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Isolated Ground Reference Two

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Isolated Reference Ground

An equipment technician wants me to install an isolated ground (three ground rods) for some new electronic equipment we received from overseas. The technician states that the purpose is to reduce ground noise so that the software would operate properly, and he wanted the reference ground to be independent of the building ground (isolated). In addition, he did not want me to install an equipment ground from the panel because he felt that this would defeat the purpose of the isolated reference ground. I'm told that if I don't comply then the equipment warranty would be voided.

1. What is the purpose of the isolated reference ground?

2. Am I required (by the NEC) to bond the reference ground (three ground rods) to the building structural steel that's part of the grounding electrode of the building?

3. Is there any danger if I don't install an equipment ground from the panel to the equipment?

Mike Holt's Beginning Comment: Almost all of the comments I received were wrong. Therefore, I edited the best responses I received to include at least the correct comments.

From: Karl Riley

1.      The installation you describe (no equipment ground) violates NEC.

2.      A separate electrode not bonded to the building electrode system will cause havoc with the electronic equipment if there is any nearby lightning strike or if in a storm a surge is produced on the grounding network. Your isolated grounds may be at thousands of volts different from the building grounding point and everything in the building referenced to it. Fire is a real possibility. You can also expect small voltage potentials in normal usage, which will corrupt the ground reference the equipment may be relying on.

I would ask the vendor for a document, which states that he, accepts liability for any results of an installation, which violates NEC. Nevertheless, I trust that your local AHJ would not pass it. There's no harm to install the rods as long as they are tied into the building's grounding electrode system. That's my take.

From: Nelson, Ron

This is a violation of the code if you don't install an equipment ground from the panel to the electronic equipment. It is common for some foreign manufacturers to void warranties if you connect the reference ground to the building grounding system. The company is in the middle. There can be damage to electronic equipment when an isolated system is installed.

From: Scott E. Thompson

I have had this situation several times with equipment vendors. They claim that bonding the reference ground to the building grounding electrode system will force noise into their equipment and no connection to system ground will drain off noise. Because it's possible for power system to have a ground fault to this equipment, then it should have an equipment ground.

Check out the IEEE Emerald book for further information on this subject.

What is the purpose of the isolated reference ground?

A: Mostly it is. It also helps limit the transient voltage / current surges - spikes generated on site. When branch circuit runs through metallic conduit, the conduit tends to "drain" EMI - RFI noise to ground, which leaves the I.G. "clean." if a ground loop is created, then noise will be generated to electronic equipment.

Is there any danger if I don't install an equipment ground from the panel to the equipment?

A: Yes, electric shock.

From: Dela, Sonny, GLENDALE, Engineering

What is the purpose of the isolated reference ground?

Reduce electrical noise (electromagnetic interference) on the grounding circuit; see Sections 250-96(b) and 250-146(d).

Is there any danger if I don't install an equipment ground from the panel to the equipment?

It is dangerous if the equipment is not properly grounded. Section 250-96(b) permit to have an isolated ground and this should also serve as your equipment ground. Fine print note (FPN) says that even you have an isolated ground to this equipment the raceway still need to be grounded.

IEEE Std 1100, section discuss their recommendation for IG circuits.


This is a common situation with equipment manufacturers. You need to convince him that an isolated ground is one that terminates at the service equipment or the sources of the separately derived system. If you install the "isolated ground" that he is asking you to install, you will not comply with NEC.

From: Harbaugh, Don

What is the purpose of the isolated reference ground?

Noise; see Section 250-96 (b)

Is there any danger if I don't install an equipment ground from the panel to the equipment?

Yes, 250-96 (b) allows a non-metallic spacer in the conduit of a separate branch circuit supplying only this machine. It does require insulated equipment grounding conductor with the branch circuit conductors and attached to the machine. It does require that the conduit ahead of the spacer be grounded. This was a change in the '93 NEC. It does not allow this machine to be ungrounded.

From: Mike Herman,

Short version: this tech guy is an idiot.

What is the purpose of the isolated reference ground?

Noise is highly exaggerated; it's usually caused by the equipment itself actually.

Is there any danger if I don't install an equipment ground from the panel to the equipment?

If they wish to isolate it completely to those new ground rods, no way no how.

I have built my share of factories that use high tech software and hardware they all have a GOOD ground. Some may have an isolated ground but it is always in compliance with the NEC.


I will make this short but sweet. Don't install this as requested. Read article 250 of the NEC. This type of installation will violate several articles'.

What is the purpose of the isolated reference ground?

The manufacture thinks that this will reduce noise, but it may actually aggravate it.

Is there any danger if I don't install an equipment ground from the panel to the equipment?

