EMI on source and sink sensors

[Azek]

Hi,

Lack of wire pairs in IS cable. Used EXdei (4-20MA output) transducers (source) which were installed in the field to connect three PT-100 sensors via non-IS cable, through WAGO AI modules down to PLC. When starting the motors driven by VFD, the feedback from the sensors becomes unstable and way above the actual value. Could not find anything obvious to attenuate the noise. However, the feedback from other (mainly pressure transmitters) sensors with different configurations (sink) installed on the same equipment are pretty stable.
I wonder why source-configured sensors are so susceptible to EMI and why EMI is so merciful for one and aggressive to the other ones?

Appreciate your help beforehand.

 

[danw2]

Depends on where and how the cabling runs and what type of cable is used, as to whether the cable picks up electrical noise.

The couple sites I picked at random for intrinsically safe cable had 3 types of I/S cable, only one of which used twisted pair. All 3 cables were shielded. So even I/S cable is not necessarily twisted pair, which is what is typically used as instrumentation cable.

If the PT's use shielded twisted pair, then they're much less likely to pick up electrical noise.

 

[lacajun]

I learned decades ago to use TSP, with an overall shield, for analog and digital signals. Some industries don't do that, which is puzzling to me. They believe that voltage signals are immune to EMI.

Pamela K. Quillin, P.E.
Quillin Engineering, LLC
NSPE-CO, Central Chapter
Dinner program:

http://nspe-co.org/events.php

 

[hacksaw]

What is the RTD model number and design dimensions.