I will be wiring a church audio system and I'll be using MC cable and isolated ground recepticles. I know there are many pros and cons about the practicality of IG but I decided to go with it anyway. When I install the recepticles in a metal box do I still need to run a EGC to the metal box separate from the IG because of the metal box? If so, how do you guys and gals do it with MC? Thanks for your oppinions even if you think I am nuts.

Blessings, Mark

Mark,

Yes you still have to ground any metal box.

Use a plastic plate on IG outlets, if you use a metal plate it is possible for it to contact the metal box connecting the isolated ground to the equipment ground.

12/2 MC IG will have a 2 greens 1 with a yellow stripe. I do not know if available where you are.

12/2 Hospital Grade type AC cable (if you can substitute for mc) use the outer jacket for equipment ground the insulated ground for isolated ground

12/3 MC remark red to green with yellow band.



[This message has been edited by iwire (edited 03-28-2003).]

Iwire beat me to it

Since Type MC cable is not listed for grounding,you will need 2 separate grounding conductors in the cable to set up an isolated ground receptacle.One of the grounds will have to be bonded to the box.

Can red be reidentified for green?

[This message has been edited by txsparky (edited 03-28-2003).]

Forgeting about the pros and cons, if you have decided to do this, you have to keep an eye for the small stuff. After all the work you do to keep the grounds separate it is incredibly easy for something small (like a metal plate) to tie the two back together.

We do a lot of office cubes with both IG and regular outlets, all it takes for the two systems to be tied together is one person to plug the florescent light that is fastened to the grounded frame of the cube into a IG outlet and you now have isolated ground jumped to the equipment ground through the cord of the light.

I see very few times where the two grounds really stay separate.

[This message has been edited by iwire (edited 03-29-2003).]

2002 250.119(B) allows remarking conductors green or bare in a cable without regard to size.

Bob



[This message has been edited by iwire (edited 03-28-2003).]

mv,- when i install IG receps, i use 12/3 rx or m/c cable. I tape the red green, and attach it to the device. The bare and or green EG, i use to bond the metal boxes.

This is copied from Bryants website and says the plate screw is isolated from the ground terminal on the device


Characteristics of Isolated Ground Wiring Devices
Isolated ground devices help limit electrical noise on the equipment grounding circuit by providing two grounding paths for the installation. The device grounding circuit terminal and grounding contacts are isolated from the grounding path of the metal outlet box, device mounting means, and metal cover or wall plate, thus establishing a separate, dedicated equipment grounding path for connecting noise-sensitive equipment.

250.119 (B) says "where the condiions of maintenance and supervision ensure that only qualified persons service the installation. "

That statement doesn't apply to most church maintenance men




[This message has been edited by txsparky (edited 03-29-2003).]

Thanks txsparky, I did not know that.

So now the need for a plastic face plates would be 406.5(B) "metal face plates shall be grounded"

Great,
Thanks. You guys anwsered alot of questions. I will check my supplier for 12-2 MC IG. Also, I considered using 12-3 and taping the red but I didn't think this would be an acceptable method. I appreciate the responses.
Blessings, Mark

By txsparky


I understand what you are getting at, but that is a pretty vague code, that is left in the hands of the inspector to determine.

In my area "commercial type" buildings or buildings with management usually will be treated as supervised by management who will hire electricians to do the work, much like the exception to 230.72(C)

But I agree that some inspectors would consider this installation outside the scope of 250.119(B)

Bob,
I believe the 6-32 plate mounting threads are electrically common to the mounting yoke. I will ring one out Monday to be sure and post back.

Nick you are 100% correct, after reading you post I grabbed one out of my truck, and yes the plate screw is connected to the yoke, so I will shut up about the metal plates.

As wrong as I was about the plate issue I still say it is easy for the IG to compromised in even with the most careful installation, by the end user.

If the prints call for IG I try my hardest to make that happen and it is a pet peeve when I see the building occupied and the IG end up jumped to equipment ground.

I totally agree. The end user is the weak link in the system. No matter how hard the owner tries there will always be someone plugging something in that will compromise the IG.

Just priced out 12/2 M/C IG cable. Unless in the spec sheets, I'll buy green tape.

Some links:
http://www.ecmweb.com/advsearchresults.asp?qry=isolated+ground&SiteID=13&MagazineID=31&CodeID=&selectSearch=mag%3B31

Just some food for thought. The most common corruptions of the IG are RS-232 cables used to inter-connect printers, modems, and CRT’s to your computer. RS-232 cables contain ground conductors and when your device is supplied by a different receptacle either standard or IG outlet, the whole IG system is compromised.

Another item that is over looked is IG should be dedicated. If there are multiple outlets on a branch circuit you defeat the purpose of an IG outlet. The only problem an IG circuit can prevent is common mode noise on the ground. And when IG’s are multiple you create common mode problems
One final point is do not waste money on installing an IG circuit in a residence. Best bet is to install a dedicated branch circuit. Reason is simple; in a residence a dedicated circuit is IG. Hence no conduit, plastic box mounted a wood frame member, and ran with NM.