Using an AC relay for DC switching, safe limits

Question

I have an relay that's rated for a 250VAC, 16A load. Would it be possible to also use the same relay for say a 25VDC, and less than 1A load?

From here, relay power ratings - AC vs DC , it seems to imply that that due to arcing, an AC relay would be able to support switching DC as long as the DC voltage is about 1/10 of the AC rating.

Would this apply as a general rule even if DC values are not explicitly stated on the relay's spec sheet, as in, there won't be long-term damage from implementing relays in this manner? The particular relay I'm using is "G5LE-1A-E" with a rated load of 250VAC @ 16A, from here: https://www.digikey.com/products/en?keywords=%09Z2619-ND

Answer 1

No, this is not a general rule. The AC rating and the DC rating rely on different relay qualities.

The AC rating allows for the arc that forms during relay opening to be extinguished by the voltage zero crossing, rather than the opening of the contacts. So the AC rating often says more about how long it can withstand an arc of a certain power level for, rather than how effective it is at interrupting the current.

The DC rating must allow for interruption of the full current at the full voltage rating. So to meet the DC rating, the relay must not only withstand the arc that forms as the contacts are opening, but also open far enough to extinguish that arc.

This is why you see with some relays, and indeed with the relay you specify, that one model is AC rated and another is DC rated - in your case, G5LE-E and G5LE-G respectively. The -G variety actually specifics the contact gap width, because that's important if it's the relay's job to extinguish the arc.

Of course, all relays form some sort of gap, so even an AC rated relay will work with relatively low DC. But how low? It depends. Be especially careful if you have any inductance at all in your load circuit. Interrupting a DC circuit with inductance in it is challenging, and pernicious, because the damage it causes may not be obvious at first.

Answer 2

From a safety perspective, its unclear. As you alluded to, the DC ratings are often much lower than the respective AC ratings. You're probably fine at less than 1 A for a one-off project.

If the project will require any type of safety certification (i.e. UL), you are better off getting a relay with the correct rating. Using an AC relay for a DC load will severely complicate the evaluation and will require you to take some additional responsibility.