Isolated Ground

[electricmanscott]

I installed some isolated ground receptacles and circuits for some home theater equipment today. I am embarrased to say I don't even know the reasoning for this other than it was specified by the a/v contractor. Any basic info would be much appreciated as to what and why and do's and don'ts. Thanks, Scott

 

[iwire]

Re: Isolated Ground

Do all you grounding of boxes and raceways as you normally do, but at the IG outlet attach a separate insulated IG conductor on to the device grounding terminal nothing else and have it run back to the ground bar at the main bonding jumper of the system you are working on.

As to the whys, that gets beyond me, I have done Data centers with and without IG and both seem to work, no call backs.

Some will say it is mandatory, others say useless.

In any case it is difficult to keep it truly isolated from all other grounding.

 

[bphgravity]

Re: Isolated Ground

The basic reasoning behind isolated grounding to eliminate or at least reduce common mode noise. The is caused by magnetic induction on common grounding systems. This is the stuff that provides poor system quality like "hiss" and "grainy" video.

The key to these systems is to ensure complete isolation of the equipment grounding path, while still providing safety grounding on metallic componetns or the wiring method.

I strongly recommend reading up on line noise and use of isolated grounding if these systems are in the scope of your work.

 

[bennie]

Re: Isolated Ground

12-2/WG NM supplying a single receptacle, in a plastic box, the ground will be isolated.
A standard receptacle can be used. Receptacles should be dedicated, not daisy chained.

 

[gwz2]

Re: Isolated Ground

I rather believe that if Isolated Receptacle was specified and a standard receptacle was installed, it would be a case of fraud even though, for a single outlet, it electrically would be the same.

Muliple outlets, definitely fraud.

What if from a sub-panel ?

Who knows why the Isolated ground receptacle was specified, except the specifier.

[ May 14, 2003, 08:30 PM: Message edited by: gwz2 ]

 

[bennie]

Re: Isolated Ground

This is a home theater.

 

[dereckbc]

Re: Isolated Ground

If this home theater equipment was installed in a wood frame home with NM cable then IG was a complete waste. All that would be needed is dedicated circuits.

 

[don_resqcapt19]

Re: Isolated Ground

gzw,

Muliple outlets, definitely fraud.

Isolated ground does not mean dedicated or isolated circuits. You can have more than one receptacle on a single isolated ground circuit.
Don

 

[gwz2]

Re: Isolated Ground

I realize that more than one Isolated Ground receptacle can be on one circuit. I was referring to using standard receptacles on one circuit and calling that the same as "specified" Isolated Ground recep.

I feel that if Isolated Ground receps are spectified, then use the recep's with the orange triangle and install per instructions.

I am not not commenting on that the circuit "will work".

If I order a Cadillac, I do not expect a Chevy.

I am sorta thinking of the swimming pool motors being 2.5 HP installed on a circulating pump that only requires a .75 HP motor to drive the pumps at the rated capacities.

At the rated 1750 RPM ( or if rated for 3450 RPM ) that pump will deliver the rated output with either HP motor.

Paying for a Caddy when a Chevy will do just as well, but the Caddy was specified by the owner.

I don't care if it is a " Home Theater"

P.S.

If I were the owner, I would want what is specified as long as it meets the "CODE".

As an inspector, I can only check that it meets the minimum code.

Many owners do not get what they paid for if they don't have their own inspector, which should be the design engineer, which went over the plans with the owner before installation.

[ May 15, 2003, 07:17 AM: Message edited by: gwz2 ]

 

[don_resqcapt19]

Re: Isolated Ground

gwz,
I'm not questioning if the isolated ground circuit should be installed, because if that is what the contract calls for, then it should be installed. I just read your comment as saying that you can't have more than one receptacle on an isolated ground circuit.
Don

 

[bennie]

Re: Isolated Ground

My point about the type of receptacle is...A conventional receptacle, supplied by a home run, will be an isolated ground receptacle regardless of the plan.

I also don't suggest ripping off the customer. I would be ripping them off if I didn't suggest a more economical method to accomplish the same thing.

One isolated ground conductor per power supply, is the normal way to use the isolation feature. Otherwise, there is a difference in potential between the two or more devices, due to filter leakage current.

The equipment will talk to each other, if supplied by the same ground conductor.

[ May 15, 2003, 11:18 AM: Message edited by: bennie ]

 

[curt swartz]

Re: Isolated Ground

How do you install an IG receptacle using type NM cable? I have never seen any NM cable with 2 equipment ground wires.

 

[don_resqcapt19]

Re: Isolated Ground

Curt,
In an installation with nonmetallic cables and boxes, the bare EGC is landed on the green screw on the IG recp. There is no need for a second EGC.
Don

 

[dereckbc]

Re: Isolated Ground

Curt, that is the point I was trying to make. An IG circuit is a waste of time and resources if it is dedicated, ran in a non metalic raceway, terminated in a plastic box mounted on a wood frame member. It is isolated by those means, so you could simply use NM cable to a standard recept.

The only time an IG might be useful is if there is a metalic raceway, metal box mounted on a conductive surface which has continuity to ground, otherwise it is pointless.

Don I agree with you there is no code requirement fo an IG circuit to be dedicated, but to do otherwise would nulify any benefit obtained. The upstream recept would inject unwanted currents into the IG if terminated to equipment.

 

[don_resqcapt19]

Re: Isolated Ground

Dereck,
On some of the commercial jobs that I did a few years ago, they would spec multiple (2 to 4)receptacles on the same IG circuit. Usually for cash registers. Some times they would spec that the wire could not be spliced. You had to strip it, loop it around the receptacle screw and continue to the next receptacle.
Don

 

[bennie]

Re: Isolated Ground

The downside of isolated ground systems is the chance, in some cases, of an increase in impedance due to the added length.

Computers that are interconnected should not be powered from isolated ground receptacles, when the isolated ground conductor is longer than a multi-ground conductor.

 

[curt swartz]

Re: Isolated Ground

Don,

This would leave the yoke of the IG receptacle, plate screw and the metal plate if used ungrounded. I don't think NM cable could be used if IG receptacles are specified. You would need to run this circuit in a raceway, use IG MC cable or HCF AC cable.

I agree that using IG receptacles for residential work are useless and a waste of money when using standard receptacles in non-metallic boxes provide the same isolation. I tried to convince someone on the ECN site of this about a week ago.

 

[dereckbc]

Re: Isolated Ground

Don, I am not trying to tell you cannot put multiple outlets on an IG circuit, it is just not a good idea.

For instance the system you mentioned of having four daisy chained. The registars all have DC recitifiers in them. Each inject a small amount of noise current into the isolated EGC. By the time you get to the last device, the noise accumulated could defeat the purpose.

Besides the need for IG circuits has just about disappeared. Very few devices rely on ground to communicate via I/O ports. Most new devices use optic, balanced signal transmission, or back-to-back modems.

In the case of the home theater as mentioned, I suspect all the equipment uses 2-wire power plugs and no ground is involved. Does that need an IG? :roll:

 

[electricmanscott]

Re: Isolated Ground

What I did was 2 12/3 nm from panel to 2 1/2 emt sleeves down basement wall with changeover connector, to 4" deep box cantaining 2 IG receptacles. Bare grounds to box and red conductors tapped green for isolated ground. Both landed on eg bar in subpanel. This is not your average hometheater. The amps are big suckers with grounging type cords.

[ May 16, 2003, 09:05 AM: Message edited by: electricmanscott ]