Analog and Digital Wires in Same Raceway

[nightfox1925]

I have a situation wherein I have cabinet with existing wireways. I do not have any space to provide another wireway. We ahve installed existing instrument amplifiers with both 24VDC digital and 4~20mA Analog output terminals. Unfortunately we could not physically separate both signal wires due to space limitations. This wireway is just beside the instrument amplifiers.

If the Analog wires are operated at 4~20mA, 24VDC and the digital signals are operated at 24VDC and the insulation voltages for the two set of signal wires are 300V and 600V respectively, is it technically acceptable to combine them in one wireway?

By the way, this particular wireway is the one beside the instrument amplifiers and routed to a point where it terminates to two separate analog and digital wireways inside the cabinet.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[itsmoked]

It is code acceptable.
It is technically lame.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

I agree with Keith that it would certainly be best to keep them separate. Having said that, as long as the 24V dc discrete wiring is not drawing 20A trying to pull in a 1" solenoid valve, and is driving low current I/O, AND you are using well-shielded, twisted pair wiring for everything, AND you ground the shield wiring properly, you will probably get away it.

 

[gepman]

I agree with dpc. I have installed and started up tens of thousands of 24VDC I/O (always used because it was more high speed than 120VAC I/O) combined with 4-20mA analog signals and never have had a problem. The 4-20mA was always dual shielded, the 24VDC was always 16 ga THHN. There just isn't enough space and money to have every signal level separate, at least for my typical food processing client. The 24VDC was always signal level not power level. Anything that took any power (which was going to be an output anyway) was wired 120VAC and run separately.

Oilfield and power plants have different specifications and I always followed their specs at their plants but I never had a problem when they were combined at other facilities. If you look at the input impedance of a typical 24VDC Input module, the 4-20mA is probably drawing more current and usually it is biased at 24VDC.

 

[burnt2x]

Didn't have problems with the case presented here as far as I can remember. As long as the wires used are all shielded and properly grounded, no problem having both cables on the same wireway.
Without hyjacking the post, we did experience signal distortions when an additional UHF amplifier was inadvertently installed inside the interface cabinet! 4-20mA signals went muddled everytime UHF signals come.

 

[Skogsgurra]

It is also a question of how long the wires are. I have seen installations with a few meters of 24 V I/O running parallel to both 4-20 mA and 0-10 V unshielded analogue signals. No problems at all.

As long as the 24 V DC isn't PWM controlled with a high carrier frequency and the analogue signals are not mV signals and your wires are not more than 5 - 10 meters (15 - 30 feet), you will be fine.

Inductive loads on the 24 V side shall, of course, always have snubbers or diodes and sometimes it is a good idea to include a current limiting resistor if you have high wattage incandescent lamps. The coldstart current will sometimes be so high that it can disturb analogue signals. But that is very rare.

VFD motor cables and HF/UHF signals are the bad guys. They sometimes influence your analogue signals also when routed in different wireways. And shielded.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nightfox1925]

I just got confirmation that all the analog and digital signals of the amplifiers are operating at 24Vdc and the wires are at a short distance only...I'll go and get a waiver on the basis of the prevailing limitations and keeping in mind the technical side based from all the inputs I gathered here. Thank you.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata