3 phase residential with one leg 'out' - Page 10

 
Yes I do see the problem with using water pipes for sole ground..as in
my case with a bad nuetral for long time - the water pipes worked well
enough as a nuetral to keep things 'going'...- just waiting for the
right pipe to be cut...or perhaps a water meter being changed...

  tabers7823@mypacks.net wrote in


Oh yea, Water meters.  They used to be made of bronze. AFAIK
Neptune still maked Bronze body meters.  But many are
made of plastic.  That breakes the ground connection.
I seem to recall that in my parents house, there was a wire
jumper around the water meter.  That is, a wire connected
to the pipes on either side of the water meter that would
maintain the ground if the meter was removed.

 
Gordon wrote:

Those jumpers are generally required. Water company folks have been
badly shocked or worse trying to change a meter that has an unknown
ground fault current flowing across it.

 On Mon, 3 Nov 2008 13:20:02 -0800 (PST), tabers7823@mypacks.net wrote:


You have what is called split open delta service.  A conventional 120/240
single phase transformer supplies your domestic needs.  A second, usually
smaller transformer is connected in series with the first.   The neutral is
not connected and the far end supplies the 3rd or "wild leg" of the three
phase circuit.  With this configuration you have 120/240 single phase and 240
3 phase.  If the 3 phase load is significant, they will add the third
transformer to make it a full split delta.

This configuration is used when there is only a small 3 phase load, say, a 3
phase AC unit.  Above about 5 tons, 3 phase ACS are much more efficient than
single phase so it's worth the extra wiring to get three phase.

This connection is also used in situations like yours where you have machinery
that needs three phase power.  Some utilities charge extra for 3-phase service
but many others (including mine) don't.  If your utility charges extra for
3-phase then a rotary converter  may make more sense, depending on the load.
If there is no surcharge then by all means use the utility-supplied 3-phase.
As you're probably aware, it's difficult to get a rotary converter to output
well balanced voltage on all three legs plus the starting torque of 3 phase
motors is generally reduced.

What the line crew found is typical when a subsequent customer no longer needs
the three phase power. (that also kinda makes me think that there is a 3-phase
surcharge).  The pull the primary fuse on the second transformer to stop the
losses in that transformer.


I's different only that you have the split leg that supplies you with 120/240
AND 240 3phase.  Small industrial connections where the majority of the load
is 3 phase are more usually wye connected on the secondary which provides 120
volts phase let to neutral but only 208 leg to leg.  The result is 120/208
single phase and 208 3 phase.  Motors can be obtained that will run on either
240 or 208, sometimes with re-strapping so it's no big deal there.  

The problem comes with 240 rated heating appliances.  Power varies by the
square of the voltage so a 240 volt appliance (oven, water heater, etc)
operated on 208 has significantly less output.  Commercial heating apparatus
(restaurant and small food manufacturing) can be had with heating ratings of
either rating.  The problem comes primarily with home cooking appliances and
water heaters.  208 volt water heater elements are available so that problem
can be solved.  AFIK, 208 volt ranges are not available.

This is domestic problem is primarily seen in large apartment buildings that
are served with 120/208 wye power.

For some reason that I don't understand despite being a utility engineer for
half my life, some utilities are hostile to the split delta, open or close, as
you have.  Fortunately mine isn't so when/if I get my shop moved up here I'll
have 3 phase installed.





Yes.  HOWEVER.  The slang for this type of feed is either "stinger leg" or
"wild leg".  That refers to the third leg that makes up the three phase, the
one that was just energized in your situation.  That leg is around 207 volts
to ground.  If you mistakenly use it to supply a single phase 120 volt branch,
you will smoke whatever is connected to it.

By code, that leg is supposed to be marked orange and/or conspicuously
labeled.  Now that it's energized, make damned sure that there are no single
phase loads connected to it.

IN my previous place where I also had split delta three phase power, I had two
separate breaker panels, one for three phase loads and one for single phase.
That made absolutely positively sure that someone could not come along and
connect up a single phase branch to the wild leg.

The entrance wiring in my place was very old, had been severely overloaded in
the past and had brittle insulation so I didn't move it any to be able to mark
the wild leg.  I simply spray painted it fluorescent orange at all points
where it was visible.


Nothing to do with grounding.  Everything to do with the integrity of the
neutral from the transformers to your panel.  Open neutrals aren't all that
uncommon and they can do massive damage, even start fires, depending on how
unbalanced the legs are.

Consider the worst possible situation - the neutral opens and there are a
number of 120 volt loads on one leg but none on the other.  The voltage at an
outlet on the unloaded leg is 240 volts line to neutral.  Plug in an appliance
and now ohm's law applies.  The many loads on the other leg are essentially
zero ohms so practically no voltage drops across them.  Almost all the 240
volts appears across that one appliance and smokes it.

