Scada read noisy values with indusoft

[engma 0]

I use indusoft to make scada with s71200 1214dcdcdc. I measure temperature using pt100 and RTD modules 8AI. I connected and tested the sensors in lab and it was working good and scada system was working perfect. When I moved to the field and connected the pt100sensors ,the values were changing on scada with random values including the real values.i don't know where the problem. Please help

I figured that the neutral wire is shorted to earth at the field. Could this cause the problem?

I used shielded cable and I connected one of the sensors shield to ground which is the neutral but nothing changed. The connection was only from the plc side 

Edited 14 Oct 2022 by engma

[DanW 41]

Commercial RTD’s are inherently isolated to prevent ground loop voltage/current (caused by a connection to ground anywhere in the lead wire or sensor) from affecting the constant current source generated by analog input. 

A ground connection of the sensor element can cause symtoms like you describe.  Check for continuty between any lead wire and the element sheath (disconnect from the analog input while testing): it should be an open circuit, an extremely high resistance.  Make the same continuity check between each lead wire and the shield ground: again it should be an dxtremely high resistance.

if you get a low resistance reading, replace the RTD assembly, unless you can find an obvious insulation problem or short in the lead wires.

The lead wires should not run in AC power conduit or parallel AC power wiring in cable trays.  

The nature of a ground loop is such that a ground fault connection in any of the connected RTD’s can produce a problem with the other connected signals, so if more than one RTD is check the others as well.

If there are junction boxes in the field wiring, check for dirt, spider webs, insect nests, and especially moisture or water, any or all of which can provide a path between a signal and ground.  

Intermittent spikes fully upscale or downscale is typically indicative of an intermittent loose connection.   

Edited 19 Jan 2023 by DanW
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