ATO stray voltage when off?

[Rohirrimus]

Hello guys,
I have mechanical ATO
(Simple one, float switch completes the circuit when water level falls and the pump fills with fresh RO)
Today I noticed stray voltage (I accidentally touched line tester screwdriver to the water trying to poke the pumps adjustment switch with it )

what’s weird is that when ATO is off ( float is up and circuit isn’t complete) there is a voltage, but if I drop down the float switch to start the circuit voltage disappears.
i don’t understand how is this happening because if there was a leak in the float switch wouldn’t water complete the circuit and start the ATO?
I am really confused, hopefully someone can explain what’s going on. Thanks

 

[Quietman]

You didn't mention if the float switch was connected to a controller or relay. If not, then I'd recommend a mod/upgrade to include a relay PDQ as that's a safety issue. Most hobby float switches aren't designed to carry even small amounts of line current and that excess current can cause the internal switches to be damaged causing various issues including failure of ATO up to and including electrocution and fire (depending on line source). Those that are rated still aren't recommend in aquariums because even though likelihood is less, if they get damaged it's still electricity in a big saltwater tank with your hands in it.

If you do have it on a relay/controller - terrific. Then you're talking small milliamps from a control circuit without starting surges. Still sounds like there maybe some damage internally to the float. Closing the reed switch is causing all the current to flow through the closed connection so I can see why voltage changes. You may not have enough damage to complete the circuit without being in the correct position. If it was me, I'd just replace (or upgrade - single float switch for ATO doesn't have any backup protection). Troubleshooting partially damaged gear is tough as symptoms may not be consistent.

Also, strongly recommend to everyone having a grounding probe in their systems just in case.