ECN Forum Forums General Topics of the Trade General Discussion Area isloated ground receps in plastic boxes

Common 12-3 MC is easily used for IG circuits. The green is the IG, the white is N, the B (or R) is stripped back all of the way creating a code compliant bare dirty ground.
In the not uncommon situation of single phase clean power -- the hot can stay on color.
This avoids ordering trick IG 12-2 MC.

I explain IG as a DC return: It is wired up just like a neutral. The typical systems demanding IG in modern specs have internal DC power supplies producing regulated DC. The a sweet DC path is part of their voltage control logic.
As new chips are moving to ever lower voltages and 3,000,000,000+ cps any spikes -- on the DC could/ should cause errant logic. The power supply can't begin to adjust in the time frame of the microprocessor.

Bad grounds ( ground impedence ) are such a problem for modern systems that it is routine to design in waves of parallel grounds throughout cables and motherboards.

The outward manifestation of such spurious voltages may be computer lock-up. In older, east coast headquarters there have been tales of headaches so bad -- while running just PCs and the like -- that the building was retrofitted with clean power and IGs. Problem gone. ( From a manufacturer's ad IIRC )

Once burned, twice shy...It is easier to over spec than chase down the ghosts. In the above story -- they spent a fortune before scoping the problem. That's is what is so costly about dirty DC returns.

Tesla: Your method is only permitted where conditions of maintanance and supervision ensure that only qualified persons service the installation. See 250.119(B).

As already stated, in a nonmetalic box with 12/2 NM, isolation has already occured, and if we are talking about a dedicated circuit, we can't get much more isolated than that, even with standard receptacles.

Tesla do you have any references to back all that up.

What can IGs do that a equipment ground can not do.

That building you describe probably had some bad wiring, and the fortune spent installing IGs could probably have been better spent correcting the existing wiring.

In my opinion IGs are useless in almost all cases, even if we say they can do some good they become connected to the equipment grounding conductor in no time at all.

Telsa, could you please explain how the IG is used as a DC return? Seeing how all DC supplies in electronic equipment are SDS in nature, I am having trouble seing how any IG could be used as a load or return conductor, not to mention forbided by NEC

Dereck Campbell, PE
C.O. Power Engineer.

Since you've written...
Ryan_J: That is code truth.... In my work, commercial, the AHJ has no problem with it. I, myself, cannot imagine how even a tyro is going to foul up such an install....
Roger: I agree entirely. IG receptacles don't get you anything in a residential application.
iwire: I run jobs for 'catfish' feeding at the bottom end of the commercial market. The IG, and associated contract specs, stops my bosses from screwing the owner on the ground path. ( And yes they would.--- I had to withstand a screamfest from two separate owner/ bosses on wasting green wire.) The requirement for dedicated ground clear to the panel provides the owner the pretext for testing it by lifting it off the isolated grounding bar.(And, yes...they do it. They've learned by experience.) Without this spec the owner would receive a system sending the ground path regular way: through the EMT -- linked to every hunk of metal on the way....
I have built raceways
under major high voltage lines ( inducing 270V on the skin of parked cars and a hefty induction to the T-bar over the sales floor. -- Lighting crews had to pre-ground the grid every time they worked on it from a lift.)
in the path of intense microwave radiation ( with all of the side lobes/ refections triggering voltage induction beyond 40V in the EMT. This had the fire alarm tech choking.

So it is true, for elements as small as microprocessor transistors, induced voltages in EMT -- found so irregularly in commercial situations -- you can't trust the EMT as a clean ground path.
And without the IG/ dedicated ground wire system the owner can't spot when the EC is cheating. And the pressure on the EC to cheat is intense since the bid was a crusher.
As electricians we are instructed from earliest days that impedence is purely an AC property. That is completely untrue. DC systems also exhibit impedence...but only during the transition to steady state conditions. And, in our customary work, that is all that we deal with. So, to get the test question right -- we answer no to DC impedence.
But, in the world of microprocessors, DC impedence is a huge design concern. A next generation of processor is being planned with additional layers expressly for the purpose of removing right angle turns on the DC ground path. Each bend in the current contributes to impedence. The exact number of conductors on the 80 pin disk drive ribbon connector dedicated to ground returns is 36 IIRC. (Every signal line gets one.)
The Code does not speak to this issue because the volts and energies are dramatically below any fire or health hazard.

