit the arc goes out when the
voltage goes to zero (120 times per second). DC circuits don't go to zero
so there isn't anything to put out the arc, this means that you need to
employ some method in the switch to extinguish the arc.
The two most common methods are distance (too much gap and the arc can't
jump it) and magnetic blow outs. Actually magnetic blowouts just increase
the distance between the contacts by making the arc take a longer (curved)
path.
DC definitely shortens the contact life. If the switch isn't rated for the
DC voltage, it might shorten the life to one activation.
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