Independent Ground????

[5 S electric]

We have an electron microscope. The guy setting up the piece of equipment says that he needs an independent ground. Totally separated from the building ground. This sounds like a big code violation. has any body ever heard or done this? I worry about having a difference in potential and getting somebody hurt. Please let me know your thoughts.


Thanks in advance.

 

[charlie b]

My thoughts are that you are on the verge of allowing language to get in the way of communication. Ask the guy what he thinks the word "ground" means, in the context of his requirement. There are essentially two separate concepts in which the electrical trade uses that word. One is a connection to planet earth. If that is what he means, then I would wonder what point on his equipment the other side of the wire is attached. If it is the external metal case, then that is also connected to the equipment grounding conductor, which in turn is bonded at the main panel to the neutral bus, which has a wire that runs out to the grounding electrode system. So if he wants you to run a separate ground rod from his equipment, it will not be separate from the building's grounding system. Indeed, as you seem to be pointing out, if you drive a separate ground rod, you are required to bond it to the rest of the building's grounding electrode system.

The other concept for the word "ground" is the low impedance path from the case of the equipment back to the panel, so that a short circuit inside the equipment will cause the breaker to trip on high current. But as I mention above, that is already connected to the building's grounding electrode system.

So I don't understand the nature of his requirement.

 

[charlie b]

By any chance, could the guy be asking for an "isolated ground," as allowed by 250.146(D)? Perhaps he sees that phrase in his equipment's operation manual, and has misinterpreted the meaning of the phrase.

 

[Tony S]

I don?t want to put a dampener on things. I?ve used this same setup and the result was far from a clean earth (ground).

250.146 D
Isolated Ground Receptacles.

Where installed for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted.

The receptacle grounding terminal shall be connected to an insulated equipment grounding conductor run with the circuit conductors.

This equipment grounding conductor shall be permitted to pass through one or more panel boards without a connection to the panelboard grounding terminal bar as permitted in 408.40, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.

Where installed in accordance with the provisions of this section, this equipment grounding conductor shall also be permitted to pass through boxes, wireways, or other enclosures without being connected to such enclosures.

Informational Note: Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system and outlet box.

 

[Strathead]

One of the most abused concepts I have come across over the years. Many times, I have especially seen it to this day with Point of Sale systems, they require an isolated ground. When you ask the right questions they conceive this to be a orange receptacle with a ground wire bonded to the box and run back to the branch panel without any other grounds tied to it except at the ground bus of the branch panel. A real isolated ground can be looked at as another grounded conductor just like the neutral that is bonded at the same point as the neutral, but intended to provide the ground path for equipment designated. It is important to note that when providing a real isolated ground, the boxes conduit and metal parts leading to the equipment must be grounded with a normal grounding conductor. ( I know conduit can serve). This often means running the isolated ground through panels all the way back to where the service is. An alternate is to provide an isolation transformer to create a separately derived system.

Worse, to add to the confusion, there are some who want a bond to their metal enclosures that is directly connected to earth, and they want you to use this as the NEC required ground. Where I have seen this mostly is audio contractors, who have this concept of shielding and radio interference. This group is dead (pun intended) wrong and you must refuse to do this.

The first is often unnecessary, but if it makes them feel good fine. I have seen engineers spec out isolated ground receptacles and almost always fail to indicate the required equipment and extra wiring required in the distribution system to create the isolated ground. Over the years I have come to the concept that the main real purpose for isolated ground was, sensitive electronic equipment uses various voltages, 3v, 5v etc. to ground to operate, these voltages need to be pure, and transient grounds voltages can affect them causing data transfer errors, parity errors etc. Most computing equipment the general and commercial public uses today is not sensitive to these transient voltages and the extra work isn't necessary, but if they want to pay for it...............

