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Separate Instrument Earth Pit
Separate Instrument Earth pit not available


I am working on a fuel farm automation project of Airport. We have supplied SIEMENS PCS7 as DCS system. I came to know after visiting site that there is no separate earth pits are available for instrument system. All the earthing system including lightning are connected to a single grid. I am using IS, Non IS and FF devices and have separate busbars for each in the DCS panel. Since a common grid is there, I am getting continuity between all. Now the construction work at site is already completed. Is there any way I can make sure safety of my system and instruments and provide isolation?

Thanks in advance

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Why do you believe your system is unsafe?

Why do you believe your (sub?)systems need isolation?

Do you realize the whole concept of earthing is to bring everything that is grounded to the same electrical potential?

It's amazing to me how many think of the earth (i.e. the dirt) as a humongous reservoir of neutral electric charges... but then want to tap it at several distant points to provide isolation. You do in fact you provide a degree of isolation, because dirt is far, far from a perfect conductor.

However, as a result, during lightning strikes there are measurable voltage gradients between different earth surface points. This will cause currents to flow at high frequency over any electrical pathway, where the amount of current is inversely proportional to the resistance of the pathway. This includes all pathways, mechanically connected (most notably those with low-impedance) plus those that are inductively and/or capacitively coupled.

Considering the preceding, how is it more safe to have earthing points separated by distance and mechanically isolated with a measurable voltage gradient between them versus one large earthing grid that is mechanically connected both above and below the earth surface?

Now on top of the preceding, which do you give a higher priority: 1) the safety of your system, or 2) the safety of personnel?

Thanks smart for your response.

As far as basic concepts of earthing are concerned, I do accept having lack of information particularly when it comes to electrical. Based on my past experience I have learned as a thumb rule that there is requirement of three earthing system i.e. plant earth (connected to structures for lightning protection), electrical earth (for motors), instrument earth (for DCS and field instruments), and we always keep the clean earth separate from electrical. Since If you have an earth fault, a transient voltage spike will appear on the earth bar which could cause electronic instruments to malfunction? But if I refer the standards according to IEC61000-5-2 as well as NFPA, it is not recommended to separate electrical and instrument earth.

I was going through blogs and forums and found many conflicting opinions over this issue, mostly prefer isolation with electrical earth. As advised by you, if I connect them, will there be ground noise for instrument signals? Also inform the resistance value to be obtained for the same?


Let's start with whether you are under IEC or NEC(nfpa) purview?

If under the latter, all grounding electrodes present at a building or structure are required to be bonded together to form the grounding electrode system. That solves that :).

Being from the USA, I am not familiar with IEC "earthing", but I am aware it permits disassociated PE electrodes for TT and/or IT systems. When all is said and done, I have to wonder why...? Seems to me the NEC system functions just fine with all electrodes bonded together!!!

At this point I refer you to the following webpage:

Scroll down on that webpage to the section titled "Measurement of the earth-electrode resistance". This is the USA-accepted basic method for grounding electrode resistance testing, and suffice it to say, it will prove earth is far from an ideal conductor. It is not an exhaustive treatise on grounding electrodes, but the main reason I refer to it is as a demonstration that resistance exists between spaced grounding electrodes, and that resistance between otherwise-isolated electrodes is usually greater than between solidly-connected electrodes.

Now let's say you have an electrical earth electrode at one point and an instrument earth electrode at another some distance away. There is a lightning strike inline with these electrodes at some distance away from either. That strike is extremely high voltage, current, and frequency, which gets dissipated through the earth surrounding the strike point. because earth has resistance, there will be a voltage gradient impressed between the electrical earth electrode and the instrument earth electrode. Now let's say the two electrodes are solidly-connected together with an adequately-sized copper conductor. What will the voltage gradient between electrodes be for an otherwise same scenario?

Can we liken this to a bird sitting on a 100,000V power line? The voltage experienced by the bird is only going to be the current times the resistance (ohm's law) between the points where its feet make contact, right? Consider what the most common cause of permanent electronics malfunction is other than heat.

Regarding your comment "a transient voltage spike will appear on the earth bar", that may be so. However, because voltage is the potential between two points, what is the other point? With [somewhat] isolated electrodes, you actually create the another point. With solidly-connected electrodes, there is no other point, or should I say any other point on the grounding system will have such low resistance or impedance, that any voltage reading should be of negligent consequence.

That said, yes, faults do occur. But we can minimize and mitigate.

On the matter of electronic "noise", that typically has nothing to do with the grounding electrode system IMO. I believe that's more a matter of bonding and/or shielding practices. I'll relinquish that discussion to another member more versed in the specifics... :)

I think that about covers it for me... but I'm open to further discussion if need be.


Thanks for a precise reply. I agree to this, but want the precise sub-clause of the NEC and preferably any handbook reference that defines this. This i need to present to a client to support the point of view that you have mentioned.

The statement that you have specified:

"all grounding electrodes present at a building or structure are required to be bonded together"

Is this straight out of NEC? Can you specify sub-clause? I tried to do a quick search but could not locate it in the NEC copy that I have.

Thanks for the help!

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> Is this straight out of NEC?

The following is a direct quote from 2014 NEC:

250.50 Grounding Electrode System. All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(8) shall be installed and used.

Exception: Concrete-encased electrodes of existing buildings or structures shall not be required to be part of the grounding electrode system where the steel reinforcing bars or rods are not accessible for use without disturbing the concrete.

Thanks Smart!

Your clarity helps alot. I have a few more questions on the subject

1. Is it necessary (or advisable) to have a separate earth electrode (triad or other system) for instrument earth, and then connect this electrode to other electrodes? Or is it okay to just have a single "system earth" electrode and terminate instrument earth cables eventually on the master earth strip (which is connected to the system earth electrode).

2. In our case, we have two separate feeders feeding the PLC panel. Each feeder has a separate TRIAD earth electrodes at the system side. There is no bonding between these two electrodes. I think both should be bonded and one "equipment earth conductor" from the master earth strip (that is connected to both electrodes) should go to the PLC panel.

3. In the standards, is there any test interval mentioned after which the earthing system resistance / proper connectivity should be measured?

Thanks for your help.

1. Totally unnecessary... and inadvisable to have separate electrodes. Just My Opinion. Bonding electrodes together make them no longer separate.

2. You can run two EGC's, one for bonding all noncurrent-carrying metal parts along the way, and an IEGC (I=isolated) just for bonding the instrument power supply. However, you would have to make certain the instrument power supply ground and all the powered-instrumentation's field wiring is galvanically isolated from all other noncurrent-carrying metal parts for such have any effect.

In the USA, we've pretty much given up on the preceding concept. It's too easy for a downstream "ground-to-ground short" to unknowingly occur and you end up tracking down where the interference is coming from anyway... IF there is interference. In a lot of instances, you cannot tell the difference in operation. A lot depends on the instrumentation you are dealing with.

3. Nothing in the NEC. There may something in the published and accepted maintenance standards, but that's not my field.


Thanks for the reference. So now I understand the NEC calls for bonding / connecting all ground electrodes at a facility together.

I was going through IEC 60364 and it has following to say

"The earthing arrangements may be used jointly or separately for protective and functional purposes according to the requirements of the electrical installation. The requirements for protective purposes shall always take precedence."

Does this mean that the "earth system for the shields (aka functional earth), i.e. all the way from shield to the earth electrode can be kept separate from the protective earth

You have earlier mentioned that NEC and IEC code differs to some extent. Is this the difference you have been referring to?


If you are using IS under IEC rules, there is a requirement for the IS earth point to be connected to the neutral point of any associated power supplies through a resistance of less than 1 ohm. This is to ensure that, in the event of a fault placing power voltage on to the safe side of a barrier, the internal fuse in the barrier will blow and prevent the power voltage appearing on the field or hazardous side.

There are three types of "earth" commonly used:

1. Protective earth - this doesn't have to be connected to earth. It is intended to make sure that if there is a fault internal to a device such as a motor, the fault current is high enough to quickly trip any protection or blow any fuses. It is usually supplied by a separate conductor either within a cable or in parallel with it.

2. Equipotential earth - if a fault does occur within a device, and protection does not operate, the frame and any exposed metal of the device may be raised in voltage to around 50 % of the supply. It may be more if the protective conductor (see 1.) is smaller than the main power conductors as allowed under some codes for larger cables. The equipotential earth system is designed to restrict voltages on any exposed metalwork within arm's reach to safe levels. It also ensures that any extraneous voltages such as lightning, or caused by external power system faults, cannot cause dangerous voltage differences. Note that generic IEC requirements for hazardous areas also place special demands on equipotential bonding systems.

3. Instrumentation earth - this system is designed to make sure that circuits sensitive to low voltages are not affected significantly by voltages or currents in external circuits. Main measures are screening and similar effects, but relatively small currents flowing in instrumentation systems can cause problems. (I have seen a large valve caused to close due to a lead function responding to alarm lamp currents in a shared conductor.) To avoid problems, the simplest thing is to design an instrument earth as a tree, with common elements from say one PLC all grounded to one branch, another PLC to a separate one, and so on. These branches are then in turn connected to a stem - so the common earth points for all PLCs would be tied together to a PLC earth, for example. Another earth point may be used for DCS.

