# Electrical switch rated for 20Amps at 220VAC. What is the max equivalent DC voltage/current? **URL:** \ **Category:** Factual Questions\ **Created:** [April 10, 2013, 12:19pm UTC](https://boards.straightdope.com/t/electrical-switch-rated-for-20amps-at-220vac-what-is-the-max-equivalent-dc-voltage-current/655314 "2013-04-10T12:19:21Z")\ **Posts on this page:** 4\ **Page:** 2
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [April 13, 2013, 12:28pm UTC](https://boards.straightdope.com/t/electrical-switch-rated-for-20amps-at-220vac-what-is-the-max-equivalent-dc-voltage-current/655314/21 "2013-04-13T12:28:35Z")
> [@ZenBeam](#): > > Why use another example? Why not just use the OP’s case: 12 V and 3 A? No prob. Even though an arc can’t be sustained at 12 VDC, the arcing could still be substantial, simply because it’s DC. If the switch has a slow contact spreading velocity due to a “slow break” mechanism, it might be adequate for AC but not for DC. (“AC only” switches tend to have slow or indeterminate contact spreading velocities. DC and AC/DC have quick-break mechanisms.) Remember those old DPDT slide switches? The contact spreading velocity was determined by how quickly you slid the switch. Fine for certain AC applications. _Not_ fine for DC. BTW, my responses in this thread are “on the record” responses, the same as I would give to my customers at work. Off the record, the switch will probably work fine at 12 VDC / 3 ADC, though I don’t have hard data to back this up. Hence my reason for being cautious. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [April 13, 2013, 2:16pm UTC](https://boards.straightdope.com/t/electrical-switch-rated-for-20amps-at-220vac-what-is-the-max-equivalent-dc-voltage-current/655314/22 "2013-04-13T14:16:28Z")
> [@AaronX](#): > > That’s interesting. But for welding, they seem to use HF (doesn’t that imply AC?) to ignite arcs. Isn’t that contradictory? High frequencies become effectively like DC because it takes a short time for the air to deionize\*; this time can be an [appreciable fraction of a second](http://books.google.com/books?id=I3N4aQwfSr4C&pg=PA186&lpg=PA186&dq=deionization+time&source=bl&ots=MbEFEwsS2G&sig=bXfzszoyQSn01A8pCxVYFOGZdOA&hl=en&sa=X&ei=FWZpUYnXMMPz2QWF7oBY&ved=0CGAQ6AEwBQ#v=onepage&q=deionization%20time&f=false) at very high voltages (0.3 seconds for 275 kV), also strongly dependent on many other factors like current and gap length (as shown in previous posts, like the [example](http://www.youtube.com/watch?v=membSshXvvM) I used, 50/60 Hz AC can easily arc over considerable distances). \*This is also why electronic ballasts get more light output from fluorescents for the same drive power; higher frequency, [up to about 2 kHz](http://www.lamptech.co.uk/Documents/FL%20Frequency.htm), reduces the time when the gas isn’t ionized. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [April 13, 2013, 3:36pm UTC](https://boards.straightdope.com/t/electrical-switch-rated-for-20amps-at-220vac-what-is-the-max-equivalent-dc-voltage-current/655314/23 "2013-04-13T15:36:25Z")
> [@Crafter\_Man](#): > > No prob. Even though an arc can’t be sustained at 12 VDC, the arcing could still be substantial, simply because it’s DC. If the switch has a slow contact spreading velocity due to a “slow break” mechanism, it might be adequate for AC but not for DC. (“AC only” switches tend to have slow or indeterminate contact spreading velocities. DC and AC/DC have quick-break mechanisms.) > > Remember those old DPDT slide switches? The contact spreading velocity was determined by how quickly you slid the switch. Fine for certain AC applications. _Not_ fine for DC. > > BTW, my responses in this thread are “on the record” responses, the same as I would give to my customers at work. Off the record, the switch will probably work fine at 12 VDC / 3 ADC, though I don’t have hard data to back this up. Hence my reason for being cautious. The switch in question is a Hager SF119F DIN rail change-over swich. It is in page 27 of this [PDF catalogue](http://download.hager.com/Hager.ie/files_download/hager/Upload_2008/Modular_%20Automation_Ctrl_%20devices.pdf) It feels like there’s some spring-loaded mechanism inside it. No matter how slowly you try to switch it, the contacts inside go snap. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [April 14, 2013, 1:10pm UTC](https://boards.straightdope.com/t/electrical-switch-rated-for-20amps-at-220vac-what-is-the-max-equivalent-dc-voltage-current/655314/24 "2013-04-14T13:10:36Z")
> [@Dog80](#): > > The switch in question is a Hager SF119F DIN rail change-over swich. It is in page 27 of this [PDF catalogue](http://download.hager.com/Hager.ie/files_download/hager/Upload_2008/Modular_%20Automation_Ctrl_%20devices.pdf) > > It feels like there’s some spring-loaded mechanism inside it. No matter how slowly you try to switch it, the contacts inside go snap. It will probably be O.K. Just make sure you install a [snubber diode](http://en.wikipedia.org/wiki/Flyback_diode) across the load. A 1N4007 should do the trick. May also want to install an RC snubber across the switch contacts. [Previous page](https://boards.straightdope.com/t/electrical-switch-rated-for-20amps-at-220vac-what-is-the-max-equivalent-dc-voltage-current/655314.md?page=1)