why trad cabls using for RTD field wiring? can we use normal electrical power cable for RTD field cable instead of shielded triad cable?

 

You could use 3 core flex for a limited distance provided it's not run where you pick up too much noise (strong magnetic fields etc). A 3 conductor shielded cable grounded at one end is the proper way to do it.

Roy

 

RTD's require 3 wire configuration for better accuracy in measurements. So, any copper conductor( High Purity ) shall be used for this. Shielding of the cable is recommended for RFI/EMI immunity to the signals.

 

BTW, the 3 conductors should be all the same size/material as the RTD is part of a bridge circuit. It is possible to use one of a different size but you need to be careful as to which one it is.

The only reason for using electrical type cable would be if it's already installed and too hard to pull in another.

Small triad cables such as "Belden" make are very cheap. You often see RTDs connected to a multi pair cable for example where there is only a couple of RTDs and several analoge 4-20 mA signals in a junction box 2 RTDs using 3 pairs.

Hope this helps
Roy

 

An RTD circuit works on a varying resistance (as a function of temperature). If the wires used to connect the RTD to the monitor/control system are long, then they will add to the total resistance of the circuit and negatively impact the feedback.

The reason for the third wire in an RTD circuit is for "lead compensation." In other words, the control system makes a calculation about the resistance of the leads and then removes that resistance from the calculation when converting the signal to a temperature.

As has been said, the third wire (the "compensating lead") doesn't have to be the third of three conductors, in a single twisted, shielded configuration (which is best for eliminating any induced electrical "noise"). A third conductor from another, second two-conductor, twisted, shielded pair can be used as the third leg. The shield drain wire from that second pair should also be properly grounded, as well as care taken if the other conductor of the second pair is not used. The "compensating" wire should be the same size and the same length for the best compensation.

Some major OEMs have used two-conductor, twisted, shielded pairs for RTD signals for decades, using one or two conductors from another pair as the compensating leads. I've seen the compensation lead of the RTDs grounded in the junction box, and the compensation wires grounded in the same junction box with no ill effects.