One of the things that happens with my oh-so-clever sump-above design is that if power fails long enough for the return drain from the sump to the aquarium to become empty, when the pump restarts, the air in the return drain causes a slow flow until it is driven out of the line.
This, in turn, raises the level of the sump output chamber, as the normal outflow is temporarily reduced.
I have been experimenting with turning the pump on and off, and although the sump does not overflow, the level does becomes temporarily higher than I would prefer. It makes me nervous. I don't like being nervous.
Consequently, the above. To start the pump, one must press the button. If power fails long enough for the relay to turn off, power is removed from the pump and you must manually press the button to restart it.
This lets me be sure I will be right there, staring directly at the sump during pump startup, ready to turn off the pump if the level gets too high and the sump threatens to overflow.
Will this happen? I don't actually think so. But I'm taking no chances on having to explain why 20 or so gallons of salt water is suddenly pooling on the floor to Deb. Or, for that matter, having to clean it all up.
 Initially, as shown in the state the circuit diagram is drawn in, power is not applied to the pump or the relay.
 Press the momentary-contact button, and AC goes through it and energizes the relay; AC is then also routed back through the relay to the relay's own energizing connection, as well as to the pump — and the pump begins to run.
 The momentary-contact button is released, while power continues to flow through the relay poles (wired in parallel for extra current-carrying capacity) to both the relay and the pump. This keeps the relay energized, and creates a stable power-on condition. The closed relay contacts are now the only route through which power is applied to the pump.
[A] When power fails, the relay will de-energize, consequently its contacts will open, thus breaking the path for power to reach both the relay and the pump.
[B] When power returns, because the relay contacts are open, the pump remains off and the relay remains de-energized.
[C] In order to re-start the system, see .
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