Yes, there is danger to personal due unsafe voltage levels if the panel power or equipment malfunctions.


A good reference is the IEEE Emerald book, which probably deals with this kind of a situation and may even have a case history.

From: John Knighton

Mike, when we installed electronic equipment in the years I worked for industry we always used the main ground to the power panel. Some of the European equipment I worked with sometimes the ground was not connected to the secondary of the transformer to prevent false tripping on the drive systems. We did find out the best way to protect electronic equipment was to use a power conditioner so that whatever the situation the power conditioner would protect the output voltage. Noise suppressers are commonly used to reduce noise with power with high harmonics.

From: Sergio El´┐Żas Castillo Mendoza

What is the purpose of the isolated reference ground?

The insolated ground purpose is to reduce noise caused by lightning, motor, electronic equipment and anything's other equipment in systems.

Is there any danger if I don't install an equipment ground from the panel to the equipment?

The correct installation of equipment is: Bring the equipment two ground (normal ground and isolation ground). The normal ground should install chassis equipment and the isolated ground shoulder install the isolated ground equipment.

From: alan.halberstadt

All equipment with metal non-current carrying parts have to have an equipment ground. The equipment should be built such that the electronics are isolated from the case, in which case it doesn't matter if you ground the case. If there is no equipment ground, then there is no way for the overcurrent protective device to operate under a ground fault, meaning possible energized of the equipment case.

From: Coward, Neil

My understanding is code requires running an equipment ground from the panel serving the load so a fault to ground has a hardwired path back to the source for clearing the overcurrent protective device without depending on the earth as the fault current path. From the panel you run both an equipment safety ground and an "isolated" ground for the reference ground in the electronic equipment. When I have done this, made sure the isolated ground is only connected to ground back at the source, ground loops and "noisy" grounds were eliminated, and the equipment ground is still connected to the enclosure for worker safety.

In fact, I saw one installation, like described in you message that actually was giving intermittent problems with some sensitive electronic equipment. When their power source was reworked as described above, they had no more problems. When you do an installation like this you also have to be sure your customer is not shooting himself in the foot by introducing multiple grounds through the signal lines, with grounded wires or shields, connected from one piece of equipment to another.

These types of installation are addressed in FIPS PUB 94, Guideline on Electrical Power for ADP Installations, September 21,1983. This can be downloaded from Mike's site at

From: Barnett, Will

The potential hazard with trying to create a truly isolated system comes from the fact that the earth is not really a perfect conductor, nor a perfect insulator. Since ultimately both systems connect to earth, they will be coupled through some resistance. When two separated (and not bonded together) grounding electrodes exist with a finite earth resistance between them, any current flowing in the earth between the systems will cause a difference in potential between them. Ironically, this may create the "noise" or "ground loop currents" that was attempted to be avoided. The difference in potential between the electrodes creates a possible shock hazard.

A ground fault on an isolated ground will not cause the overcurrent device ahead of it to clear since there is no low impedance path back to the source. Article 250-2(d) states, "The earth shall not be used as the sole equipment grounding conductor or fault current path."

From: Eric Stromberg

Suffice to say, I've spent a good number of years on the grounding issue as it pertains to audio installations. (Churches, theaters, auditoriums, arenas). In these systems, one can "hear" the grounds. (Or more specifically, the ground loops that have circulating currents in them). I'm always humored, if not somewhat nauseated, by the immediate suggestion to remove the system ground. In my experience, the problem has always been much deeper in the system. Good, careful, roll-up-the-shirtsleeves-i'm-going-in-my-room-don't-bother-me-just-slide-me- pizza-under-the-door-every-couple-hours, approach where all the i's are dotted and t's are crossed is the real solution. Removing the system ground, to me, has always been something akin to painting over the wound of the patient with flesh colored paint to make it look better.

On a different note, I've seen a rather interesting 'solution' used in Europe. In a couple instances i saw where the computer system had it's own, single point, ground but then this ground was also connected, through reactive components, to the main building ground.


Mike Holt's Comment:

WOW, I got so much e-mail on this, unfortunately most of all the responses were incorrect. Let cover the basic requirements.

1. All equipment must be grounded by an equipment grounding conductor (green or sometimes green with yellow stripe wire) supplied with the branch circuit conductors to provide a low impedance path to facilitate the opening of the circuit overcurrent protection device [250-2(b)].

2. Where equipment is properly grounded to a low impedance path (equipment grounding conductor), the equipment can have a supplementary grounding electrode to provide a local reference, but the earth shall not be used as the sole equipment grounding conductor [250-2(d) and 250-54]. The requirement of Sections 250-50 and 250-58 that all electrodes be bonded together does not apply to supplemental electrodes as permitted with Section 250-54.

