Isolated ground circuit

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  #1  
Old 01-22-05, 12:20 PM
htguy
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Isolated ground circuit

I am using metal studs with plastic electrical boxes attached in the construction of an additional room in our basement.
How do I connect an isolated ground circuit receptacle,for a computer, using
14-3 cable ? Thanks
 
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  #2  
Old 01-22-05, 12:24 PM
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Rip out those pages on isolated grounding circuits from your Black and Decker wiring book. You don't want an isolated grounding circuit, you don't need one, and using 14/3 to get one as shown in the B&D book is a code violation.

Isolated grounding circuits are sometimes useful in industrial and commercial buildings. They are never needed in homes.
 
  #3  
Old 01-22-05, 02:23 PM
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By the nature of using NM Cable (Romex) and plastic boxes, with the circuit originating from a branch circuit breaker in your main panel, you've got an isolated ground receptacle without even trying.
 
  #4  
Old 01-22-05, 06:10 PM
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why are you running 14ga wire to outlets anyway? The cost of 12ga wire and breakers is virtually nothing more than 14ga and 15A breakers. This question especially applies if you are putting in a home office or theater (isolated ground) You could really use the extra capacity.
 
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Old 01-24-05, 08:07 AM
htguy
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Thanks for the information.
 
  #6  
Old 01-24-05, 08:37 PM
dennisjonescon.
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I ran a separate ground for my welder......I put another ground rod in the ground at the area where I plug my welder into and connected the ground wire to that. It eliminated the flicker problem I had with the lights in the office.
 
  #7  
Old 01-24-05, 09:16 PM
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It may have elminated the flicker (although I don't know exactly how), but it's both illegal and unsafe.
 
  #8  
Old 01-25-05, 10:50 AM
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Originally Posted by htguy
I am using metal studs with plastic electrical boxes attached in the construction of an additional room in our basement.
How do I connect an isolated ground circuit receptacle, for a computer, using 14-3 cable? Thanks
An Isolated Ground (IG), with or without a supplementary grounding electrode, is only used were the equipment to be served is designed to use earth ground as a signal reference point. Most computers do not need and will not benefit from having an isolated ground.

If you still feel that you want to have an isolated ground receptacle and you are located in the US then go to an electrical supply house and buy enough type AC armored cable that includes an insulated green jacketed Equipment Grounding Conductor to run your IG circuit. One manufacturere calls their version of that cable "Health Facilities Cable" The jacket of type AC cable is listed for use as an Equipment Grounding Conductor (EGC). The jacket will provide the EGC for the cable itself and any junction or outlet boxes in that circuit. The insulated green EGC will be used to bond the grounding terminal of the receptacle back to the main bonding jumper at the service equipment. A special receptacle is used that does not have the yoke that supports the receptacle connected to the receptacles grounding terminal. Working with type AC cable is a little more demanding then working with romex so you will need an AC cable cutter, usually called a roto split, or a hacksaw with a thirty two teeth per inch blade.

In Canada you would buy type MC cable that has two different insulated EGCs in it. One will have a solid green jacket the other will have a green jacket with one or more yellow stripes. You could also use flexible metallic conduit, or any other metallic raceway, with two separate EGCs pulled into the raceway.

For maximum protection from signal interference due to environmental electric noise the raceway or cable used should have a ferris metal jacket rather than a non metallic or aluminum raceway or cable jacket.
--
Tom H
 

Last edited by hornetd; 01-25-05 at 11:35 AM.
  #9  
Old 01-25-05, 10:52 AM
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I am concerned about the OP menionned plastic boxes with metal studs.

It is important that metal stud assemblies be grounded. You can achieve this by using metal device boxes attached with metal screws, or if you are using non-metalic boxes then the metal stud assembly needs to be attached to ground.

See the attached safety notice ...http://www.esainspection.net/pdf/fsn/01_04_fl.pdf

With secure bonding to ground, the overcurrent device will trip if the metal stud assembly becomes energized, thereby preventing possibly serious electric shock.
 
  #10  
Old 01-25-05, 11:30 AM
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Originally Posted by hornetd
An Isolated Ground (IG), with or without a supplementary grounding electrode, is only used were the equipment to be served is designed to use earth ground as a signal reference point. Most computers do not need and will not benefit from having an isolated ground.

If you still feel that you want to have an isolated ground receptacle and you are located in the US then go to an electrical supply house and buy enough type AC armored cable that includes an insulated green jacketed Equipment Grounding Conductor to run your IG circuit. One manufacturere calls their version of that cable "Health Facilities Cable" The jacket of type AC cable is listed for use as an Equipment Grounding Conductor (EGC). The jacket will provide the EGC for the cable itself and any junction or outlet boxes in that circuit. The insulated green EGC will be used to bond the grounding terminal of the receptacle back to the main bonding jumper at the service equipment. A special receptacle is used that does not have the yoke that supports the receptacle connected to the receptacles grounding terminal. Working with type AC cable is a little more demanding then working with romex so you will need an AC cable cutter or an

In Canada you would buy type MC cable that has two different insulated EGCs in it. One will have a solid green jacket the other will have a green jacket with one or more yellow stripes. You could also use flexible metallic conduit, or any other metallic raceway, with two separate EGCs pulled into the raceway.

