Trade (Triad ) cable and pair cable

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instrumentation engineer

difference between trade cable and a pair cable?
can i use 2 pair cable for a 4wire rtd or not?
yes sir it is triad what is the difference between traid cable and a pair cable? can i lay a 2 pair cable for a 4 wire RTD or not?
Normally we'll use pair cable wherever we want to take the analogue signals to be connected to Remote IO panels. Triad cables mostly used for RTD's. Preferably pair cable shall be shielded so that it can be grounded so as to avoid any interference of signals and make the analogue signal perfect.
Triad cable is three wires (conductors; cores) that are twisted and shielded, usually in a "plastic" cover/jacket. A triad would only provide you with three wires/conductors/cores, and a properly-applied 4-wire RTD requires 4 (four) wires/conductors/cores.

You could use two (2) two-wire (conductors; cores) twisted, shielded pair cables for a total of four wires/conductors/cores--which is what should be used for a 4-wire RTD.

To answer your question you really need to understand the monitor or device or control system you will be connecting the RTD to. A 4-wire RTD is believed to provide the best line lead compensation for the power (excitation) and signal (return) wires.<pre>
-------- Excitation (Power)
| Twisted, shielded pair #1
| ---- Compensation
| /
| /
| |
| | RTD
| |
| |
| \
| \
| ---- Signal (Return)
| Twisted, shielded pair #2
-------- Compensation</pre>
A properly connected 4-wire RTD could be connected to the four terminals of a monitor or device or control system as shown above using two, individual twisted, shielded pairs (two wires/conductors/cores in each pair).

The device you are connecting the 4-wire RTD to may require four wires/conductors/cores, may only require three wires/conductors/cores, and (worst case) may only require two wires/conductors/cores. A 4-wire RTD can be used as a 2-wire, or 3-wire or 4-wire RTD depending on how it's connected--and what the monitor or device or control system it's being connected to requires.

So, the answer to your question lies in how many wires/conductors/cores the monitor or device or control system you will be connecting the RTD to requires. In other words, does the device require a 4-wire RTD with four conductors? If so, two twisted, shielded pairs will do and a single triad (three twisted, shielded wires/conductors/cores) will not suffice.

If the monitor or device or control system only requires three conductors, then you can use a single triad very easily to connect the 4-wire RTD. You will need to deal with the fourth wire appropriately.

If the monitor or device or control system only requires two conductors, then you could use a single twisted, shielded pair, or a single triad. (You will have to deal with the unused conductors of the RTD and the triad if you will only be connecting two leads of the 4-wire RTD.)

Does this answer your question? You need to know what the monitor or device or control system you will be connecting the 4-wire RTD to requires, and then decide on how you will provide the required number of wires/conductors/cores to meet the needs of the monitor or device or control system.

I am bit new to instrumentation and can you please explain when and why we go for 2, 3, 4 wire transmitters?

What is the exact reason for adding compensation?

Are you asking about <i>transmitters</i> or <i>RTDs</i>?

This thread is about RTDs, which are NOT transmitters.

As for compensation, which is used for RTD measurement, that's done because RTDs are devices that vary their resistance as the temperature in the area they are located varies. By impressing a known voltage or known current through the RTD and the measuring either the resultant current or voltage, respectively, the temperature where the RTD is located can be determined.

Because most RTDs are located some distance from the monitor/device/control system they are connected to, wire is used to connect them to the monitor/device/control system. Wire has resistance, and the longer the interconnecting wiring the larger the resistance--which adds to the total resistance of the circuit. If that wire length is not "compensated" for, the temperature the monitor/device/control system is sensing is erroneous.

So, there is one (in a 3-wire RTD) or two (in a 4-wire RTD) compensation leads that are used by the device/monitor/control system to measure the resistance of the interconnecting wiring and then compensate for the interconnecting wiring resistance by subtracting it from the total resistance of the circuit leaving only the resistance of the RTD, which makes the measurement more accurate.

2-wire RTDs are not common in my experience; and they are not used when accurate temperature measurements are required when the RTD will be located remotely from the monitor/device/control system being used to measure the RTD resistance and calculate the temperature.

You can find a lot more information about RTDS, and many other measurement instruments and devices (temperature; flow; etc.) at I believe they have free downloads of some very good reference materials which have excellent explanations, and examples of applications.

