Reed switches can fail shorted if operated with too low a current (crystal growth), or with too high a current (welding). But an opto should take a few mA, which should be comfortably between these conditions. You might want to measure the current through the switch, just to verify the circuit is working as you think it works. On Wed, Oct 16, 2013 at 8:38 AM, RussellMc wrote: > 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. > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 Regards, Mark markrages@gmail --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .