Single Ended current with 4 wire Sensor

mahmoud shahien · May 8, 2018

Dear Experts,

Is it allowed to use 4 wire sensor 4-20 ma (external power loop) with
control logix analog card ( 1756-IF16 )with a single ended current wiring
Because in manual they put only 2 wire sensor (attached)

Thanks

chopin · May 8, 2018

That is from RA knowledgebase

mahmoud shahien · May 8, 2018

chopin said:
That is from RA knowledgebase

Thanks Chopin.
This is very helpfully for me.

danw · May 8, 2018

If there is a potential difference between the power supply ground in the field device and the (-) return on the AI, then the signal will have an 'offset' created by a ground loop caused by the difference in ground potentials. The offset will affect all the AI's, not just the one causing the ground loop.

This happens quite commonly with single ended AI's. If it happens, then you need a signal isolator (4-20mA input/4-20mA output) to get rid of the ground loop.

mahmoud shahien · May 9, 2018

danw said:
If there is a potential difference between the power supply ground in the field device and the (-) return on the AI, then the signal will have an 'offset' created by a ground loop caused by the difference in ground potentials. The offset will affect all the AI's, not just the one causing the ground loop.

This happens quite commonly with single ended AI's. If it happens, then you need a signal isolator (4-20mA input/4-20mA output) to get rid of the ground loop.

Thanks Danw for your comment, yes this happened with me before that all instruments connected to PLC their readings well and stable and when I connect specific instrument all instrument readings giving me wrong readings.
But what is the main reason of potential difference between the two (-ve) and how I can eliminate this??

danw · May 9, 2018

There are engineering design guide books written on the topic of control system grounding.

The bottom line is, it's too time-intensive and costly for people at the controls end or integration end to re-engineer plants to minimize ground loops. The practical solution is to use a signal isolator.

arkyny · May 15, 2018

All RTNs need to grounded.

Single Ended current with 4 wire Sensor

mahmoud shahien

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chopin

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mahmoud shahien

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mahmoud shahien

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danw

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arkyny

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