Another good source for basic information about MANY things is

If your question was about RTDs and not transmitters (which can be 2-, 3- or 4-wire devices), the general reasons for choosing one over the other is the required accuracy of the measurement, and the cost, including the interconnecting wiring. Twisted, shielded wiring should always be used for RTDs, and that's generally more costly than single wires/conductors/cores, even in a multi-conductor/core cable.

Hope this helps!

I want to make one more clarification here. Some temperature transmitters use RTDs as their sensor. They are the "monitor/device/control system" to which the RTD is connected. They then convert the RTD resistance input to a mA output, usually, which is then connected to some kind other kind of monitor or device or control system that converts the mA signal into temperature.

Isn't this fun?

About the only time I have seen 2-wire RTDs used is when the transmitter it is providing the input to is located very close to the location where the RTD is located, and because the resistance of the interconnecting wire is negligible there's no need to compensate for it. I have seen people try to use these transmitters with 2-wire RTDs that are located a couple hundred meters or more away, and of course, the readings are abysmally erroneous, and they wonder why.

(Some temperature transmitters use thermocouples as the input, and then produce a mA output that is proportional to the input.)

Lastly, there ARE 2-wire and 3-wire and 4-wire transmitters, but they are usually mA output devices and they produce a constant mA output regardless of interconnecting wire length/resistance (which is one reason mA transmitters are so common--no compensation is required because, in effect, they are self-compensating).

Again, the Omega website has a lot of information about many different types of sensor, their inputs and their outputs, as well as different types of data acquisition methods other than PACs (Programmable Automation Controllers) or PLCs (Programmable Logic Controllers) or purpose-built control systems (such as for turbines and reciprocating engines), and the like.

As you are new to the field, I suggest you become as familiar as you can with various types of field devices and instruments, including the lowly RTD and the slightly more high-tech thermocouple. People just consider these things to be so simple and don't pay the proper attention to their application and interconnecting wiring, thinking that they're so simple they don't require anything other than copper wire of any variety and construction. And, when the readings from these devices are bad they always blame the control system--which is usually an incorrect assignment of blame.

RTDs and T/Cs (thermocouples) are not very high tech, and have been around for a very long time, but they do have their quirks and idiosyncracies and requirements.

Another thing you want to understand about instrumentation is that interconnecting wiring is very important. You want to learn about high-level signals and low-level signals, and how not to co-mingle the two. Most instrumentation (the kind that produces variable outputs) is low-level and requires the use of twisted, shielded wiring and the shield drain wire MUST be terminated in <b>ONE and only ONE</b> place (by convention, that is usually the source of the power in the circuit, but, also by convention, regardless of where the power comes from the shield drain wires are usually grounded (terminated) at the monitor/device/control system end of the circuit and NOT at the field device/instrument end.

Good wiring practices are very important to a reliable system. Bad wiring practices will inevitably cost a fortune in lost production and man-hours in troubleshooting, and in human frustration (which doesn't fit in any cell on any spreadsheet so it's never considered when considering the total cost of a project or troubleshooting).

Learn how simple devices work, how they should be connected, and always follow good wiring practices, and you will be considered to be a very good technician by your colleagues and superiors.

Best of luck!

Thank you very much for your valuable suggestions and informations.
I have the downloaded reference materials from omega website and did not take time to read it. Let me first start studying it..
My case is very particular, I must wire 8 RTDs of three cores and I only have a 12 pair multiconductor, I would like to know how serious the mistake I would make if I use these 24 cores to wire the 8 RTDs. What other thing can I do?
- the 12 pair (total of 24) conductors all are the same size conductors, and
- the receiver is capable of 3 wire lead wire compensation, and
- you use 3 wires for each RTD
then you're not likely to have any error above what you'd have if you used triad, 3 conductor cable.

Hopefully the 12 pair are twisted pair, for noise reduction.
Thanks a lot for your answer. I would take advantage and ask by the following: In the multiconductor there is a foil and a drain wire by each pair how I should connect them when I use it for triads.
Convention says to connect all the drain wires to an instrument ground point on the receiver side, leaving the field side drain wires taped up (so they don't touch ground) and floating.