This is an emergency project in progress which was driven by an incident that occurred while I had guests visiting. Over the weekend I had an issue with my septic system. My home is located in a rural area near the intracoastal waterway in North Carolina. I have what is called a Lift Station as part of my septic system which pumps waste water uphill several hundred meters to a leach/drain field. This failed and I got no indication from the lift station control circuits (which is in an enclosure in my yard) that there was an issue. Normally a light and audible alarm would come on.
The system has 3 float switches for determining water level, 4 indicator lights, and some other components to turn on a pump when water fills to a defined level. I would like to add a monitor capability to this system which would show indications in Home Assistant when each of the four indicator lights are on. The electrical enclosure for this system sits in the middle of a field. All of the float switches connect to a panel and electronics that are located in a weatherproof enclosure. The panel run on 120VAC.
Light 1 should always be on as it simply says the panel has power. For the other three it depends on the position of their respective float switches. Connectors to the 3 float switches are labeled “A”, “B” , and “C” in the diagram. These indicate the following:
“A” indicates that the pump is currently running
“B” indicates that the water level is within working parameters and has not gone to low risking pump burn out.
“C” indicates that the water level is too high and may not be pumping water or pumping it fast enough.
Since this is in an electrical panel outside in my field and weather can be pretty bad here in hurricane season, I needed a solution that can survive less than ideal conditions of temperature, humidity, moisture, etc. There are also space constraints within the box. I really needed (prefer) a single device with four inputs which can connect to Home Assistant and indicate if 120VAC is present or not (effectively confirming the position of the float switches and water level for the first 3) at each of the four points. This device should ideally be powered by 120VAC as I do not really have room for a power supply.
So I purchased a Shelly Plus i4 for this task. Upon receipt I set it up and tested it on the bench to confirm it would do what I was hoping as described above. It seemed to work fine and operated well when I applied 120VAC to any of the switch pins. This resulted in the state changing in Home Assistant as desired.
I also noticed during my initial testing on the bench that both of the connectors on the Shelly under the “L” Label did not appear to have continuity between them. I found this odd as there is no clear indication in anything I have read that they are not connected. So I expected them to be the same. The second “L” labeled connector on the Shelly is not clearly documented and as to whether it receives voltage from outside the device or if the voltage is supposed to be the supplied from the other “L” labeled terminal inside the device?
Then I installed the Shelly Plus I4 into my electrical panel. At this point I did not connect anything to the one “L” terminal on the Shelley near the Switch Terminals. It appeared to work properly for a moment or two, however it soon changed all switch indicators to a state in Home Assistant which showed them all as detected/on even though two were on and two were not. Some investigation appeared to indicate that there was some low amount of voltage (2-9VAC for example) floating on the switch lines when the circuit was open and not connected to the 120VAC. Probably induced into the line from other lines? I assume this is what makes the Shelly I4 think there is a connection and these line are on?
I then installed a jumper between the second open “L” terminal and 120VAC assuming maybe this was what is intended in the diagrams that are around on the internet. This made no difference at all.
So I have designed the circuit shown here to interface to the existing system at points “A”, “B”, “C”, Neutral, and 120VAC. This circuit has 3 relays with one for each of “A”, “B”, and “C” connectors which should connect the Shelly switch inputs to either 120VAC or Neutral rather than leaving the lines open when not turned on. I believe this should eliminate any floating voltage problems. I assume there is nothing in the Shelly Plus I4 input design that would prevent this from working.
So to the questions:
In my circuit design the Shelly switch inputs should always have either 120VAC or Neutral applied to them, and should never be left open. Can each of the Shelly switch inputs accept the either 120VAC or Neutral without issue? I would hate to switch an input to Neutral and have the Device be damaged.
Are all the Shelly switch inputs isolated from each other in a way that if some are switched to 120VAC and some are switched to Neutral will this function properly without causing issues issues?
Can anyone confirm what the internal connection are for the second “L” labeled terminal near the switch in the Shelly Plus I4? Is it expected to supply or receive 120VAC at the terminal or can it be left open? Or can it be tied to the Neutral?
