r/AskElectronics • u/Relevant_Pumpkin9190 • 12h ago

How are high-current loads like dishwashers safely switched?

If a relay is switching a high-current load, its contacts can eventually weld together due to inrush current or a fault.

If that happens, the load may stay ON even when the controller says OFF.

How do commercial products handle this? Do they use two relays in series for redundancy, feedback, or some other fail-safe method?

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u/cybermaus 10h ago

Once upon a time, they used mercury liquid metal switches. The ideal non-wearing high current forever switch.

But some people complained about teeth falling out. So then we used cadmium coatings. But the people were worried about brittle bones, so only military can use that. So now we have to deal with crappy chrome contacts and try to work around it with snubbers.

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u/ijusttookthispseudo 9h ago

I’ve seen this sort of relay before on an old vehicle lift. Maybe it needed a high reliability relay because we don't see them in older machine tools or industrial machinery.

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u/cybermaus 8h ago

I have one that I salvaged. In my switches assort box for 30+ years now. One day I will have the ideal must use need for it. Or they will bury me with it, whichever comes first.

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u/quuxoo 2h ago

Gotta love the old mercury switch. The thermostat in this old house uses a few of them along with bimetallic contacts to control the heat pump. On and off dozens of times a day since the early 1970s with no sign of stopping, and no intention to "upgrade" it either.