5 ms·

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 d

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.

amelius 6y ago
Yes I figured that, but the 50/60Hz frequency seems quite low for that to be the case.
- Doxin 6y ago
  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.
  - amelius 6y ago
    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.
    - deleted 6y ago
      [deleted]
    - Doxin 6y ago
      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.
megous 6y ago
I don't think that's a universal effect. https://www.youtube.com/watch?v=-cM8P6-aAf4 https://www.youtube.com/watch?v=-cM8P6-aAf4
segfaultbuserr 6y ago
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).

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.

amelius 6y ago

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

Doxin 6y ago
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.
- amelius 6y ago
  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.
  - deleted 6y ago
    [deleted]
  - Doxin 6y ago
    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.

megous 6y ago

I don't think that's a universal effect. https://www.youtube.com/watch?v=-cM8P6-aAf4 https://www.youtube.com/watch?v=-cM8P6-aAf4

segfaultbuserr 6y ago

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).