I had meant signal common, not earth
ground. The DC
ground, not the AC
ground.
That is odd that the shared
ground was on the same
phase and I'm guessing the same
gauge as the hots and neutrals. Even more strange that the green wires were carrying
current and nobody noticed that something was wrong. At least they used a
J-box and metal
conduit. Otherwise would have burned down the whole building.
@DIYLED The building was built from its sub basement foundations on up beginning in late 1971 and approved for occupancy / final construction clean-up during the summer of 1973. All of the work was by union contractors including members of my local / home
IBEW local.
Whether by spec' or pure coincidence, the ou puts of all
phase A dimmers were routed from basement to the attic within one 2 or 2.5 inch I.D.
conduit.
Likewise all
phase B dimmers and
phase C dimmers.
At the basement / source end, all neutrals were connected to the common
neutral buss.
All conductors were 10
gauge TW to partially offset
voltage drop over the length of runs.
A suitable quantity of neutrals were included within each of the three conduits.
Each of the three conduits were fitted with threaded grounding / bonding bushings on both of their ends.
Each of the three conduits contained a single, TW10
gauge grounding
wire bonded to the
ground bushing at either end.
The
ground wire was NEVER intended to carry
current except during fault conditions.
When you (for whatever reason) enclose all of the conductors from the same
phase, in the same
conduit, with a random assortment of neutrals, you've created essentially an 'air core'
transformer.
Air cored transformers are more commonly employed at RF frequencies and transform FAR less
power at 60
Hertz.
None the less,
transformer action / eddy currents / whatever, resulted in a
ground loop comprised of the single TW10
gauge insulated
conductor within each
conduit looping back via the appreciably larger / much lower resistance threaded rigid steel
conduit surrounding the insulated
grounding conductor.
In the survive the battle of the
transformer coupled
ground loop currents, the single 10
gauge was predictably the loser.
Only trouble was: Apparently no one had troubled themselves to assess the odds.
Go figure / who'da thunk. Only another of the many things innocently overlooked during inspections.
Nobody was opening the basement box overhead within the poured concrete slab of the basement ceiling / 1st
level floor.
Had we not decided to utilize a spare
dimmer, via spare conductors, NOBODY'D have ever noticed the flames that erupted and burned themselves out within a 16 x16 or 24 x 24 inch
J-box anchored to a poured concrete attic wall approximately 8' above the attic floor.
Upon opening the
J-box we simultaneously saw the charred
insulation, smelt the fumes AND felt the heat rising by convection and escaping from the attic ends of all three conduits. With all three conduits ending in the same box, the heat
build up was appreciable.
None of the other conductors were showing any evidence of overheating since they were loaded well within their ratings.
At the time, I was a very junior
IBEW member in my fourth or fifth term catching calls with Hamilton's IA 129. My boss within Hamilton Place was Head Electrician Tom Taylor; Tom had an excellent relationship with the
IBEW foreman who'd built the building from undisturbed soil to the roof.
Rather than
call in an electrical inspector, Tom chose to contact the
IBEW foreman who visited the next time he was passing.
Post much humming and stroking of beards, the decision was nade to replace the supplied solid
cover with a
cover band-sawed from from a sheet of heavy
gauge expanded metal to simultaneously bar access to nosy fingers while leaving adequate open space for rising heat to
escape.
Years later, another
CD80 / 48 slot / 96 x 2.4 Kw rack was installed in the attic, on the floor and anchored to the poured concrete side wall immediately below our infamous
J-Box; a 3.5 or 4" run of
conduit was installed up the wall of a nearby, 84 step, spiral stair to supply a 400 Amp, 3
phase,
5 wire feed to
power the newly installed attic rack.
To the best of my knowledge, the
CD80 rack is still there, comfy and undisturbed in its happy little attic nest.
Toodleoo!
Ron Hebbard