Have a shorted section of Bx, melted anti short bushing. Was the connection of new romex wiring to old knob and tube lighting and replaced base board outlets. The white wires had the insulation melted together and thats why there is a spot where the white conductor has a hole in insulation.....this was found as a result of circuit not working and shorted after a space heater shut off power in adjacent breaker. but this one circuit could not be reset...looks like the return wire could have been melted through at bx jacket and anti-short was not doing its job. I believe this install has been like this for 6+ years, Would all the new base board boxes be subject to damaged romex.... there is not enough connected load to cause overload under normal conditions.
Could the damage have been caused by a lightning strike? I have seen wires melted together and I questioned the homeowner as to what he used in that room. He said nothing of a heavy electrical draw that he knew of. We ruled out heaters, heavy elec. equipment, etc. Then I asked him about lightning and he remembered that the corner of the house that we were in did get struck by lightning. It had melted wires in 3-5 receptacle boxes, but only one spot were the white and black wire actually touched.
Thanks Harold, So far it has been found in 2-3 boxes, 2 had a badly melted return wires(black charred most insulation burned away) on the section of exposed wire in the box, and the third was the section of bx cable without egc that I pictured. There was a plasma tv on the box with the most charring, but has been replaced a few years ago with lcd screen. The circuit has 42 in lcd 30 lcd (seperate room)component stereo, cable tv router, dvd, misc. lights and possibly space heaters. The owners Mother was there until she passed maybe five years ago, the existing wiring was replaced with romex wiring in 1988 with permit, but I have found that some of the work was done with the metal cable connectors at the box all the way tightened pinching the insulation, and in a couple places the cable insulation was sliced when the sheathing was removed. in these boxes it is possible the white wire was contacting the bonded box from the new romex wiring, but the bx feeding the new wiring was not bonded back to source from due to arcing. I think the wireman replaced the bx cabled baseboard outlets with romex to put in grounded type receptacles but didnt get the last three feet into box with the feed to old K&T lights and two new romex. I've also heard of some guys from other countries doing loops of wiring like old telephone wiring, I there a way of seeing a wiring loop with TDR or oscilloscope. I am planning to replace full 8/16 ITE panel with 20/40 to allow for AFCI breakers to protect existing individual and three wire circuits (5 exist. but that won't be til next month, and they run a preschool in the remodeled basement.
ALL loop circuits, being loops, come back to a common point of distribution: in a residence that would be the panel, itself.
Obvious signs that a circuit has a loop ( or ring ) structure would be double hots at each OCPD/ (fuse/breaker) time and time again.
Whenever you follow the amateur hour always check for crossed over conductors. These create unintended loop circuits, too.
1) Kill all breakers -- or at least all breakers on one phase.
2) Then power up one single breaker at a time. Crossed over hots will show up as full voltage backfeeding an open C/B. This shows up like crazy with an influence tester -- but is best determined by a 'loading' voltmeter.
[ Wiggy or Simpson analog style meters]
(DMM, with their high impedance/ chain capacitor circuits, can sometimes fake you out with ghost/ capacitive coupled conductors.)
This method does not require you to even turn a screw.
3) Where double hots are under a single circuit breaker (two conductors per one poles) it's necessary to pull one out and cap it off, with a voltmeter lead screwed under the wirenut, too. Then start throwing breakers, one by one.
ALL of the circuits (#14 & #12, especially) that have been worked over by an amateur need this treatment. It's as common as dust for ammateurs to cross-backfeed breakers and never spot a problem. Of course, having a #14 conductor double fed permits brutal overloads to pass without tripping either breaker.
As long as the OCPD scheme is not defeated in this manner, you're half-way home. Never assume that an amateur has the first clue about keeping his circuits squared away. (You're lucky if that happens in residential consruction at all.)
I have a TASCO circuit identifier. If you are so fortunate then nailing down circuit structure/ labeling the loads goes very, very quickly.
[The TASCO CMT42T injects 42 unique signals -- by induction -- down each of the hots in a panel. These can be detected whether the receptacle is hot or cold. With (Edison to 1-15R adapters and a 1-15P pigtail) one can easily inject or read TASCO signals even in lighting circuits. BTW, this gambit can't get past electronic switching fluorescent power supplies. You have to fall back on tossing breakers.]
To save steps, always bring common plug testers and extension cords. You can then watch the plug tester while at the panel -- with the extension trailing off to the subject receptacle.
Watch out for incredibly chained GFCI circuits. These tend to be totally fouled up all of the time... as in way too many receptacles on one circuit. This leads to a crazed hunt for the tripped GFCI -- which often has NO obvious linkage at all to the dead dependent receptacles.
Get used to residential circuits having no relation to logical use. Instead, the boys wired it up to suit themselves. This often means that some circuits are massively loaded -- and others just 'coast.'
As for neutral conductors being fried by lightning: they take the brunt of the damage because they are fulsomely grounded. The actual process pumps current/ electrons UP out of the ground and into the sky -- through a conducting 'hole' -- as if the sky was some gaseous semiconductor P-doped wafer.
So the jolt preferentially races up the networked whites -- and greens. The hots actually work as capacitive/ or inductive elements during the bursts, so they pretty much cruise.
Other indications that lightning was the culprit: heat damage at ferrous 'chocking' points along the critical path. This type of damage indicates extremely rapid changes in voltage state -- and to extreme levels, too.
Only massive current jolts create these weird localized melt zones.
Thanks Tesla, I'll post pictures as soon as I get my other computer up and running, this one had me stumped, and only the dead short at the bx feed, then every thing worked except I still had a short from the white to the box bond egc, the overload? probably finally melted the bx to where it was bolted and not resettable, but there was no smell of burning in any of the three boxes that had charred wires, and no soot up the wall as I have seen in other boxes that had charring.....so would a lightning flash burnoff the insulation without charring in side of box?
It e n t i r e l y depends upon how close -- and how intense.
It's kind of like being a soldier during a heavy bombardment. If the shell hits too close... no-one hears another word.
If the action is even half-a-mile away... no biggie.
Lightning is like that.
EMF travels ALL routes ... it's just that the EMF takes the least resistance route every moment in time.
It's also REFLEXIVE. THat is: the current that is flowing in that instant has a direct and prompt bearing on how the entire system will shunt the flow. This is solved in normal circuits by using GOOD conductors: copper, silver, etc. so that the juice ALWAYS stays flowing within / on the skin of insulated/ even uninsulated conductors.
In a lightning event, stuff that one would never consider suitable become part of the conducting 'network.' Electricians try and keep that network from including living humans.