Colored Electrical Outlets

IMHO IG circuits were largely black magic.

There were a few manufacturers that wanted the isolated ground to be tied *only* to a local ground rod (no connection to the electrical system). It was a major code violation and safety hazard.

I was an electrician working on the mission control center for the star wars program 20 years ago and we put ground rods under the raised flooring, around the perimeter of the building, then built a grounding grid under the flooring of the whole place. I guess they didn't want any stray electric currents affecting the multi million dollar Cray Super Computer.

TDD

The "u" ground is not connected to the "frame" ground.

A NON GROUNDED isolation transformer is for safety. A grounded isolation transformer is for"noise" reduction on the line and elimination of ground loop currents and floating grounds.

"proerly grounded" and EMT as ground is an oxymoron.

Not "in some places" -- anywhere that has adopted the U.S. National Electrical Code.

Not "at some time" -- always.

Not "was ok" -- still is ok.

The NEC specifically permits the use of metal conduit of various types to be used as the equipment grounding conductor. Bare or green wires are not necessary.

Not if it's properly grounded, it doesn't.

EMT in a dry location - OK. But with humidity, and particularly humidty and vibration, the integrity of the ground suffers in a short time. I would never depend on the EMT for a safety ground.

On 8/19/2009 8:47 PM snipped-for-privacy@snyder.on.ca spake thus:

wrote: >>

This seems to be analogous to backstabbed connections vs. screw-terminal connections: allowed by the NEC, but not as good a technique.

Think about it: with EMT, all it takes is one loose screw somewhere in the line to sever, or at least seriously degrade, the ground connection.

That is true if you use a green wire ground too. Workmanship is the key to any installation. Most of the industrial jobs I inspected specified compression connectors wrench tight. That is a pretty solid ground path. They still pulled a green wire most of the time.

On 8/20/2009 11:05 AM snipped-for-privacy@aol.com spake thus:

Yes. I forgot to mention that when I run conduit, I much prefer the compression connectors to the "make-a-dimple-in-the-tubing-with-a-screw" ones, which always seem a bit on the cheesy side to me.

The problem is, the NEC allows either one to be used as a ground - and the "dimple" is NOT a safe ground. The compression connector, wrench tite - perhaps.

Chuckle. At work, on certain data circuits, they braze or epoxy the joints on the conduit, done with compression fittings. Before they pull the copper or fiber, of course. Of course, a ground path isn't what they are after.

-- aem sends...

do they also happen to have walls covered with sheets of copper soldered together? I've worked in some places like that...

nate

Nah, the hardware itself is shielded now. But there are some storage rooms in the dungeon, now used for other things, that are covered with expanded metal mesh over the concrete block, and in the plastered ceiling. There is one room that I suspect has the sort of wallpaper you describe, but thankfully I'm not allowed in there. The paperwork to be allowed in the room in front of that room, and use the hardware there, was bad enough.

-- aem sends...

y'know, now that I think about it, once upon a time expanded metal mesh was used for lath for plasterwork. I know my high school was constructed that way; not sure why they spec'd plaster and not drywall as it was built in 1972 or 1973 I believe. Wouldn't that play holy hell with radio reception? Obviously people weren't trying to listen to the radio or watch TV inside a high school, but I'm sure quite a few homes were built like this as well...

nate

No, the wall mesh in these rooms was obvious RF sheilding. You could see where there used to be a mesh door to complete the cage. I guess they figured floor didn't need shielding, since that part of building was well below grade, with no sewers and such below it.

-- aem sends...

I'm sure the code making panel will be interested - submit a code change proposal. Be sure to include pictures of the dead bodies - it is very effective. On the other hand, the code panels are not likely to make changes based only on opinion.

"Dimple" is not a good description, as there were also fittings that used a compression tool to create a "dimple". (They still allowed?)

In addition to using compression fittings you can use non-diecast fittings (not sure what they are called). They are a lot better connection on set-screw fittings. Or compression non-diecast fittings.

I once watched the construction of a Tempest-class area: * Commercial metal studs * One layer of metal-coated 5/8" wallboard on each side, metal out * Tape seams with metal tape * Another layer of metal-coated wallboard, seams shifted four feet * Tape seams with metal tape * One layer of regular wallboard for the finish surface * Speakers to play music inside the walls * Motion detectors to detect motion inside the walls

They did the ceiling after the walls closed in, so I didn't get to see what happened there, but I'm sure it was equally impressive.