RTD Cable Sizing

[ThePunisher]

Hi all. I got three questions.

-We are using triad cables for RTD applications for PT-100 (100 ohm platinum). PT-100 maximum lead resistance is 200 ohms per lead. Does this implies that if I am using two wire or 3 wire, max lead resistance would be 100 ohms(see page 57 of manual).

-The maximum RTD cable distance is based on the length that will result to a lead resistance not exceeding 100 ohms (2-wire or 3 wire)

-What is the advantages/disadvantages of using 3 wire vs. 2 wire RTD wiring connections? We are going to use PT-100 on an ABB REM 531 motor relay.

http://www05.abb.com/global/scot/sc...577c9002b74b1/$file/REM54_tech_750915_ENn.pdf

Thank you for your kind assistance.

 

[oldagpe]

Your questions will all boil down to accuracy.

The more resistance in the leads, the less accurate the reading will be.

Maximum cable distance will be after accounting for contact resistances. Maximum cable will also account for cable configuration and electromagnetic interference.

Theoretically, a three-wire is more accurate than a two-wire, as the three-wire solves for contact and wire resistance. However, a three-wire may be unbalanced if you have common-mode interference.

My suggestion would be to determine the required accuracy first. Then determine the electromagnetic environment. Then you would be ready to engineer a configuration of sensors that would work.

 

[ThePunisher]

My concern is that our motor distance from the motor relay is around 300 meters, I would appreciate any advise on how this situation is applied to RTD wiring.

We are also thinking of placing an RTD module at site and run a 4-20mA signal to the motor relay in the substation.

Any suggestions is highly appreciated.

 

[waross]

An RTD circuit measures the change in resistance of a calibrated resistor with changes in temperature.
A two wire connection is only used when the lead resistance is negligible, such as may be the case when the temperature probes mounted directly on the mother Printed Circuit Board or directly connected to the PCB.
Connecting 200 Ohm leads in series with a 100 Ohm RTD and connecting to an RTD circuit will result in massive errors which may be more than the calibration range of the instrument. In the event that you are able to calibrate out the lead resistance you will still have measuring errors whenever the temperature of the leads changes due to the different thermal coefficient of resistance between the platinum probe and the copper leads.
The third wire allows the RDT instrument to measure the resistance of one lead. This information is used to compensate for the resistance of both leads.
I like 4-20 mA systems.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

I agree with waross. Put a transmitter at the RTD, then don't worry about the 4-20 mA signal so much. 300 meters is wayyy too far to go to an RTD unless 3-wire or 4-wire to absolutely compensate for lead resistance.

Good on ya,

Goober Dave

 

[potteryshard]

You might also look at the SEL 2600:

https://www.selinc.com/alt1/SEL-2600/

This converts a group of RTDs into a fiber-optic signal. I've used other of the 2600 family of convertors, and they work very well. The SEL 200 micron fiber is really easy to work with; no polishing... Just like terminating control cable.

 

[LionelHutz]

I think 300m would be fine if you use a quality cable with 3-conductor shielded sets in it. It seems to me most sites use a cable that is about 16 gauge for their RTD wiring. However, it may not be cost effective to do it this way.

I've found that the 3-wire connection is almost exclusively used where I travel. It's necessary to have at least 3 wires to compensate for the lead/cable resistance and since 3 wires is the minimum that's what is typically used.

Startco, GE Mutlilin and Benshaw all have RTD modules. The Startco and the Benshaw are both fairly simple DIN mount units with RS-485 communications.

We have also used a NT538 RTD module a number of times;

http://www.tecsystem.it/eng/temperature_controls_systems_for_electric_units.html

They are reasonably cheap, provide relay and RS485 connections and we've had good luck with them.