do arc snubbers increase relay DC capabilities?

[coppercone2]

if you take a AC contactor or relay, and you put a snubber across it (ARC quencher, seem that many technologies exist), can you extend the rating of the contactor when switching DC ?

[Someone]

Yes

[coppercone2]

is there a derating curve that is realistic ?

and what is the best arc quencher to do this? I see good ones go for 40$ per pole!

[MagicSmoker]

is there a derating curve that is realistic ?

and what is the best arc quencher to do this? I see good ones go for 40$ per pole!

The ampacity is the same for AC or DC when the contacts are closed, so the same derating curve applies. The reason standard relays have much lower DC current (and, usually, voltage) ratings than for AC is entirely due to the nasty habit of DC arcs persisting when AC arcs automatically extinguish at the next zero crossing.

An RC snubber can be placed across the contacts to improve the DC interruption capability of a relay as long as the capacitance is sufficiently high to absorb the energy of the total loop inductance without rising to a voltage that would cause air gap ionization (basically, CV² = LI²) and the resistor is sized to limit the peak current when the contacts next close.

Alternatively, place strong rare earth magnets on either side of the contacts like shown below:

[Seekonk]

If you have a NC/NO relay, placing a cap across those two contacts and a large ohmage resistor to the other polarity will give nearly the full AC rating of a relay on DC. The cap will reverse the current flow and extinguish the arc. According to James Dillon Cobine.

[floobydust]

What voltage and current are you looking at?

A snubber will only mitigate the circuit inductance, from which the back-EMF starts and maintains a DC arc greater than the relay is good for. It's all about contact air gap. I see blowout magnets used around 125V up.

I haven't seen resistance verses current and gap length data for arcs. We've all seen an arc being "pulled" larger and larger.

Looking at PV solar relays, 2.0mm gap and 450VDC rating but FTR-K2W is 3.2mm and 60V 30A rated