This is all very nice, but instrumentation systems, especially those using IS, cannot be kept completely isolated from the rest of the system. To meet the requirements for IS connection to the power source, the IS earth (which is generally closely associated with instrumentation systems and will include IS cable screen earth connections) must be securely bonded to the power supply neutral. The easiest way to do this is to extend the IS earth "tree" to the power supply neutral. Equipotential bonding can also be connected to this.

In other words, the earthing system needs to be divided up into different service areas. Each service area needs to be designed in isolation according to the specific needs of that service, and any earthing conductors kept well segregated from those of any other service. Once this is done, a single circuit should be used to tie each service back to a common earth point for the whole site.

While this common earth point can be an earth pit, don't rely on an earth electrode to make a good connection to "the body of earth". The quality of this, and its resistance, will depend very heavily on the underlying geology. I have worked on a plant where there was a very good earth mat beneath the plant, with a bare copper grid array spaced about 6 m apart, on top of what amounted to rocks in an old river bed. If there was any electrical fault from the external supply, or a lightning strike on distillation towers, our phone connection via direct wiring to the local exchange about 3 miles away would blow their fuses - but the site equipment survived. The earth mat under the plant provided an effective ground plane for the site, but could float to a relatively high level relative to the local "earth" potential.

While Bruce made a fine post about IEC rules and types of earthing, I'd just like to note there is a legal side to this matter regarding IEC and NEC compliant earthing/grounding requirements. It seems not all instrumentation professionals are cognizant of the legal issues.

In the USA, legality falls to the Authority Having Jurisdiction (AHJ). Most AHJ's have adopted the NEC into law. For the most part, the State is the AHJ, but some local jurisdictions still exist, or the State relies on local jurisdictions to enforce compliance. It is possible a few local jurisdictions not under state-wide adoption permit IEC earthing, but I am not aware of any.

That said, not all IEC earthing and bonding methods are compliant with the NEC. Most notably non-compliant are isolated grounding electrodes.

At best, the NEC permits an isolated equipment grounding conductor (IEGC). What that entails is, no intermediate connections of the IEGC to non-current-carrying metal parts are required. However, those intermediate parts are still required to be bonded, so one ends up running an standard EGC along with an IEGC.

IEGC circuits were quite the rage back in the late '80's through mid to late 90's. Since that point in time, with reduction of EMI-radiating electronics, better power supply filtering, and improved communications and signaling systems, the IEGC approach has all but disappeared from modern wiring systems in the USA.


sticking to this topic i had a specific query, thought your inputs could be helpful. As discussed earlier here, we have separate earthing network for instrumentation and power earth within control room/building however we have then later on connected them outside at earth grid locations i.e. in field, however now a problem arisen that our construction contractor has unknowingly connected a strip between these 2 earths right within the building at close proximity to our instrumentation panels. moreover they are defending the case stating technically the arrangement has no harms, as i have gone through a number of forums, his claim can be accepted to an extent,but i need some inputs as to why this arrangement may prove to be a concern. the likely cause that i have gathered is (If i short circuit the network close to instrumentation panel, there is a remote possibility that a surge might damage my panel although the earth pit still offers least resistance path) any other inputs to strengthen the case would be highly appreciated and worth learning.

It would help when discussing grounding if you (or whomever is asking) would divulge up front whether you are working under IEC, NEC, or some other Code... as none are equal.

However, the basics are the same. The major difference I see on the IEC side of the matter is they condone isolated grounding systems. The NEC does not. IMO, isolated grounding systems stem from the myth that electricity's source is the Earth.

Even with respect to lightning, solid earth (aka "dirt") is only one end of a conductive path. The other end is gaseous earth. Consider a capacitor connected to a current-limited gigavolt DC supply. When connected, each plate is charged, one positive, the other negative. When the dielectric breakdown voltage between plates is exceeded, current travels from plate to plate through the degraded dielectric. This is comparable to what happens during a lightning strike, but keep in mind the rest of the circuit loop is still connected... but it is a current limited source, which is why lightning fizzles out after a split second or three (usually).

With man-manipulated electricity, current seeks return to its source via all available paths. Earth (be it solid or gaseous portion) is technically a poor conductor on average. So when you discuss a low resistance (or rather impedance) path, consider whether Earth actually qualifies as low-impedance path to the source, or just a nowhere near low-impedance path.

