High voltage dielectric breakdown & layering insulators

Question

I’m going to use 230 kV as the reference voltage for the example below;

I know if I put a piece of metal too close to a 230 kV source it will breakdown the air and jump to the piece of metal causing a visible spark, even if the piece of metal is floating in air and not completing a visible circuit.

I also know that when a dielectric breaks down it drastically reduces its resistive properties and can act like a conductor.

Let’s say the dielectric breakdown of air in 230 kV is 7 inches.

If I put a grounded piece of metal 8 inches away from the source then nothing will happen other than negligible small currents from capacitive induction.

If I put a floating ungrounded piece of metal 4 inches away from the source then it will breakdown the air and cause a visible spark, energizing the piece of metal.

Now what if I put one ungrounded floating piece of metal (piece “A”) 4 inches from the source and another grounded piece of metal (piece “B”) 4 inches from the previous piece A. Together the total distance piece B is from the source is 8 inches. Would the 230 kV breakdown the air from the source to piece A (visible spark), causing piece A to become energized and then breakdown the air from piece A to piece B (visible spark) causing piece B to be energized at 230 kV and causing a fault?

Can this cycle continue if I were to put another floating ungrounded plate (piece “C”) 4 inches from A then put B 4 inches from C? (Source <-4”-> A <-4”-> C <-4”-> B) Then air to A would breakdown then air to C would breakdown then air to B would breakdown causing B to be energized and a fault?

If the above is true then here’s my following question;

If I have rubber gloves rated for 20 kV and dielectric boots rated for 20 kV, does that combine to make a total rating of 40 kV safe to touch? Or if I had say a 35 kV source would the voltage breakdown my gloves destroying their dielectric properties and then break down my boots as well? Meaning the total rated voltage safe to touch would be 20 kV since that’s the highest of any single dielectric.

I understand if we were to build a circuit with capacitors acting as insulators there would be a voltage drop across each capacitor causing the layering effect to work, making the gloves and boots a total rated voltage of 40 kV when worn together. But this does not incorporate the concept of dielectric breakdown of the insulators causing a significant change in its resistivity.

Thanks for the help, please provide details and reasons for why your answer is what it is and if possible please explain the theory behind it. Thanks!

Answer 1

HV is trickier than simple addition, but in your first question, when breakdown happens between A and B, due to the air becoming a conductor, B is now roughly at A's potential, which means that breakdown will also occur between B and C.

To put it clearly, in your question, B is a stepping stone for the voltage.

Now for the second question, PLEASE DO NOT DO THAT

and for two reasons :

• The reasoning above
• Your life is on the line, do not allow any doubt

To elaborate a bit, in your question, you are B, what do you think will happen when (i said "when", not "if") the insulator breaks down?

Regarding your circuit with capacitors as insulators, your reasoning is only theoretically valid. In the real world, the capacitors have parasitic parallel resistance whose value can vary wildly, which means that the voltage will not be shared evenly between them, which can lead to failure.

As a conclusion, I do not know what you are trying to achieve, but please seek professional training and advice.

Answer 2

This should be self-explanatory why insulation does not add up. All insulators are dielectrics and we model all dielectrics including air as capacitors.

^{simulate this circuit – Schematic created using CircuitLab}

The breakdown is rarely abrupt. It changes from 3kV/mm dry flat smooth surfaces to 1kV/mm as you suggested. It then reduces 500 V/mm with high humidity or morning dew. This often causes a tiny corona of quiet ticking sounds on glass insulators on distribution lines. But if the insulators are really dirty then surface creepage reduces the partial discharge threshold even more. In extreme HVDC and HVAC annual insulator cleaning has to be done by helicopter and service guys in Faraday cage suits and this still generates an arc. Small birds are just lucky they don't have high capacitance coupling to the air. Maybe that's why you only see little birds on HVAC lines.

What happens is any dust or contamination with humidity causes a partial discharge like ESD then it repeats as you get closer rapidly like a unijunction oscillator with more voltage or a Geiger counter with more radiation and then it breaks down zap. When it is completely void of molecules of dust then its breakdown is immediate or almost.

Lightning has a streamer of partial discharges prior to the big zap but there's no warning there. If you feel the hair on the back of neck standing up or tickling then you know you are in a high E field range of a high DC voltage. Old CRT TV sets would do this on the leaded glass, especially after just being turned off, but there the glass provides good insulation but still allows static buildup on the outside.

Answer 3

I'm not sure, but if the spark appears at 7 inches or less from the voltage, and you put piece "a" 4 inches from it, it would have been 4/7 of the power. Since 7*4/7=4, it would work for piece "b" but not piece "c". Hope this helps! ps: I still can't seem to figure out the other question