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Dear All,

As far as I know we can not connect the earthing connection of Intrinsically Safe instruments to common clean earth where the shield of other instrument cables and other electronics are connected. In other words, Intrinsically Safe Earth should be used for IS signals only and not to be connected galvanically to other earth loops.

Is the above statement correct or we can share the clean earth with Intrinsically Safe signals?

Regards,
Hamid

There is no such thing as "intrinsically safe earth." All grounding points must be connected to a common earth grounding point. That said, you MUST be sure that your grounding point is actually at earth ground potential. Building steel is famous for not being a suitable grounding point.

Even copper bus bars that make up a building buried earth grounding grid may not be at true earth ground potential. The only way to validate your earth grounding point is to test it with a megger. If you have a high ground water table, and your grid is submerged in the ground water, you are OK. This is rare in most non-coastal locations.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +.978.764.4728
Fax: +1.978.246.1270
E-mail: RCaro@CMC.us
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Dick Caro

Dear Dick,

I was working in a gas plant Construction project with ENI (Agip) where the DCS was by Yokogawa.

In the earthing principle by Yokogawa it was clearly shown that there were separate earths:
1.IE Electric Ground (Instrument Earth)
2.IPE Primary Earth Ground (Instrument Protective Earth)
3.PE Plant Earth Loop
4.ISE IS Electronic Ground (Intrinsically Safe Earth)

The first 2 earths had their own loop andthey were galvanically connected to PE while ISE was totally separate and was not connected to PE.

Asking me, there must have been some sort of logic behind it, assuming Yokogawa and ENI do not do useless practices.

Regards,
Hamid

Hamid,
Yokogawa was being very conservative. In the ideal, a grounding grid submersed in ground water provides a zero resistance between all grounding points, and cannot form ground loops between grounding points. If the grounding grid is not in ground water, then different grounding points may actually have resistance between them. If there is any resistance between grounding points, then ground loop currents may happen.

The Yokogawa practice is designed to keep ground loops formed by any imbalance in the electrical power wiring from finding a ground in the instrument system, if the ground points have any resistance between them. This is the situation for dry earth grounds, but is not necessary if the grounding points are all submerged in ground water.

Dick Caro

Hamid,
First, check the requirements for earthing in an intrinsic safety system in your locally-applicable standards. The IEC 60079 series requires a resistance of less than 1 ohm between the IS earth of a barrier system and the neutral point of the relevant AC power supply system.

This is to ensure that in the event of a short between the AC supply system and the low-voltage feed to the barriers (on the safe side) the protection will operate and the voltage rise of the IS earth system during the fault will be within bounds. This requirement is usually met by a pair of large-diameter earth connections between the IS earth and the neutral point or the earthing point for the power supply neutral.

The earth loop resistance can be checked by disconnecting one of the cables at one point and measuring the resistance across the break. IEC60079 also requires that IS cable screens be connected to the barrier earth. Secondly, the IS earth is usually also connected into the instrumentation system and needs to be connected to the instrument earth or common line. For EMC reasons, there must be only a single connection between the instrumentation earth system and any other services.

The best way to do this is to construct a tree with the root being a single connection to earth common to all - power, IS, instrumentation. Power connections are one branch off this: instrumentation and IS another. The instrumentation and IS systems are separated at a single point.

Instrumentation earths should be further branched into digital and analogue sub-systems. Any connection to the rest of the world by a physical connection into a water table or well or a rod driven into the soil does not really change this - unless you are dealing with a power station, where earth faults on remote lines need to be taken into account. The metalwork on a site - including enclosure boxes, motor frames, and pipework - needs to be bonded together to form an equipotential mesh and also connected to the site earth point.

However, there may be a large voltage difference on points connected to this equipotential mesh, and large currents circulating around it. Earthing is a topic fraught with mystical beliefs such as keeping instrumentation and power earths totally segregated yet physically connected. Go back to first principles and mak sure the requiremenst are defined and satisfied.

Bruce.

all the separate grounds are typically connected together at the service point.

System suppliers often insist on separate earths thinking it will eliminate interference but at the end of the day all the earths should be tied together.

Bruce's excellent reply covers most aspects of the subject but I can send a copy of an old paper written by Chris Towle "A definitive guide to earthing and bonding in hazardous areas" to anyone that wants more reading material and contacts me off-line.

Steve
syates at mtl-inst. com
www.mtlblog.com

Thanks Steve,

I just downloaded the file from MTL.

In fact there are a dozen of nice docs there where you can download by free registration at MTL.