For maximum protection from signal interference due to environmental electric noise the raceway or cable used should have a ferris metal jacket rather than a non metallic or aluminum raceway or cable jacket.
--
Tom H
I've worked with isolated grounding systems in the telecommunications industry since 1987 and my first impulse would be to tell anybody that wants to do this in a residential application is that it can't be done correctly. We had enough problems with electrical inpsectors in new CO's and the NEC says it doesn't apply to these facilities.

There are just too many connections for a home owner that is not trained in isolated grounding to consider. Are you going to have TV cable in this room? In a true isolated ground zone, do you know the shield on your coax would have to be grounded in the zone and disconnected from the ground block at the NID? Otherwise, you have introduced a second ground point to the system and defeated all the work you have done.

Instead, buy a good surge protector. Then ponder this, in order to get UL approval there must be neutral to ground protection in the surge protector, but doesn't this violate the rule that the neutral and ground will only be tied together at the SE panel?
 
  #11  
Old 01-25-05, 11:55 AM
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Originally Posted by Blizzard
I've worked with isolated grounding systems in the telecommunications industry since 1987 and my first impulse would be to tell anybody that wants to do this in a residential application is that it can't be done correctly. We had enough problems with electrical inpsectors in new CO's and the NEC says it doesn't apply to these facilities.

There are just too many connections for a home owner that is not trained in isolated grounding to consider. Are you going to have TV cable in this room? In a true isolated ground zone, do you know the shield on your coax would have to be grounded in the zone and disconnected from the ground block at the NID? Otherwise, you have introduced a second ground point to the system and defeated all the work you have done.

Instead, buy a good surge protector. Then ponder this, in order to get UL approval there must be neutral to ground protection in the surge protector, but doesn't this violate the rule that the neutral and ground will only be tied together at the SE panel?
That last question has a simple answer which is no. What the code forbids is the bonding of the grounded current carrying conductor (neutral) to ground on the load side of the service disconnecting means. Having a cord and plug connected device that has some portion of it's circuitry connected to the EGC is not a connection within the premises wiring system. Many of the things we use in homes today have a ground connection in their circuitry. The most important point is that the devices in question are listed by an electrical testing laboratory and are therefore immune from AHJ scrutiny.
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  #12  
Old 01-25-05, 03:24 PM
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Plastic box, NM cable and originates from main panel = IG circuit. The metal studs do not require bonding, however they to require a plastic bushing (where you go through) to prevent chaffing.
 
  #13  
Old 01-25-05, 08:56 PM
dennisjonescon.
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John......you have me curious.....why would it be unsafe to shorten the ground distance and isolate it to a different location. What could I expect should something go wron with the way I have my welder ste up at the moment?
 
  #14  
Old 01-25-05, 11:10 PM
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Grounding or bonding?

Originally Posted by dennisjonescon.
John......you have me curious.....why would it be unsafe to shorten the ground distance and isolate it to a different location. What could I expect should something go wrong with the way I have my welder set up at the moment?
The term Equipment Grounding Conductor(EGC) is in fact a misnomer. The actual purpose of the EGC is to bond the non current carrying metallic parts of an electrical installation back to the neutral of the service at the main bonding jumper. This is done to cause the current to flow back to the source at a rate that will trip the Over Current Protective Device (OCPD). The code language is clearer when it describes the purpose of the EGC as,

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
(2) Grounding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.
(3) Bonding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.

If you read the section carefully you will see that the purpose of the EGC is actually to be "connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path." The last sentence of section 5 forbids using the earth "as the sole equipment grounding conductor or effective ground-fault current path." That does not mean that you cannot have a supplementary grounding electrode connected to the EGC as long as the EGC is run back to the bonded buss bar or enclosure cabinet of the panel from which it originates.

250.54 Supplementary Grounding Electrodes.
Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53(C) or the resistance requirements of 250.56, but the earth shall not be used as the sole equipment grounding conductor.
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  #15  
Old 01-26-05, 06:14 AM
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You cannot isolate an electrode on a premises. It must be connected in some fashion to the grounding electrode system. The code states "the earth shall not be used as the sole equipment grounding conductor" to interconnect electrodes.
 
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Old 01-26-05, 07:33 AM
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This raises some interesting questions. We all know that the earth cannot be used as the "sole" EGC. In fact, we know that the earth is a pretty darn bad conductor. Tom suggests that this "does not mean that you cannot have a supplementary grounding electrode connected to the EGC." However, I thought there was some code that limited you to a single point of grounding (that being the panel housing the main disconnect), although this single point can be multiply connected to the earth (e.g., grounding rods, plumbing pipes).

So let's assume that all ECGs are correctly terminated back to the panel. Can you now just pound an isolated grounding rod in any place you want and connect it to the EGC at an outlet? I thought no, but Tom suggests yes. And Ron says that you "cannot isolate an electrode on a premises." Ron, can you cite the article that says this?
 
