Article 800.xxx - Trying to Understand Communications

[sirdle]

I don't work much with communications equipment and I'm not sure which code articles apply or even where to start looking.

I often see PLC cabinets installed in Tank Farms. In addition to 120V ac and 24V dc equipment, these cabinets might include:

A) PLC-based RS-485 communications module connected to a remote Modbus instrument.
B) PLC-based TCP/IP module connected to a din rail-mounted Ethernet switch.
C) Power-over-Ethernet switch connected to a remote spread-spectrum radio.
D) Protocol converter to switch between Ethernet and RS485.

1) Are any of these devices considered "Communications" equipment?
2) Would the cables connecting them be covered under Article 800, 810 or 830?
3) Or would these be considered to be extensions of the Class 1, 2, and 3 circuits covered under Article 725?

Thanks

 

[don_resqcapt19]

It is my opinion that all of those are covered by Article 725.

 

[hbiss]

Good luck. The real answer is that the NEC isn't that advanced yet. If you don't believe me try and determine from the NEC what a simple piece of CAT5e connecting two computers is classified as. My opinion is that the wiring is communications since all but POE involves no appreciable voltage or current.

-Hal

 

[esobocinski]

Good questions.

1 and 2. These are probably not covered by article 800. They're communications circuits and equipment as defined by common sense, but the "Communications circuit" definition in 800.2 clearly requires a connection to a larger communications system and that isn't happening here; the radio doesn't count. By extension, I don't think you have "communications equipment". Maybe it would if you connected a phone line or external Internet connection. Inconsistent, yes, but as hbiss says, code hasn't caught up to any of this.

2. 810 and 830 (and 820) don't apply at all inside the cabinet. You don't have any radio or television signal wiring. The Ethernet connected to the radio is data with current-limited power, not part of the antenna system. Technically, 810 probably applies to the radio itself, but the only rules that might be relevant would be if the radio uses a remote antenna.

3. All of these are class 2 and class 3 circuits as defined by article 725 and I agree with Don that they're covered. One could argue that B and D might be exempt from 725 if they stay wholly within the PLC cabinet and could be considered integral to it. In other words, I'm pondering the PLC cabinet as a "device or appliance". I'm not sure that's legit, and it might depend on cabinet design, but that's how I read it. Of course, as a practical matter you'd use the same methods for A, B, C, and D and you'd keep all the separations needed for article 725 compliance to keep it all safe, so it doesn't really matter whether 725 applies to some or all of these circuits.