Grounding

[leomiranda]

I’m in doubt about grounding instruments. I have EExd instruments and IS (intrinsically Safe) Instruments. I saw a long time ago one project showing 2 grounding bars, one for EExd instruments and another one for IS instruments. Should IS instruments grounded in a separated bar? Is there one standard saying this?
Sorry for my poor English!!!

Best regards

 

[roydm]

It's probably good practice to use a separate ground so that any ground fault currents don't show up in the field. I think though that they will end up at the same common ground point (i.e. connection to the ground mat).

Roy

 

[JLSeagull]

I don't know that this is the latest version, however, ISA 12.06.01 3.14 intrinsic safety ground system states the following:
"a grounding system that has a dedicated conductor isolated from the power system, except at one point, so that ground currents will not normally flow, and that is reliably connected to a grounding electrode in accordance with Article 250 of the NEC ®."

 

[JohnnyHS]

May be a bit late for me adding my 10 pence as i have just seen this post (but may be usseful for others dropping by) but some installations run separate earths, one for the motors and heavy equipment and all the non IS equipment and also run a seperate second IS earth also. Sometimes called "dirty" and "clean" respectively. This is to absolutely maintain the integrity of the Intrisic Safety system as the motors and heavy equipment that can use the joint earth (if the IS components were connected to it) in a fault condition may exceed the IS cicuit maximum ie more than 1.2V or 100mA on the earth. if this was to occur then technically the IS circuit would be compromised and would not be Intrinsically Safe anymore. (The concept of IS being to not have enough energy to cause a explosion/ignition) whereas the other ATEX protection methods contain said explosion so you can ramp up the voltage/current as much as you like.



Johnny H-S BSc(Hons) MInstMC

Engineers like to solve problems. If there are no problems handily available, they will create their own problems.

 

[ScottyUK]

"...the other ATEX protection methods contain said explosion..."

Well, that's only really true of Ex 'D' isn't it? All the others rely on preventing the explosion occuring: 'E' and 'N' basically pray to God that nothing involving sparks will happen in among their cheap and flimsy innards otherwise it's Game Over, and most of the others ('M', 'O', 'P', 'Q') use some means or other of preventing the explosion happening by preventing the spark coming in to contact with the fuel-air mix.


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If we learn from our mistakes I'm getting a great education!

 

[JohnnyHS]

Sure scottyuk right on the money. I should have said EExd contains.

This is a good link for explanations for those i have confused!

http://www.gfienterprises.co.uk/Atex.html

Johnny H-S BSc(Hons) MInstMC

Engineers like to solve problems. If there are no problems handily available, they will create their own problems.

 

[Ulon]

Hi all,

Johnny HS reply has already give the answers. Ijust want to add a bit more.

Typically, all electrical equipments shall be gorunded on the dirty earth and all instrument shall be ground at clean earth.

But if we have mix of Flameproof (EEx d) and IS (EEx i) instrument, we need two clean earth (even saperate cables), one for EEx d instrument and another one for IS instrument. Tppically, the IS clean earth shall be at the IS barrier.

Main thing is:

Flameproof = Explosion shall contian inside the secured area

IS = current limiting. By doing this, spark cannot accours.

 

[JLSeagull]

Provide separate bars for the IS ground and other ground systems. Those ground bars are then tied to the grounding grid. This is often done with a triangular arrangement with three earth rods that are tied together via copper conductor. One rod is connected to the IS ground, one to safety grounds like system cabinets etc. Perhaps the isolation transformer ground ties to the same sfaty ground or a third rod. The third rod ties to the ground grid if such exists. Also, if three rods don't provide less than one ohm earth resistance, branch from each rod to additional rods.

One of those IEEE color books covers grounding details too.