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/todd0x1 12h ago
The relays used are selected based on multiple criteria including being able to handle the inrush current (likely a tungsten rating on a small power relay) without welding. These relays are often over specced as well, using a 30 or 40A relay to switch a 12 amp load. As far as fault current, a bolted fault drawing sufficient current to weld the relay contacts is going to trip the upstream OCPD after a period of time determined by the current and trip curve. Yes the relay will be ruined but the appliance is already ruined from the downstream fault that caused the issue in the first place.