 

[kwired]

One of the most abused concepts I have come across over the years. Many times, I have especially seen it to this day with Point of Sale systems, they require an isolated ground. When you ask the right questions they conceive this to be a orange receptacle with a ground wire bonded to the box and run back to the branch panel without any other grounds tied to it except at the ground bus of the branch panel. A real isolated ground can be looked at as another grounded conductor just like the neutral that is bonded at the same point as the neutral, but intended to provide the ground path for equipment designated. It is important to note that when providing a real isolated ground, the boxes conduit and metal parts leading to the equipment must be grounded with a normal grounding conductor. ( I know conduit can serve). This often means running the isolated ground through panels all the way back to where the service is. An alternate is to provide an isolation transformer to create a separately derived system.

Worse, to add to the confusion, there are some who want a bond to their metal enclosures that is directly connected to earth, and they want you to use this as the NEC required ground. Where I have seen this mostly is audio contractors, who have this concept of shielding and radio interference. This group is dead (pun intended) wrong and you must refuse to do this.

The first is often unnecessary, but if it makes them feel good fine. I have seen engineers spec out isolated ground receptacles and almost always fail to indicate the required equipment and extra wiring required in the distribution system to create the isolated ground. Over the years I have come to the concept that the main real purpose for isolated ground was, sensitive electronic equipment uses various voltages, 3v, 5v etc. to ground to operate, these voltages need to be pure, and transient grounds voltages can affect them causing data transfer errors, parity errors etc. Most computing equipment the general and commercial public uses today is not sensitive to these transient voltages and the extra work isn't necessary, but if they want to pay for it...............

And even more important is the fact that if something goes wrong and the only ground fault return path is via the earth it leaves us with a high possibility of hazardous voltage present on conductive surfaces of that equipment. Earth resistance is seldom low enough to allow for high enough current flow to operate an overcurrent device - especially for under 300 volts to ground systems.

 

[JRW 70]

Scanning Electron Microscope

Scanning Electron Microscope

Years back (not too far), there were no special requirements
for the SEM that I recall other than a very good surge protector.
We used it to look at crystalline structures for quality control.
And to the best that I can recall, the SEM just used the buildings
common ground through the UPS and surge protector.
It may be good to look at the actual installation inst. rather
than relying on second hand information.
A rod at the SEM while connected (or worse not) to the
EGC may cause damage to this sensitive and expensive
piece of equipment.

JR

 

[5 S electric]

My thoughts are that you are on the verge of allowing language to get in the way of communication. Ask the guy what he thinks the word "ground" means, in the context of his requirement. There are essentially two separate concepts in which the electrical trade uses that word. One is a connection to planet earth. If that is what he means, then I would wonder what point on his equipment the other side of the wire is attached. If it is the external metal case, then that is also connected to the equipment grounding conductor, which in turn is bonded at the main panel to the neutral bus, which has a wire that runs out to the grounding electrode system. So if he wants you to run a separate ground rod from his equipment, it will not be separate from the building's grounding system. Indeed, as you seem to be pointing out, if you drive a separate ground rod, you are required to bond it to the rest of the building's grounding electrode system.

The other concept for the word "ground" is the low impedance path from the case of the equipment back to the panel, so that a short circuit inside the equipment will cause the breaker to trip on high current. But as I mention above, that is already connected to the building's grounding electrode system.

So I don't understand the nature of his requirement.

No this tech wants a separate ground not tied to the building. He wants to disconnect the EGC feeding the equipment and rely on the ground rods. This building was just put up and i am turning this problem over to the general contractor. I do not feel safe having a separate ground not bonded anywhere. I tried talking him into an isolated ground. He would not go for it. her thinks that he will still pick up noise. I really don't agree with what this tech is proposing.

 

[GoldDigger]

No this tech wants a separate ground not tied to the building. He wants to disconnect the EGC feeding the equipment and rely on the ground rods. This building was just put up and i am turning this problem over to the general contractor. I do not feel safe having a separate ground not bonded anywhere. I tried talking him into an isolated ground. He would not go for it. her thinks that he will still pick up noise. I really don't agree with what this tech is proposing.

