Instrument Cable Vs Control Cable

Some DCS/PLC modules can handle large enough loads to be considered as power loads (e.g., small heaters and motors). So your final paragraph is not fully correct.

I don't know of an absolute line that divides signal, control, and power cables from one another. Mikerho's answer is a good start.

Noise (electrical interference or EMI), causing spikes in the electrical signal, is usually only a problem with low voltage, and typically millivolt signals that are susceptible to deviations in supply voltage and rely on these types of changes to operate the switches inside a PLC control mechanism...High or medium voltage lines that may experience fluctuations due to load or supply fluctuations will vary the electromagnetic field in close proximity to the wire location, this can trigger unwanted switching in the PLC and/or interference to a communication cable...There are strategies to minimize these types of problems...

..." The ansi/tia/eia-568a commercial building wiring standard of the Telecommunications Industry Association (Arlington, VA) is the most widely accepted performance specification in North America:"...

..." The Telecommunications Distribution Methods Manual (1996 Edition) of bicsi (Tampa, FL), in Chapter 4--"Horizontal Cabling Systems," has a section on "Avoiding Electromagnetic Interference." To avoid emi, all pathways should provide clearances of at least

- 1.2 meters (4 feet) from motors or transformers,

- 0.3 meter (1 foot) from conduit and cables used for electrical-power distribution,

- 12 centimeters (5 inches) from fluorescent lighting.

Pathways should cross perpendicular to fluorescent lighting and electrical-power cables or conduits."...

http://www.cablinginstall.com/articles/print/volume-5/issue-11/crosstalk-feedback/ask-donna/combating-electromagnetic-interference.html

<...there is a separation requirement...Is there any codes/standards which clearly demarcates the instrument and control cables...>

It will be documented as a site standard applicable to that particular installation. That document cannot be seen from here.

Generally, it is good practice to separate high-power-carrying cables from those carrying out analog and digital control, which is why the applicable site standard needs to be found, read, inwardly-digested, and applied.

The specs divide the Instrument cables in to two type of signals
1) Level 1 - Analog signals of less than 50V and discrete instrument signals of less than 30V
2) Level 2 - Switching signals greater than 30V, analog signals greater than 50V, and 120-240 AC feeders less than 20 Amps.
There is a separation requirement between Level 1 and Level 2 cables
On the other hand, spec say there are no minimum separation requirements between power and/or Control conductors for DC or AC circuit voltages less than 1000V, provided the insulation is rated atleast 600V or 450/750V.
Now If I consider Level 2 cables as "Control cables"", I can route them in same as LV cable ladder, raceways.

Oh, good. The specification has been found.

The biggest problem in all installations with computer aided operation is noise. It is useless to have a system where back EMF from colapsing magnetic fields that can generate much higher voltages than the operating voltage to give you unwanted inputs to your computer system and cause erratic unwanted actions. Can you imagine a spike cause a computer to start a hydraulic action that can destroy a item you just made. Most of the time the rule is if you are not sure look at the specifications and if that is not strict enough go for the more expensive option, better to have a smooth running operation than to have a crew of technicians trying to find the reason for spurious unwanted happenings.

I work with bad specs such as these regularly, as I design control systems for pharmaceutical process skids. I would say the intent of the spec is to keep 24VDC away from AC power. The cabling with 230VAC on it is definitely a power cable, if it is running something, which at 230VAC it most likely is. And yes, a solenoid on 230VAC would have a power cable to it and be kept away fro 24VDC instrumentation cables. They should drop that control terminology - it creates confusion. Basically the cables are instrumentation or power.

Yes - check with the engineer who cut and pasted this spec together, from 30 in the repertoire from 20 years ago, and verify, but I'd bet $100 the requirement is as stated above - keep 24VDC away from AC power.

1 key to reducing noise interference on signal cables to to cross power cables and wiring at 90 degrees so that the lines of flux are perpendicular and less likely to inductively couple.

Selk,

Another factor to consider, which you mentioned (but didn't discuss) in your second post is the source of the power on the cable. In the NEC, power sources for "Remote Control, Signalling, Power-Limited Circuits" are divided into Class-1, class-2, and class-3, and are discussed in article 725. For each of these, there are limits on where the cables can be run.

Class-1 generally is a maximum of 30V and 1000 VA (~1000 watts). If in conduits and cables, they are only allowed if there is a functional association between these conductors and other circuits. 600V insulation is required in cable tray but barriers are required if there is no functional association with other circuits.

Class-2 and Class-3 are defined by the label on the listed power supply as either class-2 or -3. These are with more restricted installation rules.

Lets assume you are dealing with class-1. Now, the question becomes one of protecting your process and equipment from unwanted electronic noise and the risk of damage from faults or unforeseen events. The advice of others, already given on this forum, is good. If your circuit is an on/off device such as a relay or PLC output, the risk of electronic noise affecting this device is fairly low. However, if the device is operating at logic-level voltages, then noise can be a significant problem; shielding with the shield grounded at one end only becomes a desired part of the installation. If the circuit is an analog input or output, shielding and separation from power cables and particularly from VFD line and load cables becomes very important. Inside a control enclosure, keep the PLC's discrete I/O and analog I/P away from each other (don't intermix modules side-by-side, if possible). Run their cables in separate paths and separate wireways within the enclosure. Cross your instrument cables at right-angles to power cables when they cannot be kept separate (because the electromagnetic fields around the power cables have the lowest coupling onto the instrument cables when run at right angles). Maintain a separation of at least a foot, whenever possible.

If your panel has a horn or buzzer, NEVER connect it directly to a PLC output--always place an interfacing relay between it and the PLC, because the mechanical action of the horn or buzzer induces noise on the wires from the PLC output and will create interference within the PLC that can affect totally unrelated inputs and outputs or the program itself (trust me on this, I have seen it happen).

If you are the person designing the control system, you can write instructions that require the installer to keep these cables you are describing separate from power cables, if you wish. If you are the installer and are trying to figure out what to do because such instructions are not present, then read all the posts and go with those that seem to be most likely to minimize noise and future problems.

--JMM