Marshalling Cabinets Design

[aslanoza]

Hi All
I'm a new member on this forum and really hope to get some tips and of course share my knowledge and experience.
The first one from me, is with regards to DCS/ESD marshalling cabinets.
On various projects I've seen that digital and analogue marshalling cabinets were split, and in some project they were combined. I've been looking through API 554 standard and there is nothing about it over there. Is there any particular reason to split marshalling cabinets by digital or analogue type?

Many thanks for your advice

 

[FrancisL]

There is a long discussion and many photos of such panels here

http://www.plctalk.net/qanda/showthread.php?t=8991

Francis

http://www.controldraw.co.uk

http://www.s88control.blogspot.com

 

[aslanoza]

Thanks Francis
Looked through few pages but didn't really manage to get the answer.
Is there any reason to segregate analogue with digital marshalling cabinets.

 

[controlnovice]

There are technical reasons to separate analog from digital (noise, mostly), but one should be able to do this in the same cabinet with enough distance between the two.

The analog and digital are usually in separate junction boxes in the field, with multi-conductor 'home run' cable to the marshalling cabinets. Running to two separate cabinets may make conduit layout easier.

Ergonomically, for the technicians, one might choose to keep the two signals in separate cabinets, so there is no confusion with troubleshooting, parts, installing new, etc...

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This is normally the space where people post something insightful.

 

[aslanoza]

Thanks a lot controlnovice
Then there is another question with regards to IS and non IS.
Are there any reasons to segregate them in different cabinets?
My understanding that it is easier from the cabinet layout and constructin pont of view to segregate them, but I do not really see any technical reason for not combining them.
thanks a lot

 

[controlnovice]

Again, there are limitations to how close you can run the IS and non IS wiring. You can put them in the same cabinet if you can maintain the separation, but wouldn't be able to put them in same conduit. I don't have the NEC close by, but I believe you might find some information in Article 720 and 725.

Also, on the digital vs analog: if both are 24vdc, then they can be run together. If digital is higher voltage, i think you can still run the analog with it up to a point, but not recommended. I like to keep them separate for clarity.

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This is normally the space where people post something insightful.

 

[JLSeagull]

Check ISA 12.02.01, 12.06.01, ISA TR12.2 and ISA (IEC) 60079.27 etc. for details.

My IS experience is rare as few of my clients want that for their normal process. The IS separation rules were significant to avoid higher voltage and current to exceed the IS rules. Some tricks seem to exist without the one-ohm grounding system etc. that existed decades ago.

I would absolutely separate IS analog from digital signals; and likely IS from not-IS.

I can power a non-IS transmitter with 24 Vdc 35 amp power supply. With the IS loop the barrier etc. (and not checking the standards) limits the signal current under about the 100 mA level. With adjacent pairs in a junction box or marshaling cabinet seems like trouble.

Again, I would have to check current standards to get this straight and don't have time.

 

[aslanoza]

Thanks a lot
Let me do some research and I'll post back on what I've found
Regards

 

[lacajun]

We kept them separate for the maintenance hands, ease of system design and layout, ease of construction, ease of commissioning, etc. Then you have those interposing relays, with 120 VAC on one side, and maintenance hands generally don't like having 24 VDC mixed with 120 VAC in cabinets. They like knowing it's mostly one or the other in a cabinet. They also like them to be clearly marked.

We tried to run separate cable trays to keep them separated. In large plants, it's easier and you don't have to worry about tens or hundreds or thousands of loops adding up to enough power to make a difference.

However, there were exceptions to the rules and the rules were bent or broken but not often. If it is not a safety issue, economics can rule.

I would keep IS separate from non-IS. I worked in one plant that used IS so I cannot comment about how prevalent different wiring practices are. Hopefully they follow NEC.

 

[rbear]

To make it simple. I have worked in many different industries. Petro/Chem, paper, etc for over 25 years. For the most part you want to keep small DC and mA signals separate from AC due to EMF noise and for safety reasons as well. When a tech is working in a cabinet he knows what the voltage and current is and does not have to guess. This is practiced throughout the industry for the most part. The big companies, refineries and chemical plants use this practice. You also want the home run cables to be in a separate tray a barrier in the tray does not always work. I have experienced first-hand seeing EMF noise crossing over into analog 4-20mA wiring; client kept replacing transmitters ever year or two until they discovered that when they loss the 120 Volt power for their digital signals the transmitters they thought were drifting worked (no drift). It was very costly to change all the wiring at this point better to be safe from the start.