What started this fire?

[electric-ed]

I learned about a strange case of a house fire that occured recently.
It was discovered that the service grounding electrode conductor had become disconnected (or burned off) at the first ground rod, just below and a bit to one side of the meter location. It wasn’t fastened and was dangling in the wind.

At every point where it had touched the concrete foundation a spall of concrete was blown off, and at every point where it had touched the wood steps there was a burnt area, and at one point had set the steps afire.

The earth was dried out (baked, he called it) surrounding the ground rod. A fire started at the oil-fired water heater, which was connected to the outdoor (grounded) oil tank by a 3/8 inch copper tubing oil line.

What do you think happened here?

Ed

[twh]

I'll guess Rule 10-406(4) was not followed and the oil line became energized.

[rmurphy]

I'll make a guess. First there must have been a dead short...perhaps on the utilities side of the service. the first grounding electrode might have had a weak connection or perhaps the ground would not/could not conduct enough to blow the fuses. When the Groung wire did break off it would have the same potential as whatever was causing the short. Everything bonded to this system would also have this same potential. therefore the bonded oil tank with the copper line would bring this short directly to the furnace and perhaps oveheating the small conductors to the point of fire. I'm sure I'm no where close but it's always fun to guess...

[electric-ed]

Millvalemurph is getting close to the answer.

I thought we would get more responses to this question. This is an actual occurance that was described by a friend who investigates fires for an insurance company.

I will wait a few more days before I post the answer.

Ed

[rmurphy]

Ok Ed. Lets have the answer. It's seems your the only one who knows how this fire started. Other than lightning or something majorly wrong with the ballast in the burner I'm stumped.

[electric-ed]

I really thought we would get more reponses on this.
Here is what they think the answer is at this point. If there is a change I will post it here.

The investigators believe that a winding-to-winding fault developed in the Power Co. transformer. Because the secondary neutral is normally tied to the grounded primary neutral, that should blow the high-voltage fuse in the primary cutout.

If the ground tie between the primary and secondary neutrals on the pole was open/broken/missing, the fault current would try to return to the primary neutral through the service grounding conductors, through the earth to another pole, and up the "butt-plate" ground wire to the primary neutral.

They believe that the service ground wire at the house burned off at the first ground rod. The fault current then tried to get to ground through the oil line to the grounded outdoor oil tank, burned off the 3/8 inch copper oil line and ignited the oil.

Ed