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Difference between Instrumentation and Control Cable
Engineering and workplace issues. topic
Posted by zarah on 12 November, 2009 - 1:16 am
Is it possible to use the instrumentation cable (which is stranded) for controls such as Motorized Valve close command?


Posted by Roy Matson on 12 November, 2009 - 3:29 pm
Short answer Yes

Long answer - if the cable is rated for the voltage and any signals you put on it don't interfere with other signals on the cable.

For example if you have a multi-pair instrument cable with a number of 4-20 mA signals it's quite OK to put discrete 24 Volt signals in the same cable. You could probably get away with low power 110 VAC signals but I wouldn't do it as you may get noise in the adjacent 4-20 signals and it would present a potential shock hazard for techs used to dealing with low voltage.

You could use a pair to pick up a DC relay coil if you need to drive an AC motor.

A word of caution with large coils and solenoids - these can cause large voltage spikes in adjacent cables when switching so you need to apply some form of spike suppression.

Hope this helps
Roy


Posted by zarah on 15 November, 2009 - 10:20 pm
Hi Roy,

Thanks a lot for your reply. Actually the application is just 24VDC to close the motorized valve. And I'd like to use other pairs for getting the status of the valve such as open position, close position, electrical availability alarm, etc. Frankly, I don't have a good grasp on what is the difference between a control cable (which is solid wire) and an instrumentation cable (stranded). I normally use instrumentation cable for 4-20mA signals and the control cables for digital signals. But now that I need more than 3 cores for the motorized valve (I need 10 cores), but the cable entry won't accommodate more than 1 cable, I'm thinking of of using the 5-pair instrumentation cable instead. Do you have additional information on the difference of these two cables?

Thanks for the help.

Regards,
zarah


Posted by otised on 16 November, 2009 - 11:41 am
The difference between control and instrumentation cable is mainly in use. Either can be solid or stranded. Back in the "good old days" control cables (for motors) needed to have larger wire than instrumentation cables in order to handle the larger currents required for the motor starters. They were typically terminated under screw terminals, and solid wire makes this termination easier. The instrumentation cables were smaller diameter and frequently made of stranded wire which is more flexible. Today, with greater use of electronic starter controls, it is mainly switchgear (breaker) control wiring that needs the larger diameter wire.

The other difference between the 2 types is that the instrumentation cable is typically a shielded (screened), twisted pair. This construction serves to minimize "crosstalk" (inductive coupling) that causes erroneous readings for the instrumentation. The control cables, whose cicuits operated at 125 VDC, 110 VAC or 220 VAC levels were generally immune to this, and so did not require the shielding.
Today, when the control signals are run at 24 VDC, the shielded twisted pair construction is advised for them as well.

You still need to be careful about level separation, but as long as you are dealing with low DC voltages (28 V maximum), resistive loads, and using shielded twisted pair cables, you can combine the control and instrumentation cores into a single cable where needed. (It is still better practice to keep them separate.) You also should keep inductive loads (like solenoids and relays) separate from the instrumentation, since they can create high inductive spike voltages when they are de-energized.

You still need to choose a wire size sufficient to handle the maximum current and insulated for the maximum voltage. For the control wiring, be careful about using a single common wire for multiple devices - it will need to be sized for the total current.

Hope this helps.


Posted by roy matson on 16 November, 2009 - 6:28 pm
To add to Otised comments re solenoids, yes they can be a problem. I have found for 120 VAC solenoids a Metal Oxide Varistor or for DC coils a reverse biased diode clears up the problem.

It sounds as though your cable choice is OK. I think most instrument cable is rated 300 Volt. Just think about which wires are carrying current and try to arrange the supply and return in the same pair so they tend to cancel each other out.

For something like position switches you could common up the hot but it's probably easier to use a pair per switch anyway.

Good Luck
Roy

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