Also, a typical an earthing electrode system is used as a voltage reference only for man-manipulated electricity. Otherwise, you would have a floating electrical system, other than during a fault condition. The only instance where an earthing electrode system serves any purpose for lightning protection is when air terminals and down conductors are connected to it.

> we have separate earthing network for instrumentation and power
> earth within control room/building however we have then

> ...offers least resistance path) any other inputs to strengthen
> the case would be highly appreciated and worth learning.

When you say "instrumentation" do you mean something like a stable DC voltage reference for you system, or do you mean instrument shielding?

Instrument shielding should probably be connected to the equipment ground at the cabinet to supply the shortest possible path back to the source of the noise.

Control system DC references will often have a separate ground wire back to your single point ground or grid. Every ground connection and every point in the ground system has a finite impedance and short duration voltage gradients will appear at the connections during faults. Any noise or fault currents at the new closer point of connection will cause Common Impedance Coupling with your control system reference being the "victim circuit".

If that fails, you can always ask them what they think an isolated ground is for. Sure all points in the ground system are supposed to be equipotential, but then why do we build isolated grounds in the first place? They will either be forced to say the entire industry doesn't know what they are doing, or that they'll remove the connection and connect to your equipment ground.


One main issue that designers of isolated electrode systems fail to include in their theories of operation is that "dirt" earth is also capable of voltage gradients... and quite wide ones at that when their are lightning discharges nearby. What this amounts to is if you do use an isolated electrode system, your power source must also be isolated, and if grounded, grounded to this isolated electrode system. If the power system is not isolated, you could be subjecting your electronics to a gradient in excess of thousands of volts.

Under the NEC I'm fairly certain such an isolated system is not permitted in the same building as other power systems.

I am sorry about not mentioning the background. I am working in INDIA and as per norms IEC standards are being followed here. I am myself from Instrumentation domain and a newbie, hence find a few concepts here tough to digest(understand) but neverthless I am making my attempt.

First of all thanks *Smart_S* for the concept of earth itself being at 2 potentials (dirt & air). common learning from industry for me has been to consider earth as the least resistance point and hence given consideration for being used as reference.

*Frank* thanks for providing a slightly understandable answer. I presume its because you are also from a similar domain, but I am a little shaken by the concept you preached right now.

I'll let you know what a typical circuit back here is like as you mentioned yes we have isolated earths within our control panel

1) Protective earth/Dirty earth/Power earth: for earthing the cabinet/enclosure current leakages this includes even earthing external body of power supplies/IO card racks which immediately gets connected to electrical earth grid within control room

2) Instrument earth/ Clean Earth: for earthing shield cables only, supposing we are using a 2 wire transmitter configuration where power rides on signal. here the negative or as you said DC reference is not earthed at all. it travels from IO card thru JB up to transmitter -ve. where as the shield cable running from JB right up to control panel, which is earthed on instrument earth bar and then both the earth get connected outside control room.

2 out of 2 members thought this post was helpful...

> First of all thanks *Smart_S* for the concept of earth itself being at 2 potentials
> (dirt & air). common learning from industry for me has been to consider earth
> as the least resistance point and hence given consideration for being
> used as reference.

Using as a reference is fine, but considering earth as the least resistance point is an industry fallacy that's best dispelled ASAP, for everyone's sake.

Simple proof. Drive two ground rods a couple to several meters apart. I'm willing to bet there is nothing short of running a wire between them to get the resistance between them to less than a few ohms. Yes there are "treatments" you can use to get the resistance pretty low. Many need maintained. But I think you get the point.

Now consider a same-spaced (linearly) third rod... hit by a lightning bolt. Use electrical formulas to determine the voltage potential at each rod (no wires or treatments between them). This is what happens when you have isolated earthing electrodes.

When you run wires between them (i.e. bond them all to each other), the potential voltage gradient is minimized. During a lightning strike nearby, the earth in the vicinity of the electrodes is raised closer to that of the strike voltage... but if everything is referenced to this all-electrodes-bonded system, the potential difference is simply that of the voltage system and not the strike voltage.

So your question would then be, how do I provide a clean reference for instrumentation. Logic has us conclude we should use separate grounding conductor branches, and an insulated conductor method for any "isolated" branch. Such wiring was quite popular in the US through the 80's and 90's... and quite possibly necessary during those times. Advances in electronics and restrictions on RFI/EMR have made it essentially a waste of money to implement such elaborate grounding methods in new installations... in the US.

As for IEC, such methods still have proponents... What else can I say!

By W.L. Mostia on 9 November, 2015 - 8:10 pm
1 out of 1 members thought this post was helpful...

