A question about arcs...

[adrianh]

Here is a question that has been running around my head for a while:

Lets say you have an electrical installation that supplies 100V DC with a MAXIMUM current of 100A. No trip switch nothing, those are the only constraints.

Now, lets say an arc is created by holding two ends together for 1 nanosecond and then seperated. Lets assume that the cables have no resistance for simplicity sake. The arc is created through the ionization of the air between the ends as the ends are seperated (I presume). Lets say the arc is present for 1 nanosecond and after that point the distance seperating the two ends is too far to support the arc.

Question 1
What would the current and voltage be for the nanosecond when the arc jumps. I can only assume that the resistance of air would be negligible but what about the resistance of the plasma created which is the arc?

Question 2
Would this arc run through the circuit as a transient? (I suppose it would)

Question 3
Would a trip switch be able to react fast enough to catch the transient and to stop damage from occuring in the associated equipment?

Question 4
What would the ratings be of a suitable trip switch?

Question 5
Would the addition of MOV's on the power supplies of the equipment in the circuit help?

Question 6
Ok, now lets assume that the arc is held for 100ms. What would the effect be in terms of voltage and current?

Question 6a
What would the effect be on a trip switch?

Question 7
Would the arc jump in a vacuum?


Ok, I don't know the answers to any of this but I find it interesting to think about. The reason I started wondering about all of this is because my wife saw a youngster jump starting his mothers car. He kept touching the jumper leads together and making sparks. I thought that this was a really bad idea because I thought that it could blow the ECU due to the transients running through the circuits of the car. Ok, I must admit, I find wondering about such thing to be fascinating!

[Dave A]

The reason I started wondering about all of this is because my wife saw a youngster jump starting his mothers car. He kept touching the jumper leads together and making sparks. I thought that this was a really bad idea because I thought that it could blow the ECU due to the transients running through the circuits of the car.

I've been told that with the electronics typically found in a modern car, it's best to leave the headlights on in the power source's car when jump starting another for (effectively) this reason.

[AndyD]

He kept touching the jumper leads together and making sparks.

This could easily damage electronics depending how well protectd they are.

Your questions make a lot of assumptions that can't or don't happen in real life. Evan if the wiring has zero impedance DC power source or any power source for that matter has an internal resistance that under normal conditions doesn't affect the circuit behaviour. When there's a very high current or a short circuit this internal resistance becomes one of the limiting factors of the fault current that can flow.

The mechanics of a short circuit become very complex in real life. At the point of contact very high temperatures occur very quickly. At the actual point of contact the copper or steel vapourises so the actual contact point isn't an actual contact any more, it's actually a varying arc across a gas/vapour which has a very definite resistance to the current flowing similar to an arc lamp.

Another limiting factor is that when a short circuit current flows even very large wiring and busbars become highly magnetic and attract/repel each other. These forces can be large enough to break panels apart under fault conditions. This conversion of electrical energy into other forms of energy is also actually a load which limits current. A similar effect is observable in a motor, if you resistance test the windings with a multimeter the resistance bears little resemblence to how much current will actually flow when the motor is connected to the supply. The internal magnetism of the windings as well as their resistance limits the current flow.
Not sure if this answers any of your questions.

[Justloadit]

To add into this, the longer the cables, the more aggressive the arc, as the cables enter into the realm of acting as an inductor for that short period, and actually build up magnetic energy which gets released at a far higher voltage. This is the shock the kid was feeling when he disconnected the cable, he obviously was connecting himself to both the cables unknowingly, and with the released energy was now flowing through him.

Yes MOVs will help, but for DC circuits you actually want to use low value capacitors, to absorb the energy being released. So capacitors in the range of 1nF to 100nF at 250V rating would help reduce the arc to negligible values. The value of the capacitors is directly linked to the length of the cables. Adding small values of capacitors in series for this type of application is far better than one capacitor of the total value. In other words, using 10 capacitors of 10nF each in parallel will work far better, than having one capacitor of 100nF. The reason is simple, the smaller value capacitor individually react to absorb the energy than a single larger value, its the sane as sayting that to move 100litres of water, will be easier to have 10 trips of 10 litres of water.

[Sparks]

While motor cars are the topic and nowadays all have loads of electronics. Most guys have felt a jolt from a plug wire. I recall from years ago baffling would be motor mechs by smothering a 6 cylinder simply by putting my arm across all six plugs while the motor was idling. Surprisingly there is no jolt. The motor loses power and cuts out. It has been years since I did it and with today's electronics I do not know if it will still work :-)