AAI143-H DCS Analog Input Module Ground Loop Suspect

T

Thread Starter

topik23inst

Hi Every body,

I've problem with AAI143-H DCS Analog input module. I have 2 Ea radar level model 3301 rosemount for monitoring level on sphere tank. both level channel connected on same AI module (AAI143-H - isolated) on DCS yokogawa to radar level on sphere tank. values of both level always bounce if 2 level online. if we take offline one of both level (it just +24VDC disconnected) and -24VDC still connected to level transmitter, the other level will be steady and values corrected. otherwise, If I disconnect -24VDC too, it will affect to other level with bounce and values not corrected.

We've IE (instrument earth) & PE (protective earth) connected together. i still headache of this design. why that different grounding system joined together with jumper cable? on marshaling cab, I look that IE is bar with isolator which aims to separate so as not connected, but on the other side there is a cable that connects from IE to PE... hahaha..

I suspect, that any some ground loop on this system. but has any one can explain to me or some solutions for this issue, because I still confused what will we do for ground loop.

Thanks for reply,
Taufik
 
Check out this thread that involved ground loop issues with a Yokogawa DCS and its AAI143 analog input card:

http://control.com/thread/1410929813#1410929813

According to Mr. Fawzy's research "AAI143 last number is 3 so the card is common isolated 'the channels have multiplexer and share the same photo-coupler' ".

Apparently, some Yokogawa analog input cards are more isolated than others and you're seeing the effects of whatever degree of isolation or non-isolation that input card actually has.

My suggestion is to use a loop isolator module on one or both of the problematic loops.
 
Hello Taufiq,

Let me answer on your both points:

1. Linking PE & IE is a practice followed (mostly in Middle East)in order to minimize the effects of Lightning strikes to the Plant Structures/Control rooms where the Control systems are installed. During the course of such incidents differences of Earth potential between systems(Electrical systems) not interconnected at some point shall form an easy discharge path to low impedance grounded systems like DCS/SIS/PLCs and subsequent consequences to system malfunctioning/severe damage.

In order to overcome this situation NEC suggests for Linking PE & IE together. However it is not accepted unanimously by any Process automation OEMS hence they suggest to follow IE(Clean earth ie. on low impedance) & PE(Dirty Earth) separation to avoid accidental earth leakage current ingress/EMI/Easy earth loop return paths from High voltage systems like Electrical systems to Process Control systems like DCS/SIS/PLCs in line with IEEE Std1050-2004.


2. Preferably a Signal Loop isolator with IS spec if required shall minimize/eliminate the problems of such ground loops.

Good Luck
 
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