Elimination of Induced Volts in Compensating Cable


Thread Starter


Recently I faced issue of receiving high mv in my TC card of DCS leading to BAD IO. When I checked thermocouple installed, it was fine and showing 10mv while I was receiving varying voltages above 80 mV on terminal of TC card of DCS. TC is of K type and compensation cable is used to connect TC to DCS. Compensating cable type is also right.
Is it Ungrounded reference thermocouple?

U have to ground shield/drain wire of compensation cable at one end. either in field or in DCS.
Many applications use unshielded compensation wires/cables. The difference is that these wires/cables are properly isolated from high-voltage/high-current wires/cables that WILL cause induced voltages in nearby wires/cables.

Even if a wire/cables is properly shielded, if it's run in close proximity to or in the same cable tray/conduit as high-voltage/high-current wires/cables there can still be the odd erroneous signal because of induced voltages. Even switching 24 Volts at several amps can induce unwanted voltages in properly shielded cables if they are run in close proximity to each other--especially if they are in the same conduit or cable tray for long distances.

One of the most common mistakes people make is to run T/C or RTD wires to/from AC motors in the same conduit as the motor power wiring/cabling. It's convenient, and most motor manufacturers even bring all of the temp sensor leads into the motor's junction box--but they DO NOT intend nor do they expect the installer to run the temp sensor wiring in the same conduit or cable tray as the power wiring.

Most times when problems like this occur, it is improperly grounded (earthed) shield drain wires (they should only be grounded/earthed at one end of the cable). But, even when the shield drain wires (if used) are properly grounded if the wire/cable is not properly segregated from high-voltage/high-current wires/cables there will be problems.

And, again, many T/C applications do NOT use shielded compensation wires--BUT they are properly isolated/segregated from wires/cables which will cause noise. One can't just put any wire into any conduit or cable tray with any other wire or cable without regard for the type of signal or power being carried by all the respective wires/cables in the conduit/cable tray. This is one of the biggest causes of nuisance control system problems--improper installation and routing of wires/cables, in addition to improper grounding/earthing of shield drain wires.

Lastly, not every temp monitor or control system can work with grounded T/Cs or ungrounded T/Cs. In addition to Type K or Type T or other types of T/Cs, they can also be grounded or ungrounded--and the type of device you are connecting the T/C must be able to work with the sensor you are using, and that includes the grounding "type" in addition to the T/C type. You need to check the specifications of the input module/monitor to see what it's capabilities are--it could be that you have connected a type of T/C that is incompatible with the input/monitor. If this is the case, even the BEST wiring practices won't fix the problem. In such cases, I've had to use a transmitter between the temp sensor and the input/monitor to "isolate" the type of sensor from the input/monitor. Of course, this means the input will have to be moved to a 4-20 mA input, and that non-compensating wire will have to be used (though I have had some success "cheating" and re-using the compensating cable for a short period).

Hope this helps!