Switches. Voltage ratings lower for DC. Why? - Page 1
Forum section: electronics / general

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 22, 2016, 07:48:25 am
content: How come some toggle switches I am looking at can have a rating of 0.5A @ 250Vac however also are rated 1A @ 24Vdc?

author: djococaud
author_rank: Regular Contributor
timestamp: March 22, 2016, 08:32:42 am
content: Hi ! Simply because of DC is more prone to arcing (on switch opening) and self destroy at high currents. AC is more tolerant because the voltage cross 0V 100 times per seconds and the arc auto extinguishes shortly...

author: djococaud
author_rank: Regular Contributor
timestamp: March 22, 2016, 08:34:37 am
content: Oops ! just re-read your post...And your values...My answer don't make sense here (should be the contrary)...forget it !

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 22, 2016, 08:37:23 am
content: Quote from: djococaud on March 22, 2016, 08:34:37 amKind of answered my question, thanks.

author: AntiProtonBoy
author_rank: Frequent Contributor
timestamp: March 22, 2016, 08:41:41 am
content: Quote from: Ed.Kloonk on March 22, 2016, 07:48:25 amMostly to do with isolation and safety. There is also power load considerations.

author: calexanian
author_rank: Super Contributor
timestamp: March 22, 2016, 06:48:10 pm
content: My mind went to isolation and leakage as well. As mentioned typically the situation is reversed and switches have a higher AC rating do to arcing and pitting more with DC. Must be a facet of the particular switch design.

author: boffin
author_rank: Unknown
timestamp: March 22, 2016, 07:54:04 pm
content: Quote from: Ed.Kloonk on March 22, 2016, 07:48:25 amIt's already been said, but an AC arc will extinguish itself 100-120 times/second, and as the contact gets further apart during an off throw, and it takes more voltage to start vs maintain an arc at a given distance, the arc lasts a lot shorter under AC, hence causes less damage to the switch

author: calexanian
author_rank: Super Contributor
timestamp: March 23, 2016, 02:54:57 pm
content: Quote from: boffin on March 22, 2016, 07:54:04 pmHave a closer look. This switch is rated in an opposite manner. quite strange.

author: SeanB
author_rank: Super Contributor
timestamp: March 23, 2016, 04:39:45 pm
content: Probably only because of arcing increasing wear on the contacts. Must be very slow switching, so AC arcing during switch off can cause excessive heating and burn off of the contacts. DC the arc will not be so bad at this low current.

author: Psi
author_rank: Super Contributor
timestamp: March 23, 2016, 08:02:56 pm
content: Could be a mistake. Is it a brand name switch or a china noname?

author: Shock
author_rank: Super Contributor
timestamp: March 25, 2016, 06:14:52 am
content: Size and design make the difference.https://web.archive.org/web/20071017190656/http://www.eaa.org/benefits/sportaviation/0502_switches.pdf

author: TerraHertz
author_rank: Super Contributor
timestamp: March 25, 2016, 07:23:55 am
content: Quote from: calexanian on March 23, 2016, 02:54:57 pmHow so? 1A @ 24VDC is a lot less power than 0.5A @ 250VAC.The rates likely take into account the typical electrical environments - high AC voltage, and low DC voltage. And why are those typical? Because high DC voltage is f*cking dangerous since DC arcs don't self-extinguish. A DC arc starting in a switch will destroy the switch, then walk along the cable destroying that too.

author: JoeN
author_rank: Frequent Contributor
timestamp: March 25, 2016, 07:27:30 am
content: Quote from: TerraHertz on March 25, 2016, 07:23:55 amJust another reason Edison was wrong and Tesla and Westinghouse were right.

author: mikerj
author_rank: Super Contributor
timestamp: March 26, 2016, 01:34:42 pm
content: Quote from: TerraHertz on March 25, 2016, 07:23:55 amContact devices such as switches and relays usually have a lower DC current rating than their AC current rating, for reasons others have explained. The OP's switch is the opposite way around.

author: Shock
author_rank: Super Contributor
timestamp: March 26, 2016, 09:16:00 pm
content: Quote from: mikerj on March 26, 2016, 01:34:42 pmWhat are you basing this on? The rough 10:1 AC to DC conversion calculation for switches?e.g 250VAC 0.5A MAX = 25VDC 0.5A MAXSo the OPs switch at 24VDC 1A MAX is higher than most switches would be for DC?