On Friday, January 5, 2018 at 12:28:57 PM UTC-5, Mr. Luddite wrote:
On 1/5/2018 11:37 AM, wrote:
On Fri, 5 Jan 2018 05:34:25 -0500, "Mr. Luddite"
wrote:

I agree the harmonic problem is just on wye but that is the most
common configuration, 208 or 480.
I think the problem first showed up on 480/277 wye systems where they
had a building full of 277v electronic ballasts.
Then it started showing up in those 208 systems that were feeding 120v
lines in cubicles through the internal wiring with a 5 wire feeder as
PCs started replacing terminals with ferro power supplies and space
heaters.

Delta is always funny stuff, depending on where or even if you land
the ground. Corner grounded is probably the easiest to confuse the
novice. It will look just like single phase until you get your meter
out. (2 pole breakers, white wires etc)


Not familiar with that. The majority of the systems we built were
designed to run on 480v, 3 phase. All the main breakers, fused
disconnects, motor starters, etc. were three pole. Ground was run
separately. We used a dedicated control transformer in each power
cabinet to generate whatever the low voltage control wiring was ... 120v
in the early days but later 24 volt. IIRC, the control transformer on
the 120v systems was a 480v/240v step down with a center tap, providing
a neutral.




Did you ground your 480 delta at all? If so where did the ground land?

I understand your control circuit voltage can be 120 if you want. That
is just a class 1 control circuit. Most people are familiar with class
2 but they are both controlled by the same article in the NEC.
Class 1 just looks like regular line voltage circuits because it is
not voltage or current limited like class 2 and 3.
The center tapped 240 delta is commonly called "red leg" or "wild
leg"because the corner between the center tapped windings will be 208v
to ground. "Red" is a misnomer tho because the NEC requires the wire
to be orange. That is very common in places where they want discount 3
phase and have a significant amount of single phase loads like those
small industrial bays. The PoCo can do it with 2 transformers,
generally the wild leg will be on a much smaller one. One "winding" is
actually open. hence another name, "Delta Veep".

http://gfretwell.com/electrical/red%...ansformers.jpg

99.99% of the time, if you see 3 transformers on the pole, it will be
wye. but I have seen one place in Key West where they had red leg
delta with 3 transformers. The only tip off was one was bigger than
the other two and it was confirmed by analysing the wiring.
http://gfretwell.com/electrical/Tran...%20_breath.jpg

Yeah my wife thinks I am crazy too, taking pictures of transformers.



Ground for the 480 Delta system service was run separately back to the
physical (metallic) ground at the panel, usually via a 6 awg wire.
Ground was not taken from any of the legs of the 3 phase Delta the way
you have described.

Many of the systems we built also had a RFI ground consisting of wide,
copper flashing to two, 8' copper rods driven through the floor and into
the ground. The rods were about 10-15 feet apart and we tried to get
close to 1 ohm resistance between them. This often required a copper
sulfate solution to be poured into the rod holes.

The reason for the exotic grounding had nothing to do with safety. Many
of the systems utilized a RF transmitter running at 13 Mhz. The load
for the RF transmitter was a plasma (ionized partial pressure gas)
generated within the vacuum chamber.

We had specially designed "matchboxes" that allowed load impedance
matching of the 50 ohm transmitter output to the very low impedance of
the plasma. Other systems used an electron beam operating at 10,000
volts. A stream of electrons are emitted from a filament and focused
magnetically onto various metals or dielectrics that were vaporized by
the beam and deposited on optics in very carefully controlled thicknesses..

Anyway, there were often some arcs and sparks within the chamber with
either of these deposition methods that would raise hell with some of
the very sensitive measurement instruments. The elaborate grounding,
using a wide conductor (flashing) is much better at RF and EMI quenching
of the arcs. A simple, round ground wire has too much inductive
reactance. It's only purpose on these systems was to serve as a safety
ground. Sometimes we'd have to chase our tail for a while however
because the use of both grounding systems sometimes generated a ground
loop which only magnified the RFI problem.

I'm familiar with the type of grounding system you're describing. I've working in many equipment rooms over the years with all types of RF and communications equipment, and they almost always have the copper flashing and bars for grounding the equipment.

The craziest thing I ever saw was a fire lookout tower on top of an 8k foot mountain in northern CA. It was a 3 story structure with something that looked like a chain-link fence around the roof, with huge cables coming down all four corners, then across the stone mountaintop with rods driven into the stone every so often for maybe 100 feet.

This was for lightning protection. It's not the lightning that kills the equipment, it's the difference in potential that kills equipment. So the idea is to get the building, equipment (and ground system) inside the building, and the mountaintop to all rise up to the lightning potential together, then drain back down together. They said it got struck several times every summer with rare losses. Probably pretty exciting for the person inside!

It had one other interesting feature... a scuttle hole on the roof with a ladder partially down one side of the building. That was for winter access when they used snow cats to get up there. I was there in the summer.