Floating Control Wires 1

[fiberstress]

We have several nodes in a simple 120V relay control circuit that are floating w.r.t. everything else. The voltages we're seeing is anywhere from 20-70VAC from the nodes to our neutral. At this point we concluded that voltage to ground on these nodes should see only minuscule DC voltage and practically no AC voltage.
We're deriving our 120VAC control power from 2 legs of 3ph 480. (one leg on 120 side is grounded). My question is would our true RMS meters be picking up much AC voltage from the floating nodes to our grounded leg because of this?
Thanks

 

[waross]

Either you are picking up induction from a part of the circuit that is, at the time of testing, floating with neither a connection to the hot or grounded line, or some circuit components are acting as a voltage divider.
A true RMS meter is almost certainly an electronic meter. The extremely high input impedance of many of these meters makes them able to read induced voltages that would be swamped out or shorted out by an old analogue meter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Inductive OR capacitive coupling.
You can easily be seeing this with a modern meter. Grab a drop light and connect one prong to the wire under investigation and the other prong to your local ground. Measure the voltage with it off then turn it on and see what happens. If it drops, as it should, replace the drop light with your meter set to AC current. Turn down the range until you get a useful reading. It will probably be something around 50uA. Once you have that reading you can mess around a little to see where and how the coupling is happening.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fiberstress]

Thanks guys, I like the light and measure current idea. Will try that next time we're scheduled to be out there.

The induced voltage was what I meant by the stray DC voltages (anything and everything from other equipment/components to solar flares..), but would we be reading an AC voltage? And could that AC voltage possibly be 20-70V? 1-5V is believable but 20-70V?

Another guess of ours was that we were measuring to our neutral which is grounded but still alternating?

 

[fiberstress]

Forgot to mention, we did measure amperage from our main 120V lugs to see if it was a short or just high resistance somewhere, and we didn't see anything the clamp on meter could register. We also did an IR scan with nothing to note.

 

[ScottyUK]

If you have long cables runs connected then 50V - 70V is entirely possible. You will be able to light a low wattage lamp from capacitive leakage current in a few hundred yards of multicore.

 

[fiberstress]

That's interesting. Our runs are only 30 yards at the most, though, and they used single core at install.

 

[ScottyUK]

It is a real problem in long motor control circuits which use the three-wire system (pushbutton starters) because the contactor can stay latched in through capacitive leakage current even though the stop button is pressed. Doesn't sound like this is the case for your situation though.

 

[fiberstress]

Just to update, we confirmed this weekend it was capacitive coupling. We hooked up the nodes to a light and nothing. I didn't write down the current, but it was minuscule. The AC signal makes sense to me now if it is just "bleeding" through the insulation. Thanks for the insight!

 

[OperaHouse]

If you ever pull apart a simple 120V LED lamp you will likely find a 47K-87K resistor to drain off capacitance charge. Early LED lamps would GHOST when turned off. These blead off resistorss account easily for 1/4 watt with a 3 1/2W lamp. Without them, 10uA makes them visible.

 

[itsmoked]

fiberstress; Thanks for the confirmation. Much appreciated.

OperaHouse; Reminds me of the LED floodlight I bought. It was one of the first ever. It was way, way, over a 100 bucks. I was running it with a solid state relay. Whenever I commanded off it got about 30% dimmer than when it was 'ON'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

fiberstress; Thanks for the confirmation. Much appreciated.

OperaHouse; Reminds me of the LED floodlight I bought. It was one of the first ever. It was way, way, over a 100 bucks. I was running it with a solid state relay. Whenever I commanded off it got about 30% dimmer than when it was 'ON'.

Keith Cress
kcress -

http://www.flaminsystems.com