Recently encounted an OEM installed AB-FlexIO panel where they grounded all the RTD cable shields to the 24vdc Neg rather than to the IE Ground. Is that better and any ideas why? Thanks

 

I have seen this done a few times as well. If your dc common is grounded, I guess it will work, just seems a bit odd.

Bob Peterson

 

I've seen that done with drains before, but that was in error... Check with AB, they may require an isolated ground or floating ground on that particular piece of equipment... Connecting the shield to the NEG may be the way they achieve that...

 

The shields should normally be tied to ground to provide proper function. The AB-FlexIO may have a internal connection to ground through the -24vdc, even if the unit had a floating or isolated gnd, you would probably not want to tie your shileds there. I would contact AB and ask them about this, they manufactured the unit, so I would not change anything until I knew for sure.

As to the question of it being better is a like opening a can of worms, messy, not much fun and hard to clean up. If everything is working correctly, I would not worry about it, but check with AB for your own peace of mind.

MJH

 

i suggest connecting dc_grd with shield_grd may cause troubles like mike dc noisy so it should not tie together.

 

I have seen this before as well. Generally a cost saving measure. Why have a seperate earth when you already have one on the power supply?

As you know, or you wouldn't be asking the question not the best practice for noise suppression. However with RTDs you can get away with this most of the time as the measurement method of a RTD is as robust as hell. Try it with a analogue voltage or current signal and it will bit you big time.

Regards

 

The shield should be connected to the zero-signal reference potential of the circuit. Assuming that the 24vdc Neg is the zero-signal reference potential point of the circuit, the grounding of the shield there is correct, whether the circuit is floating or grounded (the 24vdc is grounded). Please see the following reference, page 40, for a discussion of this:

Grounding and Shielding Techniques in Instrumentation, 3rd ed.
Ralph Morrison
Wiley Interscience
ISBN: 0-471-83805-5

Bill Mostia
=====================================================
William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com
Worldwide Excellence in Dependable Automation
[email protected] (b) [email protected] (h)
www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.

 

Thanks for all the feedback from everyone. Now Bill, your comment is interesting and I will try to get a copy of the publication you mention. Does the grounding as described apply only to an RTD? By this account I suppose most of us have been doing it "wrong" for years. I still question the grounding procedure considering that most floating DC systems at some point in time become "partial floating", especiallly it seems if there are thousands of feet of DC wiring in underground conduit. Thanks.

 

perhaps you could explain why you believe this?

what electrical difference is there between connecting the shield and the commons to the dc common bus and then connecting the dc common bus to ground or connecting the shields to a shield bus, the commons to a common bus, and then running a wire from each bus to ground?

Bob Peterson