12 volt electrical fire

[garry]

From looking at the picture the pigtail is less than 3' in length per the automotive wire gage rating the wire is capable of 20 amps @ 12VDC up to 7 ft.

Wire Gauge Amps Ratings for 12 volt Automotive Systems

I would consider that marginal but that is the rating.

30 years ago I was taught electrons ran on the outside of the wire and the stranded wire was used in cases where flexing was important.
However; stranded wire has a higher RF resistance example center conductors on coax is solid wire.

In this case I suspect poor connections were probably the culprit regardless of the electrons running on the inside or outside of the wire.

[dkottum]

If the wires overheated from high current flow they would be burned along much of their length. But they're only burned near the 12v socket. This indicates the socket overheated, and that was most probably caused by a poor connection in the 12v socket.

That is also the simplest explanation, seeing as there are probably millions of these in use in autos and RVs. My sense is there is nothing wrong with the wiring or fusing.

The cigarette lighter socket is a lousy connection, in this case easy to loosen during vacuuming. A vacuum would be a poor choice by the vacuum builder to install such a connector on.

I would install a new 12v socket and throw the vacuum away (not to be taken as an approval of cigarette lighter connections).

My background is troubleshooting and repair of aircraft electrical systems.

doug k

[Jammer]

Originally Posted by TG Twinkie

Jammer! It's not misinformation. 12 volts at 20 amps is 240 watts of power or heat capability. 120 volts at 20 amps is 2400 watts of heat capability. A big difference. Now that is misinformation to say they both produce the same amount of heat.

The amount of heat produced in a length of wire is I * I * R, where I is the current, and R is the resistance of the wire. The voltage between that wire and, say, ground, is immaterial.

Electrical fires caused by undersized conductors and poor connections usually result from relatively amounts of power dissipation (heat) compared to the power being transmitted. It doesn't take more than 10 or 20 watts to melt a connector, and so enough resistance to cause a 1 or 2 volt drop is enough to do it on a circuit such as this, powering a 10a or so load.

I am an electrical engineer.

Where did you go to school?

[Wayward]

Originally Posted by Jammer

The amount of heat produced in a length of wire is I * I * R, where I is the current, and R is the resistance of the wire. The voltage between that wire and, say, ground, is immaterial.

This is the most general equation:

120v at 20amps delvers far more power (watts) than 12v at 20amps.

Starting with your equation, Joules law for power (not heat because time is omitted).


But given that fixed resistance, the current depends upon the voltage applied. (Ohms law):


Substituting:


As the voltage goes up across a fixed resistance, so does the power.


Now that is what Twinkie was saying
BUT..... it is heat generated across a fixed resistance that is likely the answer to what caused the insulation on those pigtail wires to melt. HEAT = POWER * TIME


As garry said the pigtail wires appear sized properly for 20 amps over that short distance. I cannot imagine Airstream nor their suppliers making such a mistake. Looking at the picture, as dkottum points out, the insulation is not melted because the wire overheated from over-current, otherwise it would be melted along the entire length. It appears the socket or the socket terminals heated the wire up at that end. A poor connection between the socket and the inserted plug, (or a cheapo Chinese plug) could create enough resistance to heat the insulation to its melting point.

[wmarsha]

a couple questions for the OP: 1) what was burning? Just the insulation of the pigtails? 2) if you remove the fuse in question, does it de-energize the circuit that burned? 3) Did the "...melted..." vacuum plug (paraphrased) when removed do any damage to the receptacle? ie: is the receptacle dead shorted (faulted) now, or is it deformed in any manner? 4) is the vacuum still operating correctly?
5) was your wife able to remove the vacuum plug from the receptacle when she first complained/mentioned it to you, or did you have to remove it? I guess the real question is: was the vacuum plug melted into the receptacle?
My pea brain says the vacuum is the root cause; its plug caused deformation of the receptacle; said deformation caused a continuing resistive fault of the receptacle, ultimately causing the pigtails to become fusible links. Now, why the fuse did not operate is beyond me, unless this particular circuit is not protected the way we all think it is/was. I believe a 20amp fuse will operate before a 16 gauge wire will burn, but it is after all, a pea brain.

oh, and nice math stuff, it humbles me.
ol' pea-brain bill

[Wayward]

Originally Posted by wmarsha

My pea brain says the vacuum is the root cause; its plug caused deformation of the receptacle; said deformation caused a continuing resistive fault of the receptacle, ultimately causing the pigtails to become fusible links. Now, why the fuse did not operate is beyond me, unless this particular circuit is not protected the way we all think it is/was. I believe a 20amp fuse will operate before a 16 gauge wire will burn,

What is great is that if enough of us put our pea brains together on the WWW we can convince ourselves of pretty much anything - like pea soup.

I agree that it could well be a resistive fault caused by a poor connection or a maybe a faulty plug. To answer your question as to why the fuse did not blow..... the current flowing through a high resistance fault was probably less than 20 amps but high enough to slowly overheat the metal contacts of the socket and transfer that heat to the wire and insulation on the pigtails.

The 16 gauge wire was not hot because of excess current - it was hot from being physically connected to a mini soldering iron. Come to think of it the end of that wire does look like a few of my less than stellar soldering jobs.