A few years back I wrote a paper with a presentation on a primer regarding instrument grounding. It covers much of what has been discussed here and some other aspects of grounding. If anyone is interested in the paper and presentation, drop me an e-mail and I will send your a copy.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
wlmostia [@] msn [.] com

"No trees were killed to send this message, but a large number of electrons were terriblyinconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.

By W.L. Mostia on 9 November, 2015 - 8:33 pm
1 out of 1 members thought this post was helpful...

One thing I forgot is that the ISA-90 standards committee on "Recommended Guidelines for Industrial Control System Power and Grounding" has been reactivated after a hiatus of about 20 years. If you are an ISA member and want to join, send an e-mail to You can to be a voting member (only one to a company) or you can be a informational member (you get all the e-mails and documents and you can submit comments or you can lurk), just no vote on the final product. Attending meeting is desirable but not necessary. Sometimes the committee may have conference calls which you can participate in if you wish.

If you want a say in developing this standard, sign up. Uncle ISA needs you!

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.

Hi Frank,

> Instrument shielding should probably be connected to the
> equipment ground at the cabinet to supply the shortest
> possible path back to the source of the noise.

I have a question about your above comment. If I understand it correctly, you are saying that shield cables of the all the instruments that are powered up from the panel, should be connected to the equipment ground strip inside the panel?

[Equipment ground for me is the earth cable coming from the grounded system all the way to the Panel (on a dedicated earth cable). This earth cable is connected to an earth strip inside the panel which is in direct contact with the metal panel.]

This is quite different to what I have seen in the panels and will definitely clear up my concepts if such is true! In most of the panels that I have seen, shields are connected to a dedicated earth strip. This earth strip is isolated from the panel itself. All the earth strips are fed to a dedicated plant "instrument earth rod/electrode system".

But from your explanation, I gather that above is not the correct practices & instrument shield should be connected to the "equipment ground strip in the panel". Is that true?


1 out of 1 members thought this post was helpful...

As a side note, implementation of shielding connections versus bonding connections plays a major role in interference free instrumentation communications.

Shields are bonded at the power source end only (i.e. an actual termination to an outside-the-loop conductor). All else are isolated from normally non-current carrying metal parts of the system, any system.

All those non-current carrying metal parts must be bonded to the power system ground... just not via the shielding.

I would like to know some more things guys.

1) Concept of Faradays cage

2) how exactly shielding works

3) Why is it necessary to actually connect shields on isolated grounds and not power grounds if my wiring scheme implements a floating source configuration. i.e. the neutral is not connected to ground only shield wires are.

4) Do floating ground provide advantage or create more trouble? if so then why are so many organizations following the scheme (at least here in INDIA).

1 out of 1 members thought this post was helpful...

1 & 2. Plenty of info on the internet. Look it up...

3. It is not necessary to connect shields to isolated grounds... but it is advisable to connect to a stable voltage reference. That is typically "ground" but not necessary isolated ground. EMR and EMI induce electrical noise on instrument wires. Shields reflect some of the EMR and EMI noise (cage theory), some is conducted to its connected "sink" reference (typically ground), and the balance, if any, makes its way to the instrument wires... but if all is good, the balance is so attenuated that it has no effect on performance.

4. Properly implemented and monitored, ungrounded power systems offer advantages in keeping a system up and running.

The term "floating ground" is a misnomer. "Ground" does not float... otherwise would defy the definition of a "ground". "Ungrounded" is the proper term.

I'm not sure if anyone has pointed it out but your instrument shields must not be connected to ground except by the bonding conductor back to the main electrical point. They should terminate at the source on an insulated ground bar.

It's very hard to dissuade electricians from grounding the shield at the instrument which causes a ground loop and noise on the signal.
At some point you remove the bonding conductor and Ohm the bar to ground, it should be open. It only takes a short time to track down any grounded points.

What about remote panel (Eg remote PLC panel, vendor skid panels etc)? There is a shield earth bar in the panel. Should I run earth cable from this earth bar back to the single earth point which may be 100 to 200 meters away? Or should it be sufficient to connect to the earth grid cable running in the site for equipotential bonding?


>What about remote panel (Eg remote PLC panel, vendor skid
>panels etc)? There is a shield earth bar in the panel.

I think it's safe to treat a remote PLC panel as a different job and as you say ground it locally.

For sure if the home run comms are on optical fiber.

I think RS485 will be isolated enough also.

If it was a local junction box on the end of a multiconductor cable I would connect the shields all the way through so you have one shield all the way from the analog input/output to the field instrument.