Yes, I am still here, thanks for the kudos.

You have a good grasp of the mechanics there. Now let's take it a little further.

As the heat from a fire impinges upon a cable or conduit containing conductors the insulation begins to melt and char. Char produces carbon. Carbon is a semi-conductor material. The presence of carbon between the energized conductor and conductors of different polarity or ground creates a fault path. An arc through this path can generate sufficient temperatures to melt and possibly sever the copper conductors. The overcurrent protective device may or may not function as a result of the arcing.

I still use the term arc fault mapping or arc mapping.

Mark the arcs you have found. I hang colored surveyors tape on them. Also draw a scene diagram and plot the arcs. Step back and look at the patterns. Take pictures. The progression of the fire through the structure can sometimes be shown by mapping. The presence of absence of arcing in an area can point you to the area of origin, or dispute it as well. Generally speaking the arc found at the location furtherst from the source of supply happened earlier in the fire (not always, you have to take construction, routing, etc. into consideration). Beware of pitfalls.

1. If you do not know where the arc severed wire came from you cannot map it. That may sound silly, but, if you have three stories in the basement you are not going to be doing much arc mapping.
2. Do not directly compare different types of wire and installations. i.e. Arc severed NM cable to cable in conduit, or exposed cable to cable behind sheetrock. Compare apples to apples.
3. The wire does not have to be severed. Use a latex glove and run the wire through your fingers. An arc notched wire will snag the glove. The breaker may have tripped when the fault occured and the arc only notched the wire.
4. Do not confuse a fire melt with an arc melt. Do not map fire melts.

As an extreme example, let say you have a fire that you believe started in the utility room. The main electrical panel is located in the utility room and suffered extreme heat and damage to the point where your confident all circuits would have been tripped or de-energized. But, you find an arc severed wire several rooms away. The presence of an arc severed wire would now indicate the fire origin may not have been in the utility room since the circuit was still energized when the fire attacked it, before the fire distroyed the main panel. You need to be prepared to explain how this could have happened if you still want to call the area of origin the utility room.

Electrical arc survey cannot always be used, but when it can it can be extremely helpful and may limit the area you have to search. I have elminated 1/2 a building based on one arc. Every scene I examine one of the first assements I conduct is the viability of arc mapping.

The work is dirty and labor intensive (one of my names on scene usually is pigpen, the other ones I cannot mention but they are given to me by the poor souls who are assigned to help me). Pulling a bundle of large conductors out of a 4" diameter pipe is not easy. It needs to be done before the wiring is displaced. You need to pay close attention to construction and understand how the wiring was routed. You may have to mark the wire and/or conduit well, cut and remove it to another location to examine it. Marking well means you can recreate the layout and dimensions, so sections removed have to have their orientations (which way was the supply) marked. And no, the wire does not have to have an active load to arc, it just has to be energized. Don't forget, arc severed or notched power supply cords should also be included.

I hope this helps some. It is really not that hard to understand and once you try it I think you will see the value of having it in your bag of "Scientific Methods".

John