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I recently got a new septic system installed. It's a Presby/mound system whereas our old system was gravity fed to a single tank with fingers. The mound system has a pump tank and the tank is wired on a 120V 20A circuit from the main panel.

Naturally, my concern is: if that circuit loses power (for any reason), how many days will it be until I notice? During this downtime, our septic pump will not be running, and I don't want to imagine what that would look (or smell) like!

Is there an easy way to detect if that line voltage drops out AND then switch on a battery-powered indicator light? Apparently, I need a relay, but the single/double "poles and throws" part doesn't make any sense to me.

I'd like to get something like this that I can easily wire in. Most of the relays I find online are the size of automotive ones, and I can't work with that on residential wiring and boxes.

I'm picturing it like this:

enter image description here

AimFireReady
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  • How much current does the pump need/draw? – Rodo Jan 17 '23 at 19:00
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    You're showing RLY1 in series with the pump, it should be in parallel. The "feed" line goes to the pump and one side of the relay. The neutral goes to the pump and the other side of the relay. – Rodo Jan 17 '23 at 19:13
  • Just purchase a power failure lamp. They plug into a 120 VAC socket, and you are done. No need to construct anything. Costs about 12 US$. – Marla Jan 17 '23 at 19:31
  • @Marla, you mean like [this](https://www.ebay.com/itm/304741899102)? It's just a flashlight that plugs into an outlet. Hmm...simple yet elegant! Thanks for the tip. – AimFireReady Jan 18 '23 at 01:03
  • @Rodo, you're right. I see now how it should look. This level of electrical is definitely a stretch for me, haha. – AimFireReady Jan 18 '23 at 01:05
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    You might consider a fluid level monitoring system that would alert you to **any** issue with the pumping system, such as motor relay issues, jammed rotor, etc. For example, use an inexpensive flood warning device in the holding tank to be pumped, such as on of these: https://www.amazon.com/s?k=flood+warning+system – DrMoishe Pippik Jan 18 '23 at 01:10

1 Answers1

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120 V AC still available but pump stopping won't be detected with this circuit but loss of AC will. If you need to detect that too, you need a current transformer and perhaps a bit more circuitry.

You need wither a SPDT or SPST NC relay. Feel free to substitute for DP and leave one unused.

J115F31C120VACS is in stock at Digikey for 4.41 USD but is perhaps overkill for a lightbulb. Search for any relay as described above with 120 V AC coil. Much smaller ones are available but they tend to be PCB mounted.

schematic

simulate this circuit – Schematic created using CircuitLab

As for the poles and throws not making sense, the relay in your circuit above is a SPST NO. One "mechanism" with just one output available. The one in my schematic is a SPDT, configured for NC (normally closed). A double pole will have two such "mechanisms" but still just a single coil driving it.

If anyone better on English than me have a better word for it, let me know and I'll edit it. Not a native speaker. Lever? Moving thingy? Clappity cloppity?

brhans
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winny
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  • Thanks for the diagram. The poles and throws make more sense now. I definitely want a SPST NO so that the light only comes on when the 120VAC is off. (Bonus points for the good laugh I had when I saw "clappity cloppity"!) – AimFireReady Jan 18 '23 at 01:09
  • @Mithrandir Opposite, you need NC or the light will be on when AC is present. It may be easier to get yourself a SPDT than a SPST NC. – winny Jan 18 '23 at 07:05
  • I must be backwards on my understanding of NC vs. NO then. Skipping the instinctive "well, I thought..." part, your schematic shows the battery/light circuit connected so the light would be *on* right? In my use case, I don't want the light on *unless* the 120VAC circuit is OFF. Or does it mean this: The 120V circuit energizes the Relay, which "holds" the 12VDC circuit open, as indicated by the curved arrow, until the 120VAC goes off, which "releases" the relay contact to connect (close?) the 12V circuit, which activates the light. ?? – AimFireReady Jan 20 '23 at 16:36
  • @Mithrandir In my example above, when 120 V AC is applied to the coil, the lever will move in the direction of the arrow, _breaking_ the 12 V connection. Some reading for you: https://www.electroschematics.com/normally-closed-relay-switch/ – winny Jan 21 '23 at 11:13