Electrical arcing is an important phenomenon to consider in fire and explosion investigations. It is mentioned 79 times in the current edition of NFPA 921 in 6 different chapters. To help to systematically identify this phenomenon I submitted a proposal (Log 921-91 in the 2007 Report on Proposals) for the upcoming edition of 921, to include in a single simple table, all of the categories of common evidence of electrical arcing. That table is reproduced below:

Table 6.9.4 Common Evidence of Electrical Arcing

|  Category  |             Explanation             |    Alternative Explanation   |

|Unusual     |Electrical arcs will often create    |Some metal fires and          |
|light       |a bright, flickering, buish-white    |pyrotecnics may also generate |
|            |light that may be observed by        |bright, flickering light      |
|            |eyewitnesses to the fire             |                              |

|Electrical  |A rounded globule of re-solidified   |None                          |
|arc bead at |metal at the end of the remains of an|                              |
|the end of a|electrical conductor that was caused |                              |
|conductor   |by arcing and is characterized by a  |                              |
|            |sharp line of demarcation between    |                              |
|            |melted and unmelted conductor        |                              |
|            |surfaces.                            |                              |

|Conductor   |Similar to electrical arc bead,      |Mechanical notching by cutting|
|notching    |notching of the conductor appears    |tools, nails or abrasion is   |
|            |where a momentary arc occurred       |often apparent under          |
|            |leaving nearby surfaces unmelted.    |microscopic examination.      |

|Arc ejecta  |When an arc occurs, melted conductor |Drops of metal melted in a    |
|splatter    |material is forcibly ejected from the|fire including de-energized   |
|pattern     |location of the arc and, following a |conductors may create a splash|
|            |ballistic trajectory, will often     |pattern when they fall under  |
|            |splatter across nearby surfaces      |gravity and strike a solid    |
|            |solidifying into many small metallic |surface.                      |
|            |beads.                               |                              |

|Melted high |The extreme temperatures generated   |Direct impingement of         |
|temperature |by an electrical arc can melt high   |hydrocarbon flames can        |
|material    |temperature materials such as steel, |accelerate oxidation and      |
|            |asbestos and refractories which are  |occasionally melt steel.      |
|            |generally impervious to the normal   |Alloying with other materials |
|            |temperatures of a fire.              |(such as aluminum and calcium)|
|            |                                     |can significantly reduce      |
|            |                                     |melting temperatures.         |

|Melting of  |Arcing occurs preferentially between |None                          |
|electrically|closely spaced conductors,           |                              |
|conducting  |concentrates at sharp points on the  |                              |
|components  |conductors and travels along the     |                              |
|in a manner |conductors away from the power source|                              |
|only        |creating a melting pattern           |                              |
|consistent  |inconsistent with damage from heat   |                              |
|with arcing |conducted and radiated by a fire.    |                              |

As you can see in the table, I have only identified 6 categories of evidence that are strong indicators of electrical arcing. Roughly half of those categories are currently enumerated in NFPA 921 in some fashion and the alternative explanations that must be considered are hit-and-miss in the currect 921. In the submitted proposal, I also cited (from the current illustrations in 921) or provided (where none were currently illustrated in 921) photographic examples of each of the types of evidence.

The 921 committee rejected the proposal because “the photographs were anecdotal” and rejected the table listing because the entries in the table referenced the photographs (those photos and photo references have not been included in this message for brevity sake).

My questions to my colleague fire investigators in this forum are:

1. Do you think a table like this, summarizing the types of evidence for electrical arcing and listing alternatives that have to be considered, would be beneficial in NFPA 921 (even if there are no illustrative photographs referenced)?

2. Are there any other categories or types of evidence not included in my table, that you believe provide strong evidence of electrical arcing?

In reference to the second question, two possibilities raised in discussions with other knowledgeable fire investigators are 1) tripped over-current protective devices and 2) “electrical arcing sounds” heard by eyewitnesses (earwitnesses?) to the fire. The problem with breaker trips is that is primarily an indication of an over-current condition or possibly a heat effect of the fire, not an effect necessarily induced by arcing. The problem with “electrical arcing sounds” is they are not distinctive. It could be a buzz, a pop, a crack, etc., not like the distinctiveness of the bluish-white flickering light caused by electrical arcing (that all people have experienced during an electrical storm and many people have seen when someone is arc welding).

I appreciate any comments you may offer.

Michael A. Learmonth, B.A.Sc., M.B.A., M.Eng., J.D., LL.B., C.F.E.I., P.Eng.
Senior Associate, Giffin Koerth Forensic Engineering and Science

Michael A. Learmonth, B.A.Sc., M.B.A., M.Eng., J.D., LL.B., C.F.E.I., P.Eng.
Senior Associate, Giffin Koerth Forensic Engineering and Science
40 University Avenue, Suite 800
Toronto, Ontario, Canada M5J 1T1
Phone: 416-368-1700 ext. 225
Fax: 416-368-5576