In regards to my 3 phase problem; first, everything works, no smoke, no
shocks. I have 3 big wires coming into a big box with 3 big fuses (220v,
400 amp) from there it distributes to a number of smaller fuse boxes and
breaker boxes of various ratings. From there it goes to fused disconnects
on the wall by each of 50 machines. There is no ground wire other than the
conduit. Is this "D" or "Y"? There are no transformers except on the pole
outside. I get @220 between any 2 legs everywhere. In one section of the
plant, I get @220 between each of 2 legs and the conduit but nothing between
the third leg and the conduit. I think this is a symptom of a problem I had
better track down. The only scenario I can visualize is that 1 leg is
shorted to the conduit somewhere and a section of the conduit has a break in
conductivity somewhere so that the conduit on this side of the building is
floating at the potential of the 1 leg. Does this make sense? If so, I
have to find the short and the break. I guess if I find the break first,
the short will become more apparent. (Bzzzzzzttt)
<<A good friend will come and bail you out of jail...but, a true friend
will be sitting next to you saying, "Damn...that was fun."
You probably have a Corner-grounded Delta system in that part of the
shop. Go look at how it hooks up. (Thank you Google!)
You could have an open conduit ground and see odd readings - in that
case, it couldn't hurt to come in on the weekends and add ground wires
inside the conduits, and bond all the boxes. Conduit was never meant
to be a grounding conductor for large circuits, it's way too easy to
get a popped joint or a loose locknut and lose the ground continuity -
they only allowed it to be grandfathered like that because they didn't
want to force everyone to rewire old installations.
It is entirely possible and normal to have more than one power feed
to a commercial building, where the old part of the building has a
120/240 1Ph for the lights and a Corner-ground 3-phase Delta panel
solely for equipment. Later they added a "High-Leg" 120/240 Open
Delta or 120/208V Wye feeder. Then you get bigger and need to run
some 480V equipment and add a 277/480V Y or 480V Delta power system...
Some places that have grown and expanded many times often have many
different feeds and meters - you are at 102 Maple St. but expanded
into 100 & 104 over the years. Small shopping centers are built with
one feed for each unit, and a tenant comes in and takes three or four
(or ten) units in a row.
You need a ground reference on the power coming in - either a Y with
the center tap grounded, or a corner-ground on the Delta for normal
use. Ungrounded Delta systems are used in heavy industries (like
refineries) where they can't have unintended shutdowns (turn the power
off in the middle of a run and "Welcome to Bhopal!"), but they also
have elaborate monitoring systems to signal when there is a ground
fault present in the system. They can bypass that equipment on the
fly (bring in a portable pump and hoses, and go around the bad
permanent pump) and fix it the next time it is safe to shut down.
God only knows what you have in your shop, especially back east with
200-year-old buildings... Call a local electrician and have him come
figure it all out for you. He can make a "Ladder" or "One-Line"
diagram that clearly shows what panels are feeding from where, and
mark all the panels as to voltages and power types. He can also add
up all the loads and see if you have a panel overloaded, and if
another panel is underloaded you can shuffle the wiring around.
And sometimes, when the old equipment is really on it's last legs
it's better to catch it early, bite the bullet, and plan to change it
out over a few weekends. BEFORE the old panels blow - that can put
you out of business (partially or totally) for several weeks, and in
some businesses a few weeks of unplanned downtime can be terminal.
(And the cost of renting and running a big generator and hooking
everything up to it so you can stay open can sink you just as fast.)
They can also come by on a day when you're running all the equipment
hard, and check the panels for bad connections and circuit breakers
with an IR non-contact thermometer. Handy little gadget... ;-)
--<< Bruce >>--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
You probably have a Corner-grounded Delta system in that part of the shop ...
Corner-grounded Delta is obsolete.
However, *some* point in the service should be grounded, for safety's sake.
If not the center tap of a 120-0-120 transformer (the usual residential case),
then the service can be grounded through an impedance.
While it is possible that he has separate services, I am amazed at the
number of people that don't really listen to what he wrote.
It sounds to me like he is essentially correct. He has part of a
circuit that is (probably) grounded, and has one leg open, probably at
one of those fuses. IF he has rotating equipment on line, it would act
as a rotary converter, filling in the "blown" leg.
