This topic has been covered in the forum before.

The principle is to keep the instrument earthing system from "seeing" fault current events from higher power applications and the effect of these currents on the instruments during the events. Follow the links in #1↑.

Why?

The Op sad "no continuity between them." That isn't right.

Start looking for the problem. Within the panel there will be no continuity, but when the system is connected properly there will be.

Think this way and I'll use a house for the time being where things aren't quite right.

The neutral and ground are tied together at one point. For simplicity's sake lets's assume that one point is the main panel box. The ground of the main panel is connected to a minimum of one deeply burried ground rod. I made some assumptions here. It's a TYPICAL installation.

No suppose we have a n strings of 120 VAC outlets connected in parrallel and we'll call that circuit A and circuit n.

Circuit A is very unlucky and gets hit by a lightning bolt. This means that ANY devices connected in circuit A will now have a difference in potentials (ground, line and neutral) from the outlet hit to the main panel ground. Because of that difference in potential, devices can let out the magic smoke. i.e. If you had a voltmeter attached to the ground lug of the outlet that got hit and the main panel ground, there would be a voltage drop. Ground is SUPPOSED to handle fault currents.

The REFERENCE at the panel will not be affected. So all of the other n circuits, don't see a change in potential relative to the REFERENCE. The other n circuits do not see the fault current. The voltage from the furthest outlet on circuit n to the main panel should read zero because those grounds are not CARRYING the fault current.

Again, this is a simplistic explanation and it is a system free of ground loops.

There shouldn't be any continuity between these bars before wiring the panel in the plant. These two earth bars are isolated in the panel. Once a competent electrician installs the panel, they will end up connected together through the plant grounding grid.

Unfortunately many electricians don't understand the concept and leave this "isolated ground bar" well... isolated. This means that the instruments cable shields and other instruments are Left floating.

This is why, as a panel builder, we always install a very visible wire between the two bars with a large label instructing to remove once both bars are properly connected to the plant ground grid.

• Intelligent electricians who may not understand the concept will ask questions.
• The lazy ones who don't connect the clean ground will not bother removing our visible wire. At least the instruments (and the personnel) are be safe.
• There is nothing that we can do about the stupid ones who will remove the visible wire to isolate the bar without adding a proper ground.

It would have been nice if the panels from years past had been constructed that way. Then our electricians and engineers nowadays would maybe be up to speed with this old concept. I have been working with "isolated" (single point) grounds for over 25 years and we still have techs who don't get it.

The instrument earth and the supply earth meet and are connected at one point only, usually the single point at which the supply neutral is earthed. If they meet at any other point, there is created an earth loop in which a circulating current can be induced and in which other, transient events will induce errors into the instrument signal. Earth loops are a common cause of 'hum' in audio systems.

Instrument earth should be independent of all other earth. It must be stand alone for instruments only.

Ah, you postulate a fully floating instrument system. If screens and cases are not earthed at any point at all, the protection of earthing for safety and screening is lost.

I am, I am afraid, misunderstood. The instrument i.e. it's casing/screen is run to an exclusive, stand alone earth, independent of electrical earth.

Any conductor has some resistance, and any current in the Electrical earth would thereby generate a voltage on your Instrument earth if they share a common path.

Instrument earth should be independent of electrical earth and the twine should never meet. All instruments should only be connected to its dedicated independent earth. All other electrical equipments & utility should share electrical earth.

..yes and all the low impedance independent earths are bonded to the main earth at one point ( or to the same grid with no deliberate interconnects above the grid).

Where you get problems is when there are incidental bonds occurring before the principal bonding point resulting in loops. Normally the loop impedance is higher than that of the independant earths but where it gets critical even these need to be elliminated eg insulated rack cheeks on equipment to prevent PE to QE bonds.

Earthing both ends of a screened cable results in a loop. Don't do that.

Needs to look like a tree or a star not like the stitching on a soccer ball.

Sorry, in the U.S. the IEEE and the NEC don't agree with that view.

You do have the IEEE in India, don't you?

I left my Indian guide to earthing at work. I'll have to check it in the morning.

It would be interesting to have references to IEEE and NEC documentation. I know that PME schemes involve multiple earths (it's in the words) but I'm not familiar with it in instrumentation.

Instrumentation isn't specifically addressed, just the use of multiple grounds which are not bonded.

Most of my test equipment has a switch on it to select chassis or "isolated" ground at the inputs. We also have a single point ground for our entire facility.

I found this well written booklet from Erico last night while looking for something else.

Electronic system grounding is well covered and explained there in chapter 4.

Thanks a lot for providing the best Reference...Thanks

I am a little leery of manuals which are purported to be gospel guidebooks which are published by manufacturers, distributors, sales departments, etc. I found a few, shall I say...not quite correct, interpretations in that ERICO publication. The publication itself talks about customers requesting custom work and modifications, etc. It seems more like an in-depth sales brochure or bid proposal guidebook, i.e., "This is what we want to do and this is why".

Electronic system grounding (and those requirements) are also explained by the IEEE green and emerald books (and others), NFPA 70 (US NEC), CSA CEC, IS 3043, etc. In fact, those are the standards which all of those other manuals and guides must address. The ERICO book and all of the others are intended to be easy to read interpretations and explanations of standards, not standards themselves.

I have a shelf full of these books, but I have the IEEE and NEC right on my desk.

Please take these references with a modicum of caution...and use the standards side-by-side. Learn to understand the standards, then apply them logically and correctly.

That is true.

The Erico book does refer to standards in its text.

Let's just say it was a nice compendium.

Respect.