Floating Control Wires
Floating Control Wires
(OP)
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
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





RE: Floating Control Wires
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
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"Why not the best?"
Jimmy Carter
RE: Floating Control Wires
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
RE: Floating Control Wires
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?
RE: Floating Control Wires
RE: Floating Control Wires
RE: Floating Control Wires
RE: Floating Control Wires
RE: Floating Control Wires
RE: Floating Control Wires
RE: Floating Control Wires
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
RE: Floating Control Wires
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