Arc Voltage

without going deep into the equations -

it is a discontinuous voltage that depends on the load current and source at the time of the break, voltage, and the geometry of the parts.

basically - The flow of current continues in the arc as long as the path remains intact -

The gap is seen as a resistor in that path that slowly increases in value until it is so high that the voltage created by the current flow across that resistance cannot sustain the ionized path that is the arc. Then the increasing resistance jumps from the arc value to near the dry air value.

The general number for the resistance of dry uncharged air at STP (off the top of my head) is around 40KV per inch.

The arc resistance is much lower. And, the more current flowing in the path, the larger the arc "diameter", and the greater the ionization.

(Trivia bit - Switchs expecting heavy arcs often have some iron in the moving part so that when the switch moves, the magnetic field from the moving iron thru the electric field disrupts the ionized cloud and "stretches it" to increase the length and to "swirl" the ion cloud into an area where it will rapidly dissipate away from the usual path and do little harm - e.g., an open space - or in the case of big breakers, ceramic chute, etc.)

"Jim Mac" wrote in...

15kV-class gear is designed with a basic impulse level of 95-110kV, so something less than that.

BIL is "a measure of the ability of insulation to withstand very high voltage surges."

--s falke

A naive answer I suppose, but I would use a memory type scope to record and then form a basis for measuring the transient voltage across the switch contacts. Realize that the impedance of an arc can be very small, hence you may see nothing greater than a few hundred volts drop across open contacts while the arc while is still in progress, but when the arc breaks the voltage across the open switch contacts should be expected to jump up to the full supply value of

13.8Kv.

Harry C.

The question is wrong, voltage at any point "in the arc" is undefined by itself. Two points are needed. Did you mean to ask what the voltage is across the arc? A rough estimate is 440V/ft.

present in the

If you have a capacitive load (let's say a capacitor or filter bank) downstream of a breaker you may get more than one arc. A large transient recovery voltage (TRV) can appear between the contacts and restrike can occur.

Interestingly, the voltage across your caps can jump higher and higher and higher with successive restrikes. Picture opening a switch when the voltage is at full +ve peak and current ~0. The arc extinguishes right away. The capacitors are still charged to peak +ve voltage. Half a cycle later the line voltage is at a -ve peak and therefore you have 2pu across the gap between breaker contacts. If a restrike occurs, get ready for all hell to break loose. The capacitors will quickly try to change from their full +ve peak voltage (+1pu) to the full -ve peak line voltage (-1pu) which they are now connected to via the restrike arc. Being somewhat of an LC circuit (utility and cap banks), the caps will overshoot this 2pu voltage change, by as much as an extra 2pu, before current goes to zero and the arc is extinguished. Now you have caps with a stored voltage as much as -3pu(!). In less than one half cycle, the line voltage will be at +ve peak and there'll be 4pu across the gap between the breaker's contacts. Now you could get another restrike, another current surge (bringing the caps to +5pu) and so on until your caps fail spectacularly and/or some upstream protection is opens the circuit successfully. j

Yes across the arc... Thank you all for your replies. Jim

"Steve Alexanderson"