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Posted: 4/5/2006 4:50:34 PM EDT
Hello All

One of my coworkers today asked why they derate mechaincal relays and fuses in DC circuits. That is if why the current rating for DC is smaller than AC.

Now with relays I remember it being something to do with the fact that 60Hz AC is actually at less than the peak value for a large period of time. That is the the voltage is at less that 120V so that the current is less. Because there is not a constant current it is easier for the relays to break (open). Because it is not really an issue of power dissipation.

Now fuses have something to do with the voltage level to decrease the arc inside the fuse. And that the fact the AC power is off for every 8.3ms helps to stop the arc at a higher voltage, while a DC current does not have that advantage.

Am I right on this?

I am buzzed too, been drinkin so excuse the bad grammer.
Link Posted: 4/5/2006 5:21:16 PM EDT
Damn, someone on the board knows something about this other than me. Relays and fuses are pretty damn common.
Link Posted: 4/5/2006 5:37:10 PM EDT
[Last Edit: 4/5/2006 5:38:44 PM EDT by Skibane]
A DC arc is harder to break than an AC one, due to the fact that an AC cyle will eventually pass through zero voltage, thereby helping to extinguish the arc.

In relays, having a self-extinguishing arc when the contacts open is easier on them, so they can safely switch higher AC current.

In fuses, the trip current is the same value for AC and DC loads. However, the maximum permissible DC fault current is usually considerably less than its AC counterpart, again due to the difficulty of extinguishing a DC arc immediately after the fuse element melts.
Link Posted: 4/5/2006 5:39:40 PM EDT

Originally Posted By Skibane:
A DC arc is harder to break than an AC one, due to the fact that an AC cyle will eventually pass through zero voltage, thereby helping to extinguish the arc.

In relays, having a self-extinguishing arc when the contacts open is easier on them, so they can safely switch higher AC current.

In fuses, the trip current is the same value for AC and DC loads. However, the maximum permissible DC fault current is usually considerably less than its AC counterpart, again due to the difficulty of extinguishing a DC arc immediately after the fuse element melts.



So I was right in my thinking.
Link Posted: 4/5/2006 6:33:03 PM EDT
120 VAC RMS (root mean square) has a peak value of 169.68 volts - yes, for a very short period of time.

Want to do a simple derating for both AC and DC? Double the relay current rating. A 50% margin is a very good rule of thumb.

Have a 10 AMP DC appication? Get at least a 20 AMP relay.

Have a 120V 5A application? Get at least a 10 AMP / 120V relay.



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