DeltaV DI 8 channel 120V AC Isolated induced voltage problem

It sound as if you have a ground loop. My guess is that there are unbalanced 3 phase Y circuits or undersized single phase branch circuits dumping current into ground. Do your switches and sensors share a common ground or return? They shouldn't nor should the signal return conductors and signal conductors themselves be grounded anywhere except at the central monitoring point. My guess is that each conductor has less than .0005 uF of coupling to other conductors and the environment. This is a rather high impedance at 60Hz. Each sensor pair can then be terminated with a series RC to ground at the central monitor point if necessary. It means each sensor will need two wires but it sounds as if your environment needs it. I have seen over 100 volts of AC on telephone wires to ground but since the telephone signal is limited to across the pair and not to ground, noise on the line is less than 0.0001 volts.
14 guage wire seems excessivly large. How much current is switched?

Thanks for the response! The current is 2ma. The wire size is a plant standard size. This problem just appeared on Wednesday or it was noticed on Wednesday. Thanks again....

All the neutrals are tied together......

Try some 1-watt 33-68KO reisitors across the inputs of your logger.

Kbradio:
What is the DELTAV DI card? What does it do? Why haven't you
notice this problem before?

Send me your E-mail addr and fax#


<nbucska@pcperipherals.com>

Check page 19 for the specs.

Here is what appears to be working. Put a snubber of 1uF cap in series with a 100 ohm resistor to put a 2500 ohms AC equivalent load to ground to discharge this induced voltage.

The neutrals are all tied together...

nbucska,

'Delta V' is the name of a DCS marketed by Emerson, who bought out the original vendor Fisher-Rosemount a year or two ago.

'DI' is a digital input card for the above system.

Kbradio,

Have you lost a system ground connection? The sudden appearance of this problem indicates an abrupt change in the system behaviour. If you haven't changed the installation, or added any plant which is near, or routes its power near, the inputs to your DI card wiring, that is the line of inquiry I would initially pursue.






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Thanks for the responses! It appears the 60 conductor cable got wet and with the cold winter we had here in the northeast,now with a week of warm weather the cable's capacitance has changed hence the induced voltage has went up.
Added snubbers across the 12 discrete inputs to drain the induced voltages to ground through a 2500 ohm impedance a 1uF cap in series with a 100 ohm resistor. It appears the 8 channel discrete input card with its 60K ohm input impedance doen't have a built in snubber circuit across all 8 of its inputs.