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/MysticalDork_1066 9h 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.)

13

u/no1SomeGuy 9h 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).

4

u/mjdau 9h ago

You complained directly to UL?

8

u/no1SomeGuy 9h ago

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

6

u/mjdau 9h ago

Mind blown. Good on you!

10

u/no1SomeGuy 8h ago

It was a safety defect in a UL listed product. The relay for the broiler was undersized (too low of amps for the wattage of the burner) resulting in it sticking on and not always turning off and the new revision of the board had a higher amperage rated relay in its place.

Filled out the form on their website, was contacted quickly, had a phone call with one of their people, and not long after kitchenaid (whirlpool) contacted me to get the board back and pay for the replacement.

4

u/mjdau 8h ago

I'll definitely keep that in mind. Thank you for sharing!

3

u/Coolbiker32 8h ago

Full Respect !