Instrumentation and Control Circuits and Same Raceway

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Thread Starter

don best

In a project that is currently under construction, the engineer has decided to route instrumentation circuits (4-20ma) in the same conduit as control circuits (120VAC discrete signals). This is the first time that I've seen this done, and I'm wondering if this is OK. The conduit runs are in the order of 100-200 feet long. The application is for modulating flow control valves (Limitorque). My experience has been that instrumentation and control circuits should be kept separated. This is to prevent noise and interference off the analog signals. To me this is a good enough reason by itself, but its my understanding that the separation requirement may also be from the NEC. Isn't the analog signal considered a Class 1 or Class 2 circuit, therefore requiring separation to protect against fire or shock? All this isn't very clear to me, which is why I'm trying to pick your brains. Are the analog signals Class 1 circuits? Are the analog signals Class 2 circuits? Does it really make any difference if it's Class 1 or Class 2? Ignoring the noise issue, are we allowed by NEC to combine instrumentation circuits with control circuits? What if the cables have 600V rated insulation? Do you any experience combining instrumentation circuits with control circuits in the same raceway? Is this common practice? At this point, is it worthwhile to pay for another conduit run to separate these signals? I believe that some of the conduit has been installed, but none of the cables at this point. If we change the control circuit from 120VAC to 24VDC, do all the issues (interferance and NEC separation requirements) go away? This probably would be less expensive than installing separate conduit. Thank you very much for your input.
 
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Bouchard, James [CPCCA]

Don Best <[email protected]> wrote: >In a project that is currently under construction, the engineer has >decided to route instrumentation circuits (4-20ma) in the same >conduit as control circuits (120VAC discrete signals). This is the first >time that I've seen this done, and I'm wondering if this is OK. The >conduit runs are in the order of 100-200 feet long. The application is >for modulating flow control valves (Limitorque). > >My experience has been that instrumentation and control circuits >should be kept separated. This is to prevent noise and interference >off the analog signals. That has been our experience also >To me this is a good enough reason by itself, but its my understanding >that the separation requirement may also be from the NEC. Isn't the >analog signal considered a Class 1 or Class 2 circuit, therefore >requiring separation to protect against fire or shock? Probably not as you are not supplying them from approved transformers or power supplies >All this isn't very clear to me, which is why I'm trying to pick >your brains. > >Are the analog signals Class 1 circuits? Are the analog signals Class >2 circuits? Does it really make any difference if it's Class 1 or >Class 2? > >Ignoring the noise issue, are we allowed by NEC to combine >instrumentation circuits with control circuits? What if the cables have >600V rated insulation? You may combine control and signaling circuits with power as long as everything is insulated for the highest voltage found in the raceway >Do you any experience combining instrumentation circuits with >control circuits in the same raceway? Is this common practice? We have had problems and now keep instrumentation circuits separate from other output control circuits but we may combine them with input circuits because they do not cause interference >At this point, is it worthwhile to pay for another conduit run to >separate these signals? I believe that some of the conduit has been >installed, but none of the cables at this point. It really depends on your judgement as to the possible problems you may have as a result. It is less expensive to install a new run now then to have to do in later and change the wires >If we change the control circuit from 120VAC to 24VDC, do all the >issues (interferance and NEC separation requirements) go away? This >probably would be less expensive than installing separate conduit. Yes but not the noise issues. If the control circuits are only inputs (limit switch to PLC etc ) it is probably OK if it is the power for a solenoid valve or motor actuator they you could be looking for trouble
 
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Robert Dusza

Don, My experience has been to separate the two types of circuits. Sounds like a cost cutting measure by the engineer and not a very good one. We have problems with higher voltages (460V AC) in the same conduits as ma circuits. Really raises heck with the signals. If you really wanted to reduce the amount of conduit, try a fieldbus that will pick up any of the signals and route over one serial line. This is a lower cost on new construction and simplifies the wiring process. Good luck, Bob Robert J. Dusza, Jr. Treatment Manager (V) 1-860-647-3219 (F) 1-860-647-3150 E-mail - [email protected] Manchester Water & Sewer Dept. 125 Spring St. P.O. Box 191 Manchester, CT 06045-0191
 
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Alfonso Padilla

As long as the 120 VAC signals are the valve's limit switch signals running together with the 4-20 mA signal to the valve controller as I supose, you shouln't have trouble, since such 120 VAC wires will carry very low current. However if these are output signals to say, actuate a solenoid or a remote relay coil, you better keep them in separate conduit runs. Alfonso Padilla QPS Control & Automation [email protected]
 
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Colin Walker

There are, I believe, four issues here; 1. Insulation, in the land downunder you can run all cabling together only if the insulation rating of ALL cables is at or greater than the highest voltage that is run in the raceway/conduit. 2. Electromagnetic coupling, if the control or power circuits high current, then the electromagnetic coupling, (transformer effect) can and often does cause problems. 3. Electrostatic coupling, high voltages and some insulants, also INDUCTIVE LOADS, (relays, motors and solenoids) in the control/power circuits can cause big problems. 4. Signal circuit impedance, if the signal load is low the interference becomes more significant. Both electrostatic and electromagnetic coupling can be reduced by shielding of both control circuits and signal cables but this isn't the best solution. Interference is reduced by the square of the distance and separate conduits the most effective shielding that I know of. A number of times I have tried to estimate the amount of interference that an installation will produce, invariably I've been wrong. There are too many variables. I have been able to reduce interference by loading up 4-20mA and high impedance circuits, you may wish to try this in the future if problems arise. Obviously separate raceways is the best solution, Regards Colin WALKER
 
Don, The Standards and Specifications employed by my company permits signals of similar voltages to be in the same conduit or multipair cable. In general, this means that separate conduits would be required for thermocouple (mv), instrumentation (24 VDC in our case), motor control (120 VAC) and Motor power (480 - 4160 VAC). Also, if intrinsically safe or non-incindive instrumentation ratings are being applied, these must be separated from non intrinsically safe circuits of the same voltage level. We even go so far as to separated discrete switch circuits from analog signals. While some of this may seem like overkill, it has served us very well for the last 20 years. In short, DON'T DO IT.
 
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Ralphsnyder, Grayg

What I generally do is seperation by signal level / type: Communications - Ethernet, AB blue hose, etc... Sometimes include fiber-optics. Analog - 4-20mA, 0-10vdc, etc. Low level discrete - 120v and below ac and dc. Power - 120v thru 600v - in some instances you want to seperate ac and dc. Higher voltage - above 600vac. Factors: -Insulation for all conductors put together has to equal or exceed the highest voltage in that grouping. A wire carrying 24vdc in a duct with a 120vac wire needs to have insulation suitable for 120vac. -Shielding from electrical noise Grayg Ralphsnyder
 
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