You need to start at your breaker panel and check all the neutral connections.
Wear rubber or dry leather gloves (because the neutral could have voltage on
it) and tighten the connection.  If the connection was loose and the wire is
aluminum, it would be a good idea to have your service killed, all the wires
removed from the panel connections and coated with NoAlox or some other
anti-oxidant and re-tightened.

The problem with aluminum wiring is that once it starts to heat, high
resistance aluminum oxide starts building.  The wire has a high thermal
expansion coefficient and cold-flows easily.  Thus, the heat caused by the
oxide will heat the wire, causing it to expand, flow out from under the
connector, loosening it and causing more heat.  This continues until either
the oxide builds up enough to open the circuit or the connector burns up.

The next thing you can do is measure the voltage from each leg to neutral.
They should be within a couple of volts of each other.  Turn on a heavy 120
volt load and measure again.  The difference may increase a little but no more
than 1 or 2 volts.

If you see that change in voltage, it's time to call the utility and report an
open neutral.  From the utility's perspective, especially the low level
dispatchers who answer the phone, every customer is a dumb ass so you can't
just tell them that you have an open neutral.  The key words are "When I turn
on a 120 volt heater, lights in the room dim while lights elsewhere brighten.
It is getting worse rapidly."

A second cause of this problem, normally seen in old houses, is inadequate
service drop wiring.  Both my last house (about 30 years old when I bought it)
and my restaurant building had #6 copper wire feeding 200 amp panels.  2-0 is
now the standard for aluminum wire.  I can't recall the copper size but it's
almost as large.

This size wire was too small for 100 amp service and outrageous for 200 amp.
If you have small wiring like this then you need to try to have the load on
each leg balanced as well as possible.  That is, minimize the neutral current.
Here you need a clamp-on ammeter.  

With your house "turned on" as it typically is, measure the neutral current.
If it's significant, say, more than about 30 amps, move loads from one leg to
the other until the current is nearer to zero.  Many times that can be done by
swapping adjacent breakers, as adjacent breaker slots go to different legs.
The legs alternate as they proceed down the row.

Next, complain to the utility that your service drop is inadequate and that
you're tired of lights dimming when something like the AC or stove is turned
on.  If you can "talk the talk", ask to speak to someone in engineering.
You'll need to have your neutral current and each leg current figures handy
and know approximately what size your entrance drop wiring is.  If it is 3
separate wires then it's probably too small.  If it is modern "triplex" (two
hot wires wrapped around an un-insulated neutral) then it may be OK.

If you're lucky, they'll agree and send out a crew to change the drop.  That's
what happened at my restaurant.  I was able to talk to an engineer.  I have a
power quality instrument (a multi-thousand dollar instrument that measures
sags, surges, dips, and other power anomalies) and could tell him what my
worst sag voltage and neutral to ground voltage was.

In rural Cobb county (Atlanta), the co-op was not nearly so cooperative so I
had to resort to other means.  I dropped a tree across the line to knock down
the line, dragged the tree out of the way and then called in a power outage.
The line crew was pretty decent, especially once they found out that I was in
the utility biz too, so they reported the line failure as due to unknown
causes :-)

In any event, you need to get to the root of this faulty neutral and fix it.
If the neutral opens up it can cause a fire in the worst case and burn out
lots of expensive stuff in the typical case.

During that wind-storm outage in Ohio a few weeks ago I had a friend suffer an
open neutral inside his emergency generator.  We traced it to sorry-assed
workmanship at the generator factory.  It smoked about $10k worth of stuff in
his house including his entire overhead projection-type home theater system.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
I don't speak Stupid so do speak slowly.

 On Thu, 06 Nov 2008 17:25:58 -0600, david.williams@bayman.org (David
Williams) wrote:


I knew all that...being British!   It's the mention of breakers that
confused me.  The plugs always have fuses fitted (without exception)
usually 3A,  5A or 13A  although there are fuses available in a number
of other ratings - 1A, 2A, 7A and 10A.

The only plugs that have breakers as such are ones that have a RCD
integrated in an enlarged plug body, but despite that they are always
without exception fused.  They were common maybe 15 years ago for
lawnmowers and hedge cutters but are much less so now when whole house
RCD's are mandated for new installations and commonly retrofitted  in
many properties

In older installations without a whole house RCD, a dedicated socket
outlet for outdoor equipment use that incorporates an RCD's was
occasionally fitted although their cost approaches that of a low end
whole house RCD and MCB consumer unit.


--