Rosemount DP Transmitter Grounding

[InCwithoutE]

Quick question for the grounding experts. Why does Rosemount supply grounding screws with all of their 2-wire (DC) transmitters when in the thousands that I've either installed, calibrated, or commissioned, I have never seen a ground wire pulled and/or terminated.

Thanks!

 

[Skogsgurra]

Code (in most parts of the World) says that most electrical equipment SHALL have a grounding terminal.

But, given typical installations, the transmitter housing is alredy grounded to much better grounds than the PE will ever be. So that terminal is usually/sometimes left unconnected.

But, I am not sure if that is good practice - or even legal...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dcasto]

If you have an older plant you may see a floating ground running around. I'm not a EE, but here's what I understand and have seen it in action. What happens is the transmitter references ground for zero. If the ground is really .0001 volts, then you will be off on the transmitted value. Not much. At a station where we had custody transfer of 1/2 million dollars a day, the buyer replace his transmitters and didn't use the ground point because he thought the connection was good enough. At the end of the month we were off $50,000. Found the floating ground thing and we were back on track.

 

[Skogsgurra]

dcasto,

A two-wire DP transmitter uses a 4-20 mA loop, which is floating. If one of the wires wasn't connected you would not be able to even calibrate the thing.

The ground terminal is for safety grounding and should not influence measurement accuracy.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dcasto]

Thanks Skog, like I said, I'm not the EE, just know the results of some prblems.

 

[InCwithoutE]

Thanks for the replys!

Basically, what I found is that Rosemount provides the transient voltage grounding screw (internal/external) for local code purposes. What they do caution against is running that ground wire in the same conduit as the 4-20mA loop as an errant lightning strike could cause enough transient voltage on the 4-20mA loop wiring to damage the transmitter.

 

[GTstartup]

That makes no sense to me whatsover

 

[Skogsgurra]

Probably a salesperson trying something he picked up at a seminar.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[InCwithoutE]

Salesperson?

I've been working in the I&C Field for years in Petrochemical, Refining, Power, and Water/WasteWater. I'm no salesperson....but, maybe its time for a switch...I could use the raise...

Like I said above, I've never seen a ground wire landed on the internal or external ground screw.

Just to clear up any confusion, lets keep in mind I'm not talking about the *Shield* wire thats normally cut back, wrapped around the single conductors, and taped off in the transmitter housing. I'm actually talking about a ground conductor, not the shield provided in a twisted shielded pair.

I found the info right in the manual provided by Rosemount.

http://www.emersonprocess.com/rosemount/document/man/4809b00j.pdf

Page 2-43

"NOTE
The transient protection terminal block does not provide transient protection
unless the electronics case is properly grounded. Do not run the transient
protection ground wire with field wiring because the transient protector ground
wire may carry excessive current if a lightening strike occurs. Grounding the
electronics case using a threaded conduit connection may not provide
sufficient ground."

Thats out of the manual for the 3095. during a search I believe I found it in the 3051 manual as well.

 

[Skogsgurra]

So, it is a salesperson that turned into a manual's author.

But, the truth is that there could be some technical background to that. Not inside a factory building. But in an oil refinery or some other wide-spread plant where wires pick up transients during a thunderstorm.

They do so by induction or influence (if not properly screened) or potential difference between two parts of the plant and a direct hit is not needed to cause problems. Nothing survives a direct hit. No matter how your wiring is connected or where you put your wires.

But, to answer the OP, Transmitters and controllers are usually screwed down to grounded things like panels, cabinets, valves, tubes etcetera. So the extra grounding that the ground terminal provides is of little practical importance. But code probably requires a wire to be connected to it.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[InCwithoutE]

Excellent! Your bottom paragraph was what I was trying to verify. A project (SBR WWTP) that a co-worker of mine is inspecting recently installed several of these. On his punchlist, he directed them to ground the Xmtr. I mentioned to him that this wasnt a common practice as the transmitter is usually grounded directly to the pipe its mounted on or tubed from, etc...

Thanks for all the replies!