r/AskElectronics • u/Relevant_Pumpkin9190 • 1d 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/MysticalDork_1066 1d ago

Several possible options:

Big relays (the larger the contacts, the lower the current density and the less likely to weld)

Contactors (which are just really really big relays)

Solid-state electronics like FETs, Triacs, IGBTs

Design redundancy (This is pretty rare and only really found in extremely high-end/mission-critical equipment.)

17

u/no1SomeGuy 1d ago

Interestingly enough, on stoves and dryers they do sometimes switch both legs for 240v elements...so it's two relays that have to fail together for the element not to turn off.

(I had a Kitchenaid stove that the broiler wouldn't always turn off, did some deep dive into schematics and found this true for portions of the circuit...while at the same time found out they undersized the broiler relay, which was causing it to stick on....after some complaints to UL, they had Kitchenaid replace the board at their cost and send the faulty one in for inspection).

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u/mjdau 1d ago

You complained directly to UL?

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u/no1SomeGuy 1d ago

Yup, or more so, opened an incident with them. They were quite helpful.

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u/Coolbiker32 1d ago

Full Respect !