The reed switch is (somehow) being operated outside its sensible specs.
The circuit should be designed so that welded contacts are essentially
impossible.

Your circuit description seemed to suggest that the reed is driving only an
opto load. Working out the worst case current and designing for it sounds
useful.

Placing two reeds in electrical series would allow either to fail S/C
without causing a fault.


        R



> Recently the reed contacts welded together causing a possible unsafe
> condition of overflowing gasoline. I have new switches ordered, the
> question is, is the current configuration of the usual alarm circuit,
> normally powered, and a fault (could be a broken wire) open is detected
> and shuts thing down. But with welded contacts that's bad. :( It
> wouldn't be hard to make the reed switch NO, change the relay contact
> used to NO, and program the PLC for inverted action to make the circuit
> powered is alarm. Think it's about which is better scenario.
>
> Oh the system is 7 years old and working OK until now.
>
--=20
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