[conspire] 1999 called, and wants its backhoe returned

[Michael Paoli]

> 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.