Hi-pot cable testing part 2

[@CirrisInc]

You are correct that a few 9 volt batteries could be used to sustain an arc. Start by touching a wire to tinfoil and then pull it some distance away to demonstrate this effect. Paschen’s law refers to the voltage required to start an arc, not sustain one. Once an arc has started, ionized air allows it to sustain at much lower voltages. However, as stated in the video, an arc will not start in air (at standard atmospheric pressure) at any distance at less than 327 volts.

[@Graham_Wideman]

I believe Paschen's Law applies only to very specific conditions. For example, one condition is that voltage is applied to parallel plates, in which the distribution of electrons (or holes) will be uniform across its surfaces. But in the real world, the geometry of the opposing electrodes is rarely parallel plates. For example, in general the breakdown voltage between two sharp pointy electrodes is much less than between plates, because the point tends to concentrate the available electrons. There are other conditions that are also not generally met in real situations. And if you acknowledge that the 9V batteries can sustain an arc (because sustain requires only lower voltage)... how did that arc start?

[@CirrisInc]

@@Graham_Wideman The video is for illustrative purposes only. Different cables have different builds with all kinds of variables. DWV tests are affected by humidity, dielectric absorption, geometry of the metals in proximity, environment (vacuum, air, etc), and more. Our testers are designed for a particular case where wires or pins (typically adjacent) cause undesired arcs because of poor insulation, contamination, or physical damage in general.

[@stickyfox]

@@Graham_Wideman When a conductor carrying current (like a switch contact or battery terminal) is separated, there is a transient state between "closed" and "open" in which current flows through a microscopically thin section of material. That section can get very hot, even ignite and ionize the air, starting an arc that is sometimes called a "load arc." If you're shorting out a 9V battery to another 9V battery you will see that it sparks only when the connection is broken and not when the batteries are brought together (although there is some bounce that makes it seem that way in some trials, you'll definitely see it's visible mostly on breaking the connection). Adding any length of wire to the 9V also adds a non-trivial amount of inductance that will make it easier to spark, especially if the wires are separated by some distance. If you're using a battery to power a motor, you'll see an even bigger and brighter spark from the inductance of the motor raising the voltage well above 327V. I think Paschen's Law is OK only if you strictly consider the process of starting the arc and rule out any other physical phenomena that might get one started by means other than dielectric breakdown. Relays for motors and other heavy loads are designed differently for this reason. For an extreme example, check out some of the videos on YT showing power transmission stations disconnecting under load.

[@Graham_Wideman]

@@stickyfox hmm, not sure what you mean about shorting two 9v batteries together. You mean in parallel? Because there would be no voltage differences across which to spark. Or you mean in series? In which case this is just an 18v battery which you are shorting, and I'm not sure what's special about that voltage.

[@stickyfox]

@@Graham_Wideman Well, 9V batteries *are* special in that you can easily short them out. It's much harder with a AA, plus the spark is bigger! And yes, when you touch two nodes of the same voltage there's no spark.

[@findlaypc]

327V to make an arc? Anyone armed with a 9-volt battery (or several) and some tinfoil or wire can easily demonstrate that arcs are sustainable at far lower voltage. Either Paschen's law is wrong (unlikely), or your interpretation of it is.