RTD cables 3

[giuseppe]

Hi to all.

I am presently involved in a project where lot of RTD's (Resistance Thermal Devices) are installed on field equipment, particularly on transformers and electrical motors windings.
As I am preparing the Cable List for the whole field, is there any specific requirement for the cables that will be connected to these RTD's, or I can simply use a normal copper couple? Is there a requirement for screening the couple/cable?

Thanks for suggestions.

Kind regards to all.

Giuseppe

 

[DRWeig]

Hi Giuseppe,

Copper wires will be fine, please do twisted pairs to minimize differential noise. Sensors with substantial impedance (like an RTD, typically 100 or 1000 ohms) can make an effective antenna out of your loop wires, so it's best in most cases to use shielded cable. Ground the shield drain only on one end, though -- to prevent current due to ground potential differences.

As for conductor size, you can get really small if needed. RTD transmitters typically don't put more than 10 microamps thru the loop. If your runs are long, though, consider a third or fourth wire so that you can negate the wiring resistance.

Good on ya,

Goober Dave

 

[ScottyUK]

Most industrial RTD circuits use a screened 3-wire (triad) cable. The third core is used in a poor man's version of a Kelvin connection, using only one sense wire. The elctronics compensates for 2x the volt-drop measured by the sense wire. For precision work a four wire true Kelvin connection is used.


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If we learn from our mistakes I'm getting a great education!

 

[roydm]

ScottyUK is right, you must use 3 wires (triad) otherwise you will get significant errors due to different cable lengths or temperature change.
Be aware also that you may have more than one type of RTD. It's quite common for motors to use 10 Ohm Copper instead of the usual 100 Ohm Pt.
Different curves are possible also 0.0385 or 0.0392 Ohm/Ohm/°C

Roy

 

[giuseppe]

Hi to all.

thanks for the useful information. I will see how better use them then.

Great to know that you are out there, indeed...

Best regards to all.

Giuseppe