[conspire] 1999 called, and wants its backhoe returned
Texx
texxgadget at gmail.com
Fri Apr 17 14:59:02 PDT 2020
The last diagram is most instructive.
If you turn off one of the PCs, you increase the load across the remaining
PCs on that side of the N
One of the problems to be dealt with up at Sullys place will be upgrading N.
If most of your load is 240, you dont use the N much and its not a problem.
Also older buildings get away with it.
The problem comes with modern code and electrical circuit demand.
Current code calls for an outlet every 6ft around the perimeter of a room,
and every 4ft in bath, kitchen, or wet areas.
Its tempting to pop another breaker in the panel (which is probably already
at capacity) and run those extra circuits.
Now that you have the ability to draw a lot more at 120, you start
overloading the N.
Imagine a house built in the 30s, knob and tube wiring in the attic, with
the N running right down the ridge of the house.
The wiring is gorgeous.
Ove course additional circuits have been added, each time tapping into that
single N for the whole house.
These wires all lead to an old fuse panel.
At some point in time, the single meter box that connected to the fuse box
via conduit was replaced.
The new panel Includes breakers and the new meter.
A run of conduit connects this box to the old fuse box.
The fuses in the old fuse bax have been removed and there is a rats nest of
wire nuts connecting wires that once connected to
fuses to wires leading out to the bew breaker panel.
The new panel is a 100 amp panel.
There are no 240 circuits in the panel.
The N connecting the boxes is a #12 wire.
What could go wrong? (I can hear you scream from here)
Once again, in an AC circuit, the other end of the circuit will always be
180 from one end to the other.
This is IN PHASE!
An often used labeling scheme is H0 & H1.
When we refer to phases, we speak of phases as seen in the panel.
H0 & H1 are IN phase at the panel because one is the return of the other.
It IS OK to refer to the other hot as "Other leg". Thats quite acceptable.
I would say that this "phase" concept is something that escapes more than
hald the population.
AC is weird stuff, very weird.
If you dont understand it, Im not going to call you stupid. It can be a
pain to wrap your head around.
I WILL tag you on it for using the whord phase in the wrong place, but i
wont call you a dummy for it.
Phases are not too hard to get unless you throw in that N and THATS where
it starts getting wierd.
There is also somehting called a "stinger" lead which happens in "Open
Delta" wiring and Im going to leave that out of the discussion.
I will point out that 277vac is NOT stinger, however, and leave it at that.
The economy was starting to come back in 2010. Private IP space had bought
us time, but this time we were REALLY out of IPv4 addresses.
I had taken a 3 month job with a startup networking company that was
embracing IPv4 at a time when ATT was hoping it would go away.
Asia had totally burt its last IP block and the rest of the world wasnt far
behind.
Being an asian company, the aisian telecom companies discovered out IPv6 &
app level switches.
(This is where I learned to eat fruit on the bottom yogurt with chopsticks.)
We went into that hockey stick curve just as I started there.
I ended up staying 2 years, as the demand for computing resources climbed
faster than we could satisfy it.
It was an interesting time that included literally exploding chiller units,
electrical panels hot enough to burn you.
We had already expended into 2 more buildings in the complex to spread out
our electrical load.
I had installed thermal monitoring in all the labs, and wrote scripts to
alert me about alarms, and shot the data to rrdtool.
It was my birthday in 2011. I came in the main building and the "B" side
was dark.
Main building was 2 offices next door to either with a couple doors between
the suites.
I had noticed that my alerts had stopped at 4am.
I fumbled my way to my cube and measured the outlet voltege. 32 vac. How
quaint!
Grabbing a screw driver, I headed to the electrical room.
Voltages in the breaker panel for the B side were also screwy.
Then I opened the 480vac junction panel (To the horror of the VP of OPS)
OK Phase-A to Phase-B 480, cool. Phase-B to Phase-C....
WTF?!?!?!?! Where is the bussbar to Phase-C ???????
Oh, I see.... That must be the slag in the bottom of the cabinet....
I had Cupertino Electric on speed dial on my Crackberry along with the
chiller service.
This was a 400 amp bussbar, by the way.
(Did you face palm yet?)
By the way, this was not the first time that we had run part of the
building off a portable gen set.
That was the same week that I discovered the rotten roof truss under the 25
ton chiller, RIGHT ABOVE MY F***ING CUBE!
Thats quite another story.
We eventually got a new building with a huge engineerting lab, enabling the
consolidation of the company back into a single building.
It had a 3 phase 480 feed to the engineering lab at 4000 amps.
On Fri, Apr 17, 2020 at 1:23 AM Michael Paoli <
Michael.Paoli at cal.berkeley.edu> wrote:
> > From: "Rick Moen" <rick at linuxmafia.com>
> > Subject: [conspire] 1999 called, and wants its backhoe returned
> > Date: Mon, 13 Apr 2020 13:23:54 -0700
>
> > pipe) along Monte Rosa. In front of 815 Monte Rosa they hit a PG&E
> > secondary and shorted one phase to the uninsulated neutral (according to
> > PG&E). The result was that the two lines (beyond the transformer)
> > coming to our house that should have been +120 and -120 became +85 and
> > -155. Electronic stuff on the 155v phase (without a surge protector)
>
> And might've mentioned it before, once-upon-a-time. Many moons
> ago, I dealt with a kind'a similar situation at work.
> We'd recently moved into a previously occupied building.
