120 volt Control Circuits Induced Voltage

[Eldon1955]

I have a perplexing dilemma.I have a control system,120 volt,about 24 conductors in a single conduit.This system is fed with a 120 volt UPS system.The conduit run is about 40 feet.Conductor size is #14 awg.In this conduit are a #12 bond,four neutrals and 19 conductors some energized and some not at any given time.Their function at this time is virtually no load.Their purpose is to run indicating LEDs and two momentary contact n.o. contacts as well as three s.p.s.t. toggle switches.The conduit run is between an indicating panel with the aforementioned devices and a control panel with 120 volt control relays and two small contactors .This system is fed by a single circuit with a UPS system though I have the same problem when I bypass the UPS. The problem is I read weird small voltages at points where I should be reading zero voltage.The voltages vary from 20-40 volts from various points to neutral.Readings were taken with a Fluke 87V meter RMS.My system is all hardwired 120 volt of a single power source so I don't believe these strays will adversely affect me but they puzzle me.I can read 40 volts to the neutral with a conductor disconnected at both ends.Any ideas where this dilemma is originating and how do I get rid of it? Thanks ERC

 

[bmnet]

I would think the small fluctuations are a result of induced voltages. The conduit should be acting as a wave guide and thus the variations of voltages at various points.

 

[itsmoked]

Indeed those are induced voltages via inductive and capacitive coupling. Your Fluke has a high input impedance so it doesn't heavily load those wires.

Do this. Take a 60 or 40W bulb in a socket. A drop light works well. Alligator clip the drop light's plug ends - across the points you are measuring. Note your measured voltage then toggle the drop light. What do you get? If when the drop light is on you measure zero then this confirms those are nothing but 'coupling' signals.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Eldon1955]

Thank you so much for your input.I was suspicious of the Fluke and resorted to my antique "Simpson".It indicated severely diminished readings and with a load of virtually any dimension my readings are virtually zero.It was perplexing but I am content with these findings.Thanks so very much. Eldon

 

[itsmoked]

Yep! Simpsons present way more load to the test and drag the coupled voltage down.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[IRstuff]

Simpson meters have something on the order of 10K to 100K input impedances, don't they? A Fluke would have a minimum of 1M.

TTFN

FAQ731-376

 

[itsmoked]

Yes. A fluke is 10M-Ohms input impedance.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Noway2]

We had this problem on one of our existing products. AC coupling into the 120Vac control signal lines caused the optically isolated inputs to falsely activate. The problem was solved by putting a high wattage 2K resistor on the opto input to ground. We used a big power resistor, but as there was little power behind the voltage, I doubt we really needed it.