Whether or not you agree with what the tech is proposing, the NEC prohibits it for valid safety reasons and that should be the end of it.
Even if you install an isolation transformer to create an SDS feeding the machine, I think that the ground electrode used still must be bonded to the other ground electrodes for the building.

 

[charlie b]

He wants to disconnect the EGC feeding the equipment and rely on the ground rods. . . . I really don't agree with what this tech is proposing.

The tech is ignorant. This setup might reduce noise; I cannot speak to that question. But it is a "death-by-electric-shock event" that is simply waiting for its turn to present itself. It is illegal; it is dangerous; it is stupid. If this really is what the tech is asking you to do, then I suggest you do two things, in the following order: (1) Ask him to put his request in writing. I don't mean email either. I mean you should ask for a wet signature on a piece of paper. (2) Give him a reply, in writing, in which you refuse to do what he asks, and that cites the NEC article that prohibits it.

You need to not be caught in the middle of this nonsense.

 

[mwm1752]

Simple & to the point
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.

 

[petersonra]

A lot of "techs" think this way. It is pervasive in the CNC business among others.

I would suggest that you ask him for a copy of the instructions from the manufacturer on this subject.

 

[JRW 70]

Yes, Bob just above said it all... Get the REAL INSTRUCTIONS
and not depend on uninformed individuals. The SEM we used
for checking crystalline structure was wired as I described above
and it worked very well. No noise or interference whatsoever
and was installed in an NEC compliant fashion.
In my opinion you made a wise decision to keep the liability
as far away as possible.

JR

 

[Pharon]

An isolation transformer will give you a perfectly clean ground and cost less than a UPS, though a UPS (double conversion) will also protect you from power blips and surges.

 

[petersonra]

An isolation transformer will give you a perfectly clean ground and cost less than a UPS, though a UPS (double conversion) will also protect you from power blips and surges.

An isolation transformer will still have an EGC coming in and an EGC going out that are bonded together so I am not sure why that would qualify as any "better" of a ground. And you will have to bond one of the secondary conductors to that ground, and to a grounding electrode if the transformer is large enough to require it.

It will reduce high frequency noise that might otherwise be on the power lines, as the higher frequencies have a harder time getting past the transformer, but that is not about grounding.

 

[Pharon]

An isolation transformer will still have an EGC coming in and an EGC going out that are bonded together so I am not sure why that would qualify as any "better" of a ground. And you will have to bond one of the secondary conductors to that ground, and to a grounding electrode if the transformer is large enough to require it.

It will reduce high frequency noise that might otherwise be on the power lines, as the higher frequencies have a harder time getting past the transformer, but that is not about grounding.

I didn't use the term "better," so I'm not sure why you put that in quotes. An isolation transformer with proper shielding will absolutely cut down on common mode noise while also re-establishing a new neutral to ground bond point with zero potential. When people use the terms "clean" and "dirty" ground, that's what they're referring to. No need for a semantics war.

 

[kwired]

The tech is ignorant. This setup might reduce noise; I cannot speak to that question. But it is a "death-by-electric-shock event" that is simply waiting for its turn to present itself. It is illegal; it is dangerous; it is stupid. If this really is what the tech is asking you to do, then I suggest you do two things, in the following order: (1) Ask him to put his request in writing. I don't mean email either. I mean you should ask for a wet signature on a piece of paper. (2) Give him a reply, in writing, in which you refuse to do what he asks, and that cites the NEC article that prohibits it.

You need to not be caught in the middle of this nonsense.

Include in the document the fact the individual whose signature appears takes 100% responsibility for any electric shock incidents as a result of not having a low impedance equipment grounding conductor and agrees to pay all medical, funeral and wrongful death or other lawsuit claims because of this action.

Also make a note in there that that document will be required to be posted on the equipment involved so others can have the opportunity to read about the hazardous conditions before using the equipment.

Then mention you still are not sure if said document will get the installation to pass inspection, but at least will hopefully lower your liability pretty significantly.