> The (our) business included computers (mostly selling PC "clones"),
> and servicing thereof, accessories, etc. The previous tenant was a
> more industrial customer (some printing operation, if I recall correctly).
> Anyway, the room that was used for computer repair, had been repurposed
> from whatever it was before. It had workbench-like surfaces all around,
> with (nominally) 120VAC outlets a plenty readily available all around the
> workbench type working surfaces.
>
> Well, one day, things went badly. A few things semi-randomly fried on
> circuit(s) on there, when there was (otherwise) no particularly good
> reason for 'em to do so. Our tenancy being new, I didn't know the
> relevant breaker (or maybe even where they were) for the applicable
> circuits - and all else in the building seemed perfectly fine (was
> likely just one or two circuits or so for that entire room).
> So I gave the directive to the techs, to unplug or otherwise disconnect
> everything from all circuits in the room as fast as humanly possible.
> And so they did. Then we went about diagnosing the problem - as it
> still appeared limited to just the circuits in that room. Doing a bit
> of checking ... two circuits. But alas, not hangin' very close to 120VAC.
> But if one measured between the two hots from the two circuits, a nice
> reliable right around 240VAC between 'em. So, me thinks (and was later
> confirmed) - flakey neutral ... basically 2 circuits, 180 degrees
> out-of-phase (or call it center tapped one phase - for the most part
> effectively the same for the end circuits).
>
> And sometimes this is (or used to be?) a semi-common problem. In more
> industrial environments, there might be simple 2-phase like that
> (180 degrees out-of-phase), or might be more (3-phase also being relatively
> common). In many industrial settings, it is presumed, and often the case,
> that the loads across each phase are very well balanced - not only
> power/current, but phase relationships, etc. Essentially if they're
> all equal loads, and well shaped sinusoidal loads - even if V/A shifted
> a bit - so long as they're all shifted the same bit, a "convenient"
> interesting bit happens. In such case, the current on the neutral, in
> that nominal case, is very much zero (notwithstanding some noise and
> imbalances). Still pretty close to zero - much less than the hot legs,
> anyway. So, what - at least was - common practice, the neutral would be
> somewhat undersized, relative to the hots - as it would be expected to
> "always" carry less currents than the hots. I might be mistaken, but I
> think older electrical codes (and/or practice) actually allowed that /
> and/or was relatively commonly done. Well, that's sort-kind'a all
> fine and dandy ... until the loads aren't well balanced, and there's
> quite a bit more current on the neutral - can be as high as at least
> any one of the hots (or approximately that), at least in worst case
> unbalanced loading. Well, that could be a really bad thing if the
> neutral was undersized. The other related thing, is earlier well balanced
> load case, the neutral could've gone decades or more, without carrying
> hardly any current. Then the loads change (go from huge industrial
> motors, to dozens of switch mode power supplies for PCs - which have
> rather messy not-very-sinusoidal load characteristics), neutral current
> can go way up ... where that part of the electrical really hasn't been
> "exercised" like that before. So issues such as latent defects,
> could go from what was a hidden issue, to a bad issue and major
> failure. In our case, it was one of the neutral connections,
> which probably for decades before that hadn't shown as an issue,
> as it hadn't carried enough current to cause it to fail,
> but with heavier currents on the neutral, the not-so-great
> connection failed to an intermittent open ... which caused the
> two individual 120VAC circuits, relative to their now oft floating
> neutral to vary wildly anywhere from 0VAC to about 240VAC ... so some stuff
> got fried.
>
> Hmmm, maybe teensy diagram helps.
> So, let's say this is my (crude ASCII) drawing of secondary of a center
> tapped transformer. From center to each side, 120VAC, or 240VAC between
> the
> two end sides of the secondary:
> || C------------------
> || C
> || C
> || C
> || C------------------
> || C
> || C
> || C
> || C------------------
>
> With a bunch of PCs on the 120VAC circuits, we have something roughly
> like this:
> || C------------------+--+--+-----
> || C | | |
> || C PC PC PC ...
> || C | | |
> || C------------------+--+--+-----
> || C | | |
> || C PC PC PC ...
> || C | | |
> || C------------------+--+--+-----
> So far so good.
> Note also in the above, were it something other than PCs,
> or otherwise full balanced loads on each of the two legs,
> the neutral current would be (approximately) zero.
> Then the neutral goes (intermittent) open:
> || C------------------+--+--+-----
> || C | | |
> || C PC PC PC ...
> || C | | |
> || C-----X X-----+--+--+-----
> || C | | |
> || C PC PC PC ...
> || C | | |
> || C------------------+--+--+-----
> Now things are ugly. The PCs are essentially in series across 240VAC.
> And to the extent their loads across the two legs aren't highly balanced,
> they can be getting anywhere from almost 0VAC to about 240VAC. Not good.
> And, that was before most power supplies were auto-ranging. Yes, (some)
> things fried.
>
> Anyway, that's a problem. Likewise if the neutral shorts
> (fully, partially, or intermittently) to one of those two hots.
>
> Anyway, depending where the fault happens, this can happen to an
> individual (pair of) circuit(s), entire house/building, or even a
> neighborhood - really depends on the scope and coverage of the
> circuit.
>
>
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--
R "Texx" Woodworth
Sysadmin, E-Postmaster, IT Molewhacker
"Face down, 9 edge 1st, roadkill on the information superdata highway..."
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