RTD Cable Sizing
RTD Cable Sizing
(OP)
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.
ht tp://www05 .abb.com/g lobal/scot /scot229.n sf/verityd isplay/8fc 84dfe9d205 8f9c12577c 9002b74b1/$file/REM54_tech_750915_ENn.pdf
Thank you for your kind assistance.
-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.
ht
Thank you for your kind assistance.






RE: RTD Cable Sizing
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.
RE: RTD Cable Sizing
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.
RE: RTD Cable Sizing
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
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"Why not the best?"
Jimmy Carter
RE: RTD Cable Sizing
Good on ya,
Goober Dave
RE: RTD Cable Sizing
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.
RE: RTD Cable Sizing
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
They are reasonably cheap, provide relay and RS485 connections and we've had good luck with them.