# How does voltage/current limits work in switches?

I want to get a rotary switch, it says that it can handle 600 V and 40 A. I will use it to turn ON two DC brushed motors of 12 V 20 A. So, the voltage is way too high but the current is just right.

If the current goes above 40 A will the switch work? The voltage is just 12V.

Rotary switch

• No it will not. I'll leave it to someone else to explain why, though, because it's late and I should be asleep. Apr 16, 2019 at 4:07
• Do not exceed the rating. Do not apply AC ratings for a DC application. Read electronics.stackexchange.com/questions/421137/… or search this site for "relay rating" for more information. ( Apr 16, 2019 at 4:53
• Inductive loads (motors) possess substantial stored energy (at times you cannot predict well) and may cause significant arcing when the switch opens. The fact that they are just $12\:\text{V}$ so to speak doesn't really help the switch much. It still must handle the arcing. I'm not an expect on this kind of analysis, but AC ratings are usually listed as much higher than DC ratings. If the datasheet isn't explicit, your better bet is to contact the manufacturer and just ask. They may have a ready answer for you.
– jonk
Apr 16, 2019 at 6:20
• There are no DC ratings for the switch and therefore the switch cannot be used safely in a DC application. It may work, but will not be regarded safe. The fact the voltage is just 12V may help, because some metal contacts (gold, platinum) require a minimum arcing voltage to sustain the arc. But don't rely on that. Ask the manufacturer like @jonk suggests. Apr 16, 2019 at 8:03

## 1 Answer

The switch is manufactured to handle safely 40 A. It is not safe if the current exceeds this rating, as the contacts are not meant to handle this. Disconnecting the contacts while more than rated current is flowing will damage them eventually by arcing. Also the contact resistance can heat up and damage the switch when using excess current

• Question is asking about using a 600V-rated switch in a 12V application, not vice versa. Apr 16, 2019 at 5:40
• So it is, so I edited the answer to match the question. Apr 16, 2019 at 5:58
• The answer is half the truth and therefore unsafe: OP's switch is rated for 40 Amps AC. There is no known DC rating, so, you cannot safely handle it in a DC application Apr 16, 2019 at 7:50