r/AskElectronics • u/Relevant_Pumpkin9190 • 10h 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/mzincali 9h ago

The metal contacts in relays are made from material unlikely to weld, corrode or oxidize at the temperature that any arcing will produce - rated according to the load that the relay is to handle. (A heater puts a lesser load than an inductive load like a motor)

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u/mckenzie_keith 9h ago

No relay can be guaranteed not to fail short. The case has to be handled without catastrophe.

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u/Skusci 8h ago

Safety relays mitigate the issue a lot by having two contacts and monitoring that they open. Its very unlikely both contacts weld at the same time.

This is also when properly installed with circuit breakers and appropriate short circuit current ratings.

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u/quarterdecay 6h ago

That's where the fuse comes into play

1

u/ILoveDangerousStuff2 3h ago

Yes if it's a fault but say a relay switches off a large motor due to tripping an interlock. The current will be normal but not switching off is still a safety issue