I would use at least two independent redundant circuits for such a
critical application.
It is known from residential alarm experience that a common failure mode
for reed switches is to fail closed.

Perhaps a safe approach is to use one in N.C. and another in N.O.
configuration "ANDed" together, so if any of them says "stop" the pump
stops.
Another solution would be to use a different type of sensor for backup.


Isaac


Em 16/10/2013 10:22, Carl Denk escreveu:
> For my standby generator 16 gallon gasoline tank, there is a nearby 175=20
> gallon gasoline tank with a 12 VDC transfer pump to keep the 16 gallon=20
> full. The pump ground has a 12VDC 1" black cube automotive type SPDT=20
> relay that is using the NC contact that opens when the relay is=20
> energized. The relay coil is energized with a IRF9530 P-Channel Power=20
> MOSFET, which is activated with a H11B1 opto-isolator. The reed switch=20
> is a float switch  to ground, NC at normal fuel levels, and NO at high=20
> level activates the opto-isolator. There is a suppressor diode across=20
> the opto-isolator input and dropping resistor from the 12 VDC power.=20
> There is also one section of a PS2501-4 opto-isolator driven by the=20
> switch that goes to a PLC that turns on the pump (there is also a manual=
=20
> pump switch).
>
> Recently the reed contacts welded together causing a possible unsafe=20
> condition of overflowing gasoline. I have new switches ordered, the=20
> question is, is the current configuration of the usual alarm circuit,=20
> normally powered, and a fault (could be a broken wire) open is detected=20
> and shuts thing down. But with welded contacts that's bad. :( It=20
> wouldn't be hard to make the reed switch NO, change the relay contact=20
> used to NO, and program the PLC for inverted action to make the circuit=20
> powered is alarm. Think it's about which is better scenario.
>
> Oh the system is 7 years old and working OK until now.
>
> Comments appreciated. :)

--=20
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