ControlLogix: 120V AC Output putting out 87V ?

GaryS said:

Let me follow up with this
the output is an OW16 card that's a relay output ( NO leakage)
OFF)

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The card is a 1756-OW16I

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Take a look at the pdf I post above. (leakage is 1.5ma per point)

I still say its a grounding issue the snubber resister is about 200,000 ohms
there is just no way to have a voltage level that high if it's connected to any load at all
the leakage at 120VAC worst case would only be .75ma I would expect it to b far less then that anything connected would drain the voltage down to nothing. if you connected a high impedance volt meter on the unconnected wire from the output the reading would be 120VAC not 87VAC . that can only happen if the source is floating. if that's the case any voltage is possible you could even see 480VAC on that line at some point. floating grounds cause more problems and are harder to find than any other problem you will have.

sounds like a floating voltage to me as well.

I have seen the leakage from a OW16I drive a small relay but the voltage from the card contacts was the same as my source 120VAC.

As I see it, you have several issues to deal with.

1. 400 ft between buildings means only one thing, different power sources and grounding, so you must use the same power source from the interface to the other building and back. You CANNOT use power from both buildings !
NEC 70 - cannot quote book, chapter, and verse.

2. a distance of 400' between buildings means 400 ft from building A to building B. You then go thru the isolated output, then 400 ft from building B back to building A. A total of 800 ft. You must size the wire to account for voltage loss, I would say 3 or 4 wire sizes, my rule of thumb, 1 size for every 200 ft of wire.

3. wiring must be run in its own conduit.

4. wire color must me orange to signify interface wiring and power from an alternate source. See NFPA79.

5. alternate power source placards must be on the plc door.

some rules may have changed since I did design on that scale.
if I am wrong, someone please correct me

regards,
james

GaryS said:

Let me follow up with this
the output is an OW16 card that's a relay output ( NO leakage)
if you were reading 87V at the receiving end with the output ON forced or otherwise then you have a serous grounding problem that needs to be addressed.

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Not necessarily. It's not true that relay outputs have no leakage current. The typical impedance of a 120 VAC input is quite high, so this output isn't heavily loaded.