Isolated Ground

[brucestart]

I was hoping some one could clarify isolated ground systems for me.
I am wondering where in the code I can find requirements for these systems and I was hoping some of you could share your views on the PROPER wiring methods for such systems. I am mostly a residential electician and have a limited knowledge and background with this topic. Any guidance here would be nice


Thanks, Bruce

 

[ron]

Re: Isolated Ground

Isolated ground panelboards have a seperate insulated grounding conductor run back to its source with the phase, neutral and the normal grounding conductor. The individual load/receptacle has an isolated grounding conductor run back to the isolated grounding panelboard.
By the nature of residential wiring systems, each nm cable run from a receptacle back to the main panelboard, where the box is plastic, is an isolated ground since the grounding conductor is isolated from the rest of the grounding system, until it gets to its source.
Most (almost all) isolated grounding systems are not worth the trouble, as they are easily contaminated, and become more trouble than they are worth.

 

[stanley]

Re: Isolated Ground

Hi Bruce, I have to agree with Ron. If the the "isolated" ground is over 40' (#12 AWG) the isolating effect is more often than not compromised.

If you truly need isolation an isolating transformer is the way to go. Some UPS units have one built in and Sola Corp. makes excellent units. (Not plugging Sola in particular. Just from past experience.)

 

[ryan_618]

Re: Isolated Ground

Hi Stanley. I understand the concept and the installation methods of an IG system, although probably not as well as Ron and yourself. With that in mind, could you tell me why the isolation effect is easily compromised at distances over 40'? Thanks.

 

[dereckbc]

Re: Isolated Ground

The only use for an IGR is a possible control of Common Mode Noise Rejection (CMR). The expected results from using IGR are in the order of 1. no effect, 2. worse condition, 3. desired effect.

To be effective the IGR ground should originate from the N-G bond point of the source. Code does not require the IG to originate at the source.

Ron is correct in residential applications using a dedicated circuit, with NM cable and an plastic box, make the circuit IG.

An IGR circuit cannot clean a "dirty ground" or remove common mode noise, only possible keep it from entering. The most effective means to remove Common mode noise is an shielded isolation transformer, and/or a dual conversion UPS with a dedicated circuit using an EGC and metallic conduit. The standard UPS found in most residances are standby UPS and do not remove common mode noise. The

IGR's are easily compromised, by using multiple recepts on a branch circuit, and/or by the I/O with ground reference cables (RS-232, coax, etc) inter-connected between pieces of equipment on different branch circuits.

Stanley hows does an IGR circuit in excess of 40 feet compromise the isolation?

 

[stanley]

Re: Isolated Ground

I should make a small correction. At 40' the ground is still technically isolated. However, its usefulness is suspect.

The 40' is an approximation based on the capacitive coupling with the other conductors in close proximity. The capacitive coupling is where outside noise will be induced.

Derrick makes an excellent point. The RS232 cable from your PC to the printer foils all attempts at ground isolation.

There is a myth out there that the earth is a magic noise sump and if we connect all of our isolated grounds to it all our problems will be solved. This belief is shared by some of the largest CNC manufacturers in the world. To a point where they require a "isolated" ground rod.

This is a warranty item to them. It always takes a phone call and a request for a letter absolving the installer of any and all liability before they back down. We put their isolated ground in but then bond it to the EGC.

Sorry for being so long winded but this is one of those pet peeves of mine.

 

[brian john]

Re: Isolated Ground

In my opinion, most IG systems are installed after the customer, engineer, electrical contractor has had problems with another distribution system. And if the suspected system is investigated I would bet that system was not installed properly. IE. Neutral grounded downstream from the main neutral ground bond.

In my opinion in most circumstances a properly installed distribution system does not need the IG adder.

And the number of creative (polite way of saying improperly installed) IG systems we see installed
is more than I care to recall.

[ March 19, 2004, 05:13 PM: Message edited by: brian john ]

 

[stanley]

Re: Isolated Ground

Couldn't agree more Brian. Fixed many a problem just by lifting the bonding screw in subpanels.

 

[brucestart]

Re: Isolated Ground

Thanks for the input guys... you helped to clear up much confusion and misguidance. One question though is there any specific code references requiring the use of IG systems or is this typically an engineering requirment , also which occupancies will IG systems most typically be found in ?

Bruce

 

[scott thompson]

Re: Isolated Ground

Bruce;

If it's of any assistance, I have some Drawings of IG installs posted at ECN's Technical Reference Section.

This would be found at the Electrical Contractor's Network, in the Forums.

Let me know if you need a direct link.

Scott

 

[scott thompson]

Re: Isolated Ground

Just for the heck of it, here's a link to one of the IG schematics at ECN:

Page is IG Details #1 at the Tech Reference area.
Just click the above link to be taken to that page with the IG schematic.

If your not seeing the link, here it is "Raw":
http://electrical-contractor.net/ubb/Forum15/HTML/000049.html

Scott35

 

[rhombus65]

Re: Isolated Ground

brucestart:

First let me say that the above posts are top notch. I just figured I would give my opinion.

I think that IG circuits are useless in residential applications and agree that the use of an IG transformer is the only true way to create IG circuits.

I use to wire malls and large office buildings. This is an area where IG circuits can be helpful because the whole building can act as an antenna of sorts. So if you use the standard EGC as your only ground you can have a lot of problems with low impedence/ resistance loads such as computers and POS equipment.

We used IG transformers, IG panelboards, MC cable with an IG conductor and IG receptacles. It is important to use metal sheathed cable (or some type of metal raceway). Keep in mind that if you use MC cable you need to buy it with an extra conductor for your IG as the sheath of corrugated MC can not be used as an EGC.

 

[iwire]

Re: Isolated Ground

I think that almost all IGs are a waste of time, unless someone is keeping an eye on things that knows about IG grounding and what it entails, it is not long before the IG grounds become corrupted by incidental contact with other grounded items.

rhombus65 I am not putting down your work, I am sure when you leave the job the IG is isolated.

That said the smallest things can change that, in a restaurant it could be the metal enclosure of a IG supplied cash registrar resting against a stainless steel counter.

In office cubicles all it takes is someone to plug the florescent light in thier cubicle into the IG outlet for the PC. The lights have 3 wire cords and are fastened to the grounded metal of thee cubicles.

In the mall all it takes is an in house maintenance guy to grab the wrong grounding conductor in a box for a new circuit.

I install the same types of systems you install and if either you or I go back to jobs we have done and lift the IG from XO we will probably find it is grounded somewhere else. :roll:

Bob

[ March 28, 2004, 12:55 PM: Message edited by: iwire ]

 

[rhombus65]

Re: Isolated Ground

iwire:

Well stated.

I never thought that far into it.

 

[brian john]

Re: Isolated Ground

Iwire:

The proof may be in the pudding....90%+ of the neutrals we test (neutral to ground bond lifted) are ground. If that hihg a percentage of neutrals are grounded downstream from the neitral ground bond, what percentage of IG's are grounded downstream???????

 

[karl riley]

Re: Isolated Ground

Brian, a question. When you lift the neutral bond at the main and then test the neutral for grounding, how do you know that the neutral is not simply connected to another neutral which in turn is still bonded at the panel?

In other words, how do you distinguish between a grounded neutral and a neutral that is incorrectly connected to another circuit's neutral?

Karl