Yes, this is old - but if you want to get pedantic you CAN INDEED
take 3-phase power and make 6-phase power if you have a use for it.
They used to do it all the time on the old 'turn of the 20th Century'
transit trolley power supply stations.
They made a large double transformer to take incoming 2,400V or
4,800V 60Hz AC utility power with both a Delta and Wye output
windings, and through some magic you now had six phases of 600V AC
that were only 60 degrees apart in angular vector, not three phases
120 degrees apart.
Much easier to rectify that 6-phase AC power into nice smooth 600V
DC - which was done with a rotary converter "filing off all the high
spots and dumping it in the low spots" that was for all practical
concerns an autotransformer motor-generator. Rather efficient, no
drama, they just worked.
No high current solid state rectifier plants would be available for
many decades, and the early mercury tube rectifiers were unreliable
and lossy.
Orange Empire Railway Museum in Perris CA has a totally restored and
operating rotary converter station on display and available for tours.
The Auto-Start and Auto-Stop sequencing with a huge cam switch (and a
current sensing setup so it could do it all by itself as a trolley
came through the served rural district) is a true display of Rube
Goldberg Engineering - and they let me push the button, too.
Nice panel mounted open-frame contactors with arc chutes sending
controlled little gouts of flame toward the floor...
He didn't say that. The context makes it very clear that by "phase conductors"
he means what the NEC refers to as "ungrounded conductors", or, in the
vernacular, "hot wires".
I won't go into the use of EMT as a ground source.
EMT is explicitly permitted by the NEC as an equipment grounding conductor.
[2008 National Electrical Code, Article 250.118(4)]
Yes, on the solid run, but not after any couplings, connectors etc..
A Greed Wire is to be inserted in the pipe to insure a real grounding
system and each box is to be connected to this ground.
You can not use EMT or the like for a grounding source directly, it
has to have a ground wire in it and the attached equipment in the
circuit also connects to this same ground.
As for the article you popped up, I think you'll find it proteins to
the use of EMT as a grounding buss point, meaning, several grounds can
come off this point using ground clamps from a single run with no
couplings how ever, a main ground source must be bonded to this pipe.
If you truly believe otherwise, then you are practicing very dangerous
habits.
Haven't you ever heard of galvanetic issues with EMT hardware?
I work in a manufactory facility where we still have a lot of older
machines using the access boxes as the ground sources for attached
equipment with no internal ground wire feed from the main buss. I can
say in the time that I have been there, we have seen several fires from
lose EMT hardware connections causing arcs because the attached
equipment was having ground issues.
With dust,oil and paint that has been apply to these machines, it
makes a nice catalysis for a fire.
Most of the time if your lucky, the lose connects will weld them
self's long enough to force the protection to initiate.
That NEC section starts out: The equipment grounding conductor
shall be ONE OR MORE OR A COMBINATION OF THE FOLLOWING...
You CAN used the EMT as the sole safety ground source in older
installations, it is "legal" to do so - but you would be a fool to
trust it as truly safe.
I always combine them - you make up the conduit system properly as
one of the grounding paths, then run a seperate grounding conductor
inside the conduit, even if it really isn't called for - because WHEN
someone gets hurt or killed they start asking a lot of very
uncomfortable questions. And if they don't like the answers...
Electrical safety is a very odd thing - For any situation there are
many correct answers and many wrong ones. Pick one or more of the
correct ones, and avoid the known wrong ones.
Legal, but not necessarily smart. But if the customer refuses to
pay for a seperate grounding conductor to get the job done a little
cheaper, and it is legal the way they want it done, there isn't a lot
you can do.
Then again, most of those old buildings have large amounts of steel
in them as building materials, and they will serve as secondary ground
paths - unintentional, but whatever works...
yeah, I bet their GFCI's really worked reliably in wet places.
I bet workers just love how they got shocks now and then from
aux equipment.
P.S.
Old mills are grand fathered in, unless, they do updates. Any
new installations are suppose to follow code. also, years ago
it was common when switching the mill over to higher voltages, they
kept on using lower voltage receptacles. There was a transition period
allowed to give plenty of time for the switch over, mean while, they
simply did things like 480 volts in a 240 volt receptacle.. AUX
equipment would use the 208/240 etc. plugs with 480 in it.
Still today, this is being done! shame! shame!
It gets corrected when an outside entity blows up an expensive piece
of test gear - and then bills the plant for it...
There are exceptions to the rules how ever, insurance companies
love to force these old places to update their basic electricals with
modern wiring systems, cause they know these old mills are full of code
violations.! and grounding and old clothe wire is a big one these days,
along with wire rungs not out of reach or being protected via something
like a jacket (romix) or pipe with a ground in it.
Voltages below 50, can ignore most of everything that is in there how
ever, in this state, maybe others, the state government is now
attempting to enforce (more) low voltage licenses, installers licenses
etc.. of just about anything.
Just goes to show, its all about the money..
---
You should see the screw up that had to be corrected, hacked if you ask
me. we had a long run, a service installed from one of our 2000 amp sub
panel over to a machine that has a 500 HP drive in it along with some
other goodies. 3 EMT pipes were run, one for each phase, 2 1000 MCM in
each pipe. They ran a ground wire only in one pipe. Now keep in mind,
these pipes are running in parallel from the sub panel over to the
machine. Doing thermo imaging, we found 2 pipes were getting hot when
the machine was under heavy operation for a while. That started a
investigation. Its then we found that only 1 pipe had the ground wire
pulled.
All this was done by licensed contracted electricians. We do this
when the job is to large for the in house electricians.
The problem was correctly, not the way it really should of been, but
it covered it. We placed pipe ground clamps on both sides of each emt
connector and connected with copper bar. We later found out, that ground
couplers are made for cases like that in old factories that didn't have
ground wires in. oh well. Expensive as hell, just like the large amp
fuse adapters.
And the power was probably imbalanced, too. Never worked with
anything that big, but AFAICT the best practice is to have multiple
conduits in parallel, but run one conductor for each phase through
each conduit, and cut the wire so they are all matching lengths.
Each conduit would have a 750MCM of all three phases, and a smaller
ground. That way they magnetically cancel each other out on start
current Locked Rotor Amps surges.
Having one phase in each pipe puts a LOT of stress on the hangers,
every time that 500HP motor starts those conduits with individual
phases are trying to jump off the wall from the magnetic forces. And
if they manage to do it, they will short out and all hell is truly
going to break loose.
Or just give up and install Bus Duct like a practical person.
--<< Bruce >>--
