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I have a drainage pump with a level sensor. I'd like to have some protection in case of failure of the level sensor, which would leave the pump running dry while I'm not around, eventually leading to pump failure.

Basically I'm looking for something like a timer switch, but rather than starting at a programmed time and running for e.g. 10 minutes, it should start the countdown once it detects enough current being drained (say > 1 A), and cut off the power supply if it's still draining > 1 A after 10 minutes.

Does anyone know of such a product?

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  • I'm not clear on how what you're asking for is different from a simple 10 minute timer switch. – jwh20 Aug 27 '20 at 14:49
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    It sounds like you are looking for a switch that senses when a circuit is running and then shuts off the circuit with a timer. The problem is it can't shut it off "forever", so it needs an automatic reset. Gets confusing, complicated and expensive. It seems to me that either a better level sensor or perhaps two level sensors in series (so that a fail-on of one will let the other sensor operate as a single switch) would be simpler. – manassehkatz-Moving 2 Codidact Aug 27 '20 at 15:17
  • You can get cheap relays with temp sensors on wire leads that can turn on or off at a set temp. You would put the probe on your pump casing so that if it ran dry, the relay would cut out before it got so hot that it burned out. – dandavis Aug 28 '20 at 16:41
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What you want is a current transducer with a relay output. I use these to stop motors driving conveyor belts if the drive chain or belt breaks the load drops the current transformer senses the drop and trips a relay. Stopping the motor The ones I use are fairly expensive AcuAMP act200-10-s 0-10v output 0-100 amp range 96$ I know automation direct carries them I use these on small motors in about 20 places in the mill. No interest in automation direct I use there products. The sensor is adjusted for the load and when the pump runs dry the current drops and sets the output a relay is what I use to interrupt the motor. This could be used but a low level high level setup would be cheaper and make more sense.

Edit added for another method of control. you could just turn off for a delay then pulse the pump if current high it keeps running if still dry it would shutdown.

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No, I don't believe such a thing exists, if I understand your requirement correctly, which is to sense when the motor is running, and then turn off after a set time.

However, you could get a programmable timer, set to run for 10 mins, once an hour (say), and just run the whole shebang from that. That way, once an hour, the system will be energized, and if the level sensor is actuated, the pump will run for a maximum of 10 mins (less if the switch works).

Obviously this assumes that in an hour you can't go from 'full' to 'flooded' in an hour, you may need to adjust this 'duty-cycle'. It also doesn't guarantee that the pump will never, ever, run dry, but that's expensive and difficult to do.

Alternatively, just running two redundant switches in parallel, as suggested in the comments would be a better solution, probably.

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  • A current transducer with an adjustable output level exists but there are better ways because a start stop circuit would be needed. – Ed Beal Aug 27 '20 at 16:05
  • Yes, your suggestion of having the sensor shut the pump off when the load drops (i.e. it's running dry) is probably better. – SiHa Aug 27 '20 at 16:08

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