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Comment by amelius | Hacker News Reader

Can anyone explain why the contact DC voltage rating of relays is often much lower (5x is no exception) than the AC rating?

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I think it is due to arcing. With AC an arc will extinguish itself itself when the voltage goes through 0, but for DC no such thing occurs and an long arc can damage the relay contacts.

Also, an interesting oddity about relays and switches: many have a minimum current rating because some arcing is actually required, it burns off the oxidation layers on their contacts. This is called "wet switching", if you use a wet-switching relay to toggle a control voltage with negligible current flowing on the line, you can drive the relay to death prematurely due to oxidation. The solution is to use a control current instead of a control voltage, but it has higher power consumption (on the bright side, better noise immunity). There are also specialized "dry-switching" relays to solve this problem, usually for test and measurements applications (but using them to switch live circuits causes the opposite problem - arcing will damage them).

Yes I figured that, but the 50/60Hz frequency seems quite low for that to be the case.

50Hz basically means the arc is extinguished after at most 20 milliseconds, limiting how much the contacts heat up. For DC on the other hand there's no guarantee the arc will extinguish at all. You can imagine how a sustained arc would be bad for a relay.

Of course a higher frequency would heat the contacts even less, and I imagine you could get a relay rated for even higher voltages given a higher AC frequency.

But arcs only happen when the contacts are almost-but-not-entirely closed, I suppose (ionization could be a factor). This time-period is, I suppose, very short, much shorter than the 10 milliseconds of the AC half-wave.

I suspect the arc could cause the contact to stick for a while, which may increase the arc time.

At high enough voltages you get anywhere from second-long to indefinite arcing. Not to mention that the contacts are cost optimized, so the manufacturer would like to get away with as little travel as they can as to simplify the mechanism.

I don't think that's a universal effect.

https://www.youtube.com/watch?v=-cM8P6-aAf4

As the other poster pointed out arcing is one component. Another is that DC voltage equivalent of an AC voltage is the RMS value of the AC signal. So, V_AC_RMS = V_peak / sqrt(2).

AC specifications are usually in RMS equivalent voltage so 1A 10V AC is same power as 1A 10V DC. This makes the peak difference larger, not smaller.

AC is easier to extinguish (0A current 100 times / s), and I believe the reversible direction makes for better electroplating characteristics.

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