Instrumentation Earthing

in my experience there are two earth pits available in the plant. one for Signal(electronic) earth and the other protective earth. signal earth is direct from electronic earth pit. all other earth requirements can be connected to protective earth (station earth bus).this arrangement will not affect the operation of electronic system (analog system, controller etc)

There seem to be numerous recommended practices minus the reasons for the practices.

MTL's tech note TP-1121-1

http://194.203.250.243/Download.nsf/d6af6d051404caf880256c84004a0605/62c490e19b058b45002569f2004e4a25/$FILE/Tp1121-1.pdf

addresses grounding and bonding in hazardous areas. Instrumentation could be considered 'sensitive devices' discussed at the end of page 5 (figure and text below).

Figure 11 is associated with its text below, which indicates that impedances develop voltage drops when current flows (ohms law), and that impedances can be expected between grounds at different points.

Commonly, sensitive circuits are bonded to the
structure at one point so as to avoid some of
this structural current circulating through the
sensitive circuit. The general principle is
therefore to bond sensitive circuits to the
equipotential plane at one point only and to
choose the point carefully so that the current
return paths do not share a common path with
the structural currents. The principle to be
followed is illustrated in figure 11 (above).

The transient current to charge the motor
capacitance flows through the structural bond
back to the neutral star point. If the 0V of the
computer is returned to the neutral star point
Y then the common mode voltage generated
by the surge current is across the relatively
small impedance XY and is possibly acceptable.
However if the computer 0V is connected to
the structure at the point Z then the common
mode voltage is related to the large impedance
ZX and hence usually causes difficulties.

The problem associated with multiple bonds to the
structure is illustrated by the thermocouple
which is in contact with the motor. If the
amplifier does not have isolation between the
input and the computer 0V, a proportion of
the transient structural current will flow
through the parallel path created. Since the
current flows through the sensitive input
circuit of the amplifier it will cause a
measurement problem or could possibly
damage the circuitry.

The current book on the subject:
Title: Control System Power and Grounding Better Practice
Authored by Roger Hope, Dave Harrold, David Brown
ISBN-13: 9780750678261, ISBN: 0750678267
a trade paperback published by Newnes

A good portion of the book appears as the basis of an article in Design Line, in three parts (only two of which I can find online:
part 1:
http://www.industrialcontroldesignline.com/showArticle.jhtml;jsessionid=SU3G4JPAT4JYVQE1GHRSKH4ATMY32JVN?articleID=208803203&queryText=Control+System+Grounding

part 2:
http://www.industrialcontroldesignline.com/showArticle.jhtml;jsessionid=SU3G4JPAT4JYVQE1GHRSKH4ATMY32JVN?articleID=209100218&queryText=Control+System+Grounding

Thanks Buddy

Hello no problem for the transformer and it will not damage anyting, but may be distortion in signals cables.

Instrument earth and Protetive earth should be seperated.

CARL E has replied elaborately to your Doubts.<p>

You have the doubts that whether Transformer fault current will pass through the instrumentation.<p>

No.<p>

Only to avoid problem of any transient voltage appear on instrumentation circuitry we link instrument earth bar to Station earth only at on point.<p>

By this the entire Instrumentation circuitry as whole elevated to Station Earth potential and there is no possibility of any cross current flow through the Instrumentation Circuitry<p>

Therefore your present arrangement of earthing is safe<p>