wire grouping in conduit - school of thought

I swear I've seen this issue discussed here before but I can't find it using the search feature.

I'm looking for the collective school of thought regarding plc inputs and outputs of differing voltages run in the same conduit. Many clients have a preference for 24Vdc inputs and others prefer 115 Vac. All our discrete output signals are 115 Vac to the starter circuits. The question is do you run the status/alarm inputs and control outputs between the starters and plc in the same conduit? I've always felt that only low level stuff like analog had to be separated from 115 Vac wiring. Has anyone experienced noise or interference problems mixing 24 Vdc and 115 Vac status and control in the same pipe?

We routinely allow 24 VDC status and control in common conduit with 120 VAC without problem.

We routinely run 24 VDC status and control in common conduit with analog without problem.

We routinely run 24 VDC status and control in common conduit with communications without problem.

We routinely run communications in common conduit with analog without problem.

We NEVER run AC with analog, or AC with communications.

We run them all in one conduit. Communications must be sheilded properly, and we tipically use a low baud rate. On a few occasions we have been forced by job conditions to run communications for remote I/O with no sheilding, or twisted pairs etc. No serious problems yet.

Tom,

Do you do some work at sewage boosters, or treatment plants? If so what NEMA Class/Division do you typically run into? I was on a job the other day were all the controls were rotted out by the gas in the enviroment. I think somebody spect the job for the wrong class/division. It was a really bad situation. Nothing could be salvaged.

Mike.

Mike

To keep the control from sewage environment you need to remove the control to other room if it not in open space.
Maybe Explosion proof can save the control I not sure.
I use to move it far as I can.

I normally run them together BUT input, communications and analogue cables are ALWAYS run in screened cable. No problems.

I do not run inputs, outputs or comms cable with power cables (eg pump cables).

Our business is 90% wastewater treatment. Most of our systemns are in the blower rooms, and are specified NEMA 12. The panels we use are generally NEMA 4/12. For outside panels we use NEMA 4X. We hardly ever do any NEMA 7/9, so I'm not sure if that would really solve th problem or not.

The NEMA 4/12 is no problem for most of our work, and we have stuff pushing 20 years operting OK.

Where you run into problems is areas near digesters or headworks or in pipe tunnels where there are heavy concentrations of hydrogen sulphide. The biggest problem with that isn't enclosure corrosion, though. It's corrosion of any exposed copper - stripped wires and traces on circuit boards and even bus bars turn black and disappear, sometimes in a matter of months.

On some occasions I've sprayed the bejeezus out of everything with conformal coating. Those Hoffman stick on corrosion inhibitors actually work OK, but they need to be replaced frequently - fat chance. The best bet is actually to either adequately ventilate the control room or use compressed air to pressurize the enclosure.

Tom,

You hit the nail right on the head! All the exposed copper is GONE. The controls are in explosion proof cabinets. I cant cite the type off of my head, but their the type w/wide flanges and no gaskets (flame arresters) this lets the gas into the cabinets. The application is a sewage booster/transfer station. The elevator goes from ground level to the incoming side of the transfer station, about 120 ft deep. I would love to do NEMA 4/12 but the site also has a methane presents.

I have proposed moving the electrical equipment to a different location but we looked around and couldnt find anyplace suitable. Besides we have to get a waiver to move them from the machine room. Your idea regarding the compressd air might be the way to go. However the wiring going down the hoistway is also shot, along with the car controls, and limits etc.. I have to hit the books on this one and update myself. I greatly appreciate your input. I had no idea about what caused the corrosion, and now I know what I'm fighting with.

hydrogen sulphide

Thanks very much.

Mike.

what about cpvc pipe, then pour it full of a low viscocity potting compound after you pull your wires,made up all prox connections, and have tested it...monolitic wiring!!!??? I'm only half kidding...sure would keep out the gases!!!!
David

David,

Sounds like you may have had some experience. Years ago, (agout 20), I was an apprentice on a job where we installed a freight elevator in a chemical plant. The controller, all relays, was submurged in transformer oil, and all the pipes were filled with some kinda compound. All the switches, buttons, limits, etc were filled with a dilaelectric grease also.

I asked the customer for the rated Division/Class of the jobsite, and to have some testing done to determine exactly what compounds and or elements are floating aroung in the atmosphere.

What I would really like to do is to install the new controller in the existing cabniets, add gaskets, and somehow pressureize the existing enclosures, & raceways with clean air, or an inert gas, like Tom suggested. This would be dramatically less expensive than moving the controller.

Mike.

Tom Jenkins said:

We NEVER run AC with analog, or AC with communications.

Click to expand...

I wish I could change that around here. We often do that, and while problems from it are minimal, there are problems.

As far as the 24vdc and other voltages, I believe that whatever conductor you use for the 24vdc must be rated for whatever voltage the ac is.

Mike:

You are definitely dealing with the effects of hydrogen sulfide as Tom mentioned. We design wastewater treatment plants and lift stations. I've seen firsthand, cables hanging from brackets on the wall where the conduit used to be in headworks facilities (where the sewage enters the plant). Lift stations are similar. Good lift station design will include separation of wetside and dryside equipment.

NFPA 820 governs the area classification of all sewage related facilities including lift stations, valve and meter vaults and treatment plant areas. In general, all collection system facilities will need explosionproof devices unless ventilated at 12 air changes per hour. Same for the headworks at the treatment plant. Explosion-proof enclosures will NOT prevent H2S laden air from getting into the panel.

There are devices that can be attached to panels like air conditioners that will chemically strip the hydrogen sulfide from air entering the panel, but they need to be maintained. We also use only pvc or pvc coated conduit in areas like this.

H2S is the bane of all treatment plant headworks and collection systems. It stinks to high heaven and required odor control equipment, it corrodes anything metal and it also destroys the top of sewer pipes where the pipes are only flowing partially full. This can cause sewer collapses years after installation.

Tim,

Thanks for the cite on the Code. I did a google, located it and just now ordered a copy.

Thanks again.

This is a great site!!

Mike

Tim,

Are you fimillar with Ehrsam? The tags say their from Abilene Kansas. However the relays, contactors etc look like C.J Anderson. I did a google but I had no luck finding them, all I can find is a bed and breakfast with the same name a few miles outside of Abilene.

I suspect that they were bought out, or went out of busniess.

Life would be much easier if I could just buy new controlls from the OEM.

That name is not one that I recognize. Sorry.

Tim or Tom,

I'm unfamillar with "sewage lingo". This facility is not a treatment plant. It's basically a huge deep hole in the ground that has a wall dividing it into two halfs. On the lower side raw, and I mean RAW sewage enters through mains in the bottom of the hole, and drain into sedement tanks, then the sewage is pumped up to a higher level and drains out through another set of mains. Is this what you mean by the term "LIFT STATION"? The customer calls it a booster station. and the elevator is refered to as the "wet well elevator". But it seems pretty dry to me.

Mike.