It protects the pump from running dry by cutting out if pressure falls
below a low limit, usually around 5 PSI. The pressure should never
fall that low unless the pump runs dry or gets completely clogged. (Or
if you have a jet pump that looses prime)
Once the low limit trips, you have to flip a little lever on the side
to reset it.
If your well never runs dry, you don't need it, but it's cheap
protection for the pump.
You will have to use the manual override the first time you run the
pump with the new switch, since you'll have to drain the line to
install the new switch.
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