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Comment by realreality | Hacker News Reader

> Electric stoves, ovens, and other simple electric heaters.

Not really. The current would be too high on low voltage DC. And high voltage DC is dangerous.

> And high voltage DC is dangerous.

Is this also at 120V or 220V DC? Is it due to how the alternating current allows muscles to release? (Or was that just a myth?)

DC arcs don’t self extinguish like AC ones do, because there is no zero-voltage crossing phase point. For a given voltage, it makes DC much harder on relays, and DC relays are more expensive and harder to produce.

This is true even though AC peak voltage is quite a bit higher than the RMS AC voltage. 170V for ‘120V AC’ for instance.

Thanks!

It matters for switches and things releasing in a physical sense, so muscles may not come into it. Also, there are issues with high voltage DC contactors welding themselves closed in high demand EV situations because they were sized incorrectly or had poor control.

Thanks, dangerous in the sense of damaging equipment/starting a fire? (As opposed to say shocking someone)

> Also, there are issues with high voltage DC contactors welding themselves closed in high demand EV situations because they were sized incorrectly or had poor control.

Would this have have made a difference if it were AC? I think AC welding is also a thing.

AC definitionally has zero voltage 60 times a second, so when you try to "disconnect" by breaking the switch, the flowing electricity doesn't hold the switch closed. It's why when you look at relays they're rated for 12VDC or 120VAC (that and the commonality of house voltage and automotive voltage). I think the true values are probably a little higher in each, but you'd find that relays cannot break contact at 120VDC where they can at 120VAC.

Thank you! So it's the "stickiness" of DC causing these problems, eh? I wonder if there are applications where the DC could temporarily be converted to AC or turned into some kind of oscillating DC temporarily to use more hardware.

>So it's the "stickiness" of DC causing these problems, eh?

Yes, but it's more than just sticky in the sense of welded-contacts. A DC Arc is a continuous plasma that is conductive. That means the arc continues even with a significant air gap. The arc stretches as contacts are separated and yet the arc continues. That means that fuses can burn out completely but still conduct. Breaker-switches can trip and then catch fire while they continue to conduct rather than safely interrupting the arc. So fuses, breakers and relays all need to be designed specifically for DC or significantly de-rated compared to their AC voltage and amperage ratings.

> applications where the DC could temporarily be converted to AC

Yes and that involves an inverter.

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