  #17  
Old 01-26-05, 08:05 AM
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Supplementary Electrodes

The language of section 250.54 seems pretty clear to me folks. It flat says you can install an electrode at any point you need one as long as you connect it to the Equipment Grounding Conductor (EGC). This article is meant to meet the needs of certain users who must have a signal reference ground for their equipment or who have to ground to discharge static, provide lightning protection, and so forth. Take the example of the waste water lift pumps on a rural property situated on a completely exposed ridge line. Without supplementary electrodes I would be completely dependent on the EGC to discharge the induced currents from nearby lightning strikes. I would loose pump after pump. By running a ground ring around the base of each vault and attaching that electrode to the EGC of the branch circuit supplying the pumps I provide an effective discharge path for the branch circuit TVSS that is located at the junction box at the top of the vault manhole. In another article we are required to bond the EGC of the circuit serving submersible well pumps to the metal well casing. If a metal well casing is not a hell of a grounding electrode it wouldn't be specifically included in the language describing underground metal water pipe electrodes.

250.112 Fastened in Place or Connected by Permanent Wiring Methods (Fixed) — Specific.
Exposed, non–current-carrying metal parts of the kinds of equipment described in 250.112(A) through (K), and non–current-carrying metal parts of equipment and enclosures described in 250.112(L) and (M), shall be grounded regardless of voltage.
(M) Metal Well Casings. Where a submersible pump is used in a metal well casing, the well casing shall be bonded to the pump circuit equipment grounding conductor.

--
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  #18  
Old 01-26-05, 09:51 AM
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John,
250.50 says that if there is a rod (and six other types of electrodes), each item shall be bonded together. What do you define as supplementary.
 
  #19  
Old 01-26-05, 12:43 PM
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Let's try an example!

Originally Posted by HandyRon
John,
250.50 says that if there is a rod (and six other types of electrodes), each item shall be bonded together. What do you define as supplementary.
Ron
The service is built and it is fully compliant. I come along to install a piece of equipment that will generate static electricity. If I don't effectively bond that equipment to local earth ground it could ignite the material it is handling and cause an explosion. I install a grounding electrode sufficient to my purpose and I connect the Equipment Grounding Conductor of the circuit that supplies that equipment to this supplemental grounding electrode.

An example may help. I'm wiring a pneumatic unloading dock for flower receiving at a commercial bakery. There must be no chance of static discharge between the pneumatic unloader, the truck frame, or the earth beneath the truck. In order to provide static bonding to avoid the possibility of a combustible dust explosion a concrete pad has been poured on which the five axle grain hopper truck will park during the unloading process. Part of the unloading equipment will be a grounding real that connects the frame of the truck to the unloading equipment. The reinforcing steel in the pad has been double tied and is galvanized reinforcing bar. One bar has been turned up out of the pad close to the building and the unloading equipment will be bonded to this concrete encased electrode. The actual purpose of bonding the rebar in the pad is to create an equipotential plane across the entire unloading area.

The code recognizes that bonding this new electrode to the service grounding electrode system may not be practical or in some cases even desirable. In the event of a utility transformer puncture or a similar power cross we would not want the fault current from the high voltage line to be flowing in the combustible dust handling area. It also recognizes that any fault between the branch circuit and the equipment must have a low impedance fault clearing pathway back to the source of supply via the main bonding jumper. Given the voltage drop over the EGC of the branch circuit the equipment may go high relative to the earth on which the truck is parked so the bonding of the concrete encased electrode to the EGC assures that the whole area will be at the same potential until the circuit's Over Current Protective Device (OCPD) opens to clear the fault.
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Last edited by hornetd; 01-26-05 at 12:45 PM. Reason: To eliminate smilie that did not display properly.
  #20  
Old 01-26-05, 12:54 PM
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Might the EGC itself provide the bonding of the supplemental rod to the grounding electrode system?
 
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Old 01-26-05, 02:45 PM
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Originally Posted by John Nelson
Might the EGC itself provide the bonding of the supplemental rod to the grounding electrode system?
It does to a degree but it's size allows it to serve as a kind of fuse so that if there is a power cross and a large amount of current is seeking to return to the grid via the supplemental electrode and the multi grounded neutral of the power grid then the EGC of a branch circuit would likely burn clear thus avoiding the high current discharge through a dangerous or sensative area.
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  #22  
Old 01-26-05, 03:13 PM
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I agree with Tom.

The code is very distinct as to the difference between supplemental and supplementary grounding electrodes.

A supplemental electrode may be, and often is, required by code.

A supplementary electrode is never required by code, but is permissive.
 
  #23  
Old 01-27-05, 12:12 PM
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HandyRon said,
The metal studs do not require bonding, however they to require a plastic bushing (where you go through) to prevent chaffing.
The danger would be if the metal wall system is not grounded, and someone attaches a shelf to the wall stud with a long screw and penetrates the cable, all of a sudden the entire wall frame is energized setting up for a potentially serious shock.

If the metal stud assembly was grounded, then the circuit breaker would trip in this situation, removing the possibility of shock.
 
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