iwire: the NEC will accept bonding through the raceway. There is abundant evidence that microprocessors can't live with that 'fault clearing path'.
One real world example: I have an RV -- and in that RV I have a PC -- and that PC is stand alone -- not even internet -- and when my propane igniter kicked in for hot water -- the PC locked up and crashed over and over. The solution was to change locations and clean up the grounding pin. Problem gone.
dereckbc has validity but speaks mainly to signal quality. Modern systems use error correcting data transmissions and operating system firewalls. The classic signal problems have been addressed.
Falling DC microprocessor bus voltages coupled to higher speeds drives the need for cleaner grounds.
All modern regulated power supplies have feedback mechanisms working at dramatically slower speeds than the microprocessor: 3 Gig. All of the DC loads are sunk to bonded common voltage (nominal 0 Volts) and the supply voltages are biased off of it through various schemes. If -- for even a moment -- a wave of DC/AC voltage surges up into the DC sink -- the microprocessor locks up entirely. This is the malady that has people pounding their keyboards.
I call it a 'DC return' because correctly wiring an IG causes it to exactly parallel the style and form of the AC return: the neutral. It is so much easier to explain it thusly to the troops. Neutral, they understand. Otherwise, my crew is bonding green wire all over and omitting the homerun.
"SDS in nature" does not compute. Spell it
out.
I am arguing very much in parallel to derekbc except that the "noise" that he declaims seems to be coming out of signals...in my experience the trouble almost always is from major circuit loads such as igniters, elevators, motors, contactors that trigger significant wave effect distortions ( think pulsed inductions ) on proximate ground paths. And, no the energy waves do not all immediately head into the GEC. They pour out onto all adjacent conductors until the wave sinks into the earth.

[This message has been edited by iwire (edited 07-25-2004).]

So, I gather that the IG system works best when the ground is brought back to the service, is never of any use in residential applications, and could be totally eliminated by simply re-designing the electronics used, but this would be costly to the electronics manufacturer. They would much rather have the electrician eat the costs by fixing the designer's omission with elaborate IG specs.

I've often thought that electronic gadgets need to be housed in magnetically isolated metal cases (like the old anti-magnetic watch) and fed by DC batteries (i.e. the laptop). Such a plan would eliminate any stray high frequency current from being confused with valid input.

Harmonics could be eliminated as well by proper design of the harmonics producing devices. Wave chopping is cheap, but why not use the entire sine wave? The cheapest way is not the best way.

Welcome to ECN Tesla, I should have said that earlier.

by Tesla

Tesla you have to remember this forum has members from all the country (and beyond) to give a recommendation based on what your AHJ allows will not help anyone outside your area. The wording from 250.119(B) certainly leaves it open to the AHJ.

part of 250.119(B)

The opening poster describes a wood framed bank, In my opinion an AHJ could feel this building does not meet 250.119(B)

by Tesla

I also agree but Roger was not talking about a residential application.

by trekkie76

by Tesla

Unless they are lifting this IG daily and tracking down points where it has been connected to the EGC in some way it is not an IG for very long.

by Tesla

How exactly did you measure 270 V on the skin of a car or 40V in the EMT?

Between what two points and what kind of meter?

by Tesla

You run large commercial work and SDS does not ring a bell?

"Separately Derived System"


Bob



[This message has been edited by iwire (edited 07-25-2004).]

iwire...
It is true that cars parked under high voltage power lines can pick up amazing voltages...Any and all testers used by a crew of twenty DMM/ analog found the same readings on their own cars on the job site. Parking further away caused a voltage drop. Potential existed between bumpers and ground....
SDS, as in 480 delta to 208Y120, I've built many. It is not the term of art that first comes to mind in describing the internal powersupplies found in mass produced PCs. Strictly speaking I have to give it to you...their regulated DC busses are separately derived power.

Inre IG conductors: The test I described is performed - required - for each California Lotto terminal. (Watching that get fouled up is suitable for a comic thread.)

Once 'proofed' the conductor is re-bonded. It never becomes an issue again.

Inre the fire alarm tech and hair pulling over EMT induction...He and I used a battery of analog and DMM to prove that such voltages were all over the fire alarm raceway. We ran shielded cables within the EMT. EMT required by AHJ for high rise office structure standing adjacent to a major fault, the eastern twin of the San Andreas fault. ( This is the fault that is splitting Berkeley Stadium in half -- ultra slow motion. )

Somehow I think that most AHJ are going to think that a bank is going to only ever use professional licensed electricians to work on their circuits. But its always going to be a case by case situation. I've never seen one of our local 'catfish' ever order/ install trick 12-2 IG MC. The price is higher, and one must wait. The technique I mentioned is quite common out here....If it is not permitted elsewhere what can I say...every EC has to make his own call.