And contrary to what bruce said, conduit was indeed intended as the
ground. On the other hand, he is correct about how easily it goes bad.
It is possible that the circuit is not "grounded" but is just being
pulled low by an incorrectly applied load [Hot to ground].
There are several ways to troubleshoot this.
#1 is to get an electrician
#2 is to identify on your one line or 3 line (you do have one don't
you) where the affected area(s) are fed from.
#3 is to get an electrician
#4 is to shut down the fused switches, and check the fuses.
#5 is to get.....
I have read the two posts and the several replies to them. What I hear is that
you have a small industrial operation with 50 machines that are using or
connected to some type of three phase arrangement. And Ben Franklin wired it
But the first thing that comes to my mind is that you are unsure of what the
posture of the electrical system is. IOW you don't know anything about what you
have except you have a small problem that will grow by leaps and bounds at a
time when it will cost big bucks.
So get someone in there who can get a handle on things and carefully document
things. Then you should have a good picture of things and can make plans to
bring things up to the year 2004.
Otheres have suggested that you have a corner grounded delta arrangement that
in my opinion is just asking for trouble. Go to a Wye arrangement with the
resulting simplicity. Yes all the runs will be 4 or 5 wires but it will be
worth the piece of mind.
And finally until there are published specs on conduit and the accompanying
fittings use the conduit for its best purpose. Support and protection of the
electrical conductors. Run ground wires if you need ground. Have the grounding
system as carefully organized as the rest of the system.
Enough for tonight.
Could be either one. Delta or Wye has to do with the way the source or
loads are configured. Either can be distributed on 3 wires.
Ok, that removes the possibility that you have corner grounded delta
in that section of the plant.
There could be a break in the conduit, or there could be an open in the
hot leg that reads zero to the conduit (probably a blown fuse). There's
definitely a short somewhere, could be in a machine.
The first thing I'd try after a visual inspection of the conduit and wiring is
to switch off all the machines in that area at their fused disconnects. That
will prevent them from acting as rotary transformers and manufacturing a
third leg of the 3 phase which would confuse troubleshooting. It'll also
clear any short that may be internal to a machine.
Then, measuring line to line in that area of the plant, you may find that
one leg is dead. Trace that back to where it is broken (could be a fuse
in the subpanel feeding that area, could be a bad connection, could be
a broken wire).
Or if you still have good phase to phase voltage, reference one lead of
your voltmeter back to the service entrance ground. Now use the other
lead to probe the conduit. If you read *any* voltage (other than a volt
or two of stray), you're probing *beyond* the break. Work back toward
the service entrance until the voltage reading becomes zero.
Once you've located the open (either in the hot or in the conduit), don't
repair it yet. Find the short first. You know the short has to be somewhere
after the break.
Hopefully, opening all the machine disconnects cleared the short. You
can now close them one at a time while monitoring with the voltmeter
to find which one produces the short (the conduit will become "hot"
with respect to service entrance ground). Then troubleshoot that
But if the short is still present with all the loads disconnected, then
you're going to have to play detective by breaking the circuit in various
places until the short does clear, then examine the last section you
broke to find the short. (A binary search strategy can isolate this
After you clear the short, you can then go back and repair the break.
I am electrician by trade. And I refrain from giving advice. The person
asking it may not be sure of exactly what he is describing and how he is
presenting it (which may or may not be the case here). Then "we"
misinterpret it or "we" are not fully experience in that principle of the
trade and while we are attempting to give good helpful advice, and maybe
even accurate advice based on the information provided, the user may not
understand the principles or advice given and attempt to use them in his
situation, which may result in catastrophic events.
Folks, we spend hundreds, if not thousands sometimes on our equipment for
the enjoyment and pleasure and sometime yes, to make money with. Why in
the world would we be so cheap and reckless and not paid a licensed
electrician what he is worth to come out and do things right?
I have seen time and time again someone attempting to do their own work,
and while it may work, its down right scary.....and then there is the
electrician who is out of his realm, in other words no or little
experience with this sort of electrical work. Call and ask outright if
the contractor has someone with industrial electrical experience
including dry transformers.
Don't mean to upset anyone, but I would rather see these guys come back
with the ability to ask questions and still have a shop that hasn't gone
up in smoke......
The best advice, call and pay $50 or $100 to have someone professionally
look at things.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.