This means that if you want to supplement the equipment ground by adding a reference ground for the equipment (let's say three ground rods) this is permitted by the NEC and the reference ground is "not" required to be directly bonded to the building grounding electrode system. See the NFPA's comments below.

Comments from Jeff Sargent - NFPA:

"The key is that the use of a supplemental grounding electrode to provide a "local reference at a piece of equipment" is not a "system ground" as is referenced in 250-58. Note that the Code references in Section 250-58 are to the "supply system grounding requirements" for services [250-24] and separate buildings [250-32]. Section 250-54 clearly permits the use of supplemental grounding electrodes to augment equipment grounding conductors, and in reality the supplemental electrode ends up bonded to the grounding electrode system through the equipment grounding conductor."

New - From: John West

Mike, I read both your comments and those of Jeff Sargent. The only issue that can come up is the "ground loop" if the supplemental electrodes are not bonded to the building electrode. In most stand alone installations thee no problem with ground loops, however computer networked installations ground loops can be a very big problem.

Mike Holt's Comment:This might be true (ground loops) but it has no bearing at it relates to the NEC. The system designer is permitted to add a supplemental reference electrode if he/she desires and the NEC does not give any instructions on the type of electrode, its installation requirements, the conductor size etc. If the designer is concerned about ground loops (NEC doesn't care) then the designer can specify the reference ground to be bonded to the building electrode. If the designer elects not to bond the reference ground to the building electrode this is okay by the NEC, because it's not unsafe!


Code Making Panel 5 has accepted a change to Section 250-54 (regarding the supplemental grounding electrode) for the 02 NEC. It will state that " supplemental grounding electrode shall not be required to comply with the electrode bonding requirements of Section 250-50 or the resistance requirements of Section 250-56." T

Mike Holt's Comment:If the 02 NEC keeps this text, it will go a long way is resolving this issue. Looks like we're 1/2 of the way there.


After reading the comments regarding the supplemental ground, I am not sure I understand the rational for separate ground planes. I am in the telecommunications field and have seen many of these requirements (isolated ground planes), particularly from foreign manufacturers. I can think of two reasons that I would not want to see separate ground planes.

First, where RF antennas are deployed (or similar) on the exterior of a structure and subject to lightning strikes. It is desirable to shunt these currents to earth as rapidly and safely as possible and to keep all metallic part of a building at the same voltage reference to minimize differences of potential. How can lightning be shunted to the earth if separate ground planes are utilized (one for the building and a separate one for the RF antenna)?

Secondly, if a fault occurs in the electrical system to the RF antennas how can you insure the protection device will trip, particularly if the soil impedance is high?

I may have misunderstood the implications of the text, if so please clarify it for me. We never permit separately derived systems for telecommunication systems and do not know anyone that does.

Mike Holt's Comment: My newsletter about a separate ground reference plane does not apply to the grounding of "communications systems". Communication systems (Chapter 8 of the NEC) have their own specific grounding requirements, and one of them is that they "must be bonded to the building grounding electrode system". This topic is covered in detail on my website: under Satellite Dish.


I agree with the answers you gave to the question but I would like to add the following. I have encountered system designs that require a tri-ad ground for each of two electrical services in the same building. Typically in I/O buildings, one three-phase 120/208 volt, service supplying the building power and the other three-phase 120/208 volt service is for field instrumentation equipment and instruments. The design specifically states that the two tri-ad grounds (one for each service) be completely isolate from each other.

I feel the NEC requires grounding electrodes for all services in one building to be bonded together to prevent a "difference of potential" between the two grounding systems. I have been involved in trouble shooting old installations (were multiple services in one building were not bonded together) and I have read as much as 100 volts between the grounding systems.

I know that many "instrumentation engineers" do not want the service grounds bonded together, but this can lead to dangerous conditions if the two grounding systems are not at or remain at the same potential.

I just felt that some of the people asking this question might indeed have this situation in lieu of an isolated reference ground for electronic equipment.

Mike Holt's Comment: NEC Section 250-58 - Where multiple services for grounded ac systems supply a building, the same grounding electrode shall be used for all service grounding. The rule also clarifies that where two or more grounding electrodes are effectively bonded together, they are considered as a single grounding electrode. The easiest way to achieve is to use the building foundation or footer steel (Ufer electrode) [250-50(c)] or the metal frame of the building [250-50(b)] to form the grounding electrode system in accordance with Section 250-50.

For more grounding information, see Isolated Ground One.

For more grounding information, see Sensitive Electronic Equipment - ITE Standard. [PDF]