High current vs high voltage result

[RITESH KAKKAR]

Hello,
High current in wire while short blow/burn up the conductor while if there will be large voltage will there will be arcing only?

 

[Audioguru]

What is your question? You know that a high current blows/burns up things and you know that a large voltage causes arcing.
High current and high voltage causes burning/blowing things and arcing.

 

[skywalker898]

A low voltage short will cause a high current, but a high voltage arc will also cause a high current if a current limiting resistor is not placed in the circuit.

A large voltage between conductors will cause a high electric field. If that field is high enough to cause breakdown in the air, an arc will form between the electrodes, resulting in a high current that is only limited by the series resistance in the circuit.

Is that your question?

 

[D.A.(Tony)Stewart]

Hello,
High current in wire while short blow/burn up the conductor while if there will be large voltage will there will be arcing only?

The equivalent circuit of the arc like an SCR with a series ESR inverse to the current and Q=CV of the loop. So a high current will have low ESR, and high C will determine the duration of the pulse . Rise time depends on electrode inductance R/L which can be >1 pico to xx nano to <1us. The stored dielectric charge anywhere will also have an ESR which will limit the current.

So measure your ESR at 1MHz and 1kHz or compute the actual impedance of each physical part.

Given Paschen's curve if you have <1 torr vacuum, the kv/mm will rise significantly from 3kV/mm at 1Atmosphere but it will degrade to 300V/mm or so if you dont go past the critical vacuum threshold. Any dust and mositure will degrade further. so it needs to be very clean better than class 100 for a long distance or achieve 80kV without PD. BEcause airborne velocity increases in a partial vacuum, PDIV and BDV degrades quickly.

This higher velocity also increases the Coulomb discharge levels and detonation of particles, any combustible gas remaining can exceed 5000'K easily on contact with the electrodes. Generally electrons are emitted easier from lower atomic mass from a conductor in an AC field so the apparent charge looks like the field is being rectified into a rising DC static charge.

The vacuum dielectric is never ideal, so other weird things may occur in the presence of detonated trace gases or particles, whatever is not removed. 4% H2 is the LEL combustible level in the presence of even a single molecule of O2. This detonation can vaporize steel , so tungsten is preferred.

Technically we may call the arc a tunnelling negative resistance dielectric breakdown almost like a tunneling semiconductor, ... but good luck finding an EHV diode.

 

[RITESH KAKKAR]

A large voltage between conductors will cause a high electric field.

what this electric field will do?

 

[c_mitra]

...This higher velocity also increases the Coulomb discharge levels and detonation of particles, any combustible gas remaining can exceed 5000'K easily on contact with the electrodes.

Does not make sense at all- combustible gases are mostly two component- e.g., hydrogen and oxygen- the ideal mix for H and O is 2:1- and there are critical pressures for every combustible gas mixtures. Anyway, most common particles cannot detonate even at very high velocity.

Generally electrons are emitted easier from lower atomic mass from a conductor in an AC field so the apparent charge looks like the field is being rectified into a rising DC static charge.

If you look up the most common electron emitting materials, you will find them to be K, Cs, Ru, Ag etc. For vacuum tubes, the choice material is W, W+ThO2. The specific property related to elements is called ionization potential and for metals is called the work function.

The vacuum dielectric is never ideal, so other weird things may occur in the presence of detonated trace gases or particles, whatever is not removed. 4% H2 is the LEL combustible level in the presence of even a single molecule of O2. This detonation can vaporize steel , so tungsten is preferred.

W is preferred (thermal electron source) because of its high melting point (most are operated well below the melting point). Most common materials for electrodes in gas discharge tubes are not tungsten.

 

[D.A.(Tony)Stewart]

- having done much testing in 10MVA oil filled transformers, I understand dissolved gas analysis of combustible gases more than on your equipment, but ensure there is no oil vapor sucked into chamber when vacuum is release from the two-stage vacuum pump. Otherwise you will have combustible particles of dust which detonate oil vapors into combustible gases and degraded performance.

Do you measure for Partial Discharge. This occurs prior to the arc and if it occurs when slowly ramping up well below normal breakdown voltage, that is your evidence of detonation of contamination of sub-micron particles. (or in the case dry dielectrics, voids)

Sorry about tungsten, I was thinking of spark plugs with current limited carbon wire. Yes if you are hitting target with a kW, higher temp electrodes are needed. How much power do you want to arc with? lots?

When performed light strike simulations on 10MVA transformers, I noted that the big Marx Generators use a tiny low power UHV trigger from a coil to ignite the lower voltage E.g. 50kV charged rack-mounted plastic dielectric caps, which then get switched in series from the trigger arc pulse and supplied thru shock absorber sized 20 to 100 Ohm resistors rated for xxx Watts and >100kV to control the rise time and duration to simulate line strikes.

Do you use vapor-phase process to clean out chambers or just aerospace grade alcohol like we did when I was in aerospace design in the late 70's.

- ionization velocity is extremely slow while arc velocity is the speed of light with picosecond rise times (tunnelling) which is why X-Rays and gamma are generated.

Any contaminant insulating molecules will be detonated from the impact on the anode charge cloud formed before ionization.

So it is a good idea to verify your cleanliness with Partial Discharge monitoring by raising the voltage slowly to find the inception voltage PDIV and if significantly below BDV. PD pulse rates start below 1 pulse per minute and then increase rapidly with voltage. This is easy to measure with a SW or AM radio on a quiet channel. ( sounds like any electrical storm pop) or using a loop of wire to coax terminated with 50 Ohm to scope.

HTH

 

[skywalker898]

The electric field between electrodes will potentially cause an arc depending on your environment (vacuum vs air vs SF6), field strength, geometry, surface roughness, surface cleanliness, pulse length etc.

 

[c_mitra]

The electric field between electrodes will potentially cause an arc depending on your environment (vacuum vs air vs SF6), field strength, geometry, surface roughness, surface cleanliness, pulse length etc.

No arcing is possible in vacuum except by pair production (extremely large field strength) OR by electrode breakdown- field emission from the electrodes itself. Field emission is self-limiting and will not cause any arcing. This is often used for producing an electron beam. Vacuum is not the best insulator simply because of the vacuum breakdown.

 

[skywalker898]

I hope c_mitra isn't implying that the factors mentioned do not affect breakdown - because they do. The factors mentioned (environment, field strength, geometry, surface roughness, surface cleanliness, pulse length) 100% affect breakdown in vacuum.

The physics of the actual mechanism can range from pair production (unlikely without radiation or extremely high fields), to field emission, to field emission that causes localized heating on the anode or cathode which will then perpetuate the arc, or there is the clump hypothesis from Cranberg. There is plenty of peer reviewed research and experimental evidence out there for this.

I am not referring to an unrealistic pure vacuum here, but real vacuums obtained in chambers ranging from 10^-5 to 10^-10 torr. Vacuum arcs are frequent causes of failure in devices ranging from x-ray tubes to linacs.

 

[D.A.(Tony)Stewart]

I hope c_mitra isn't implying that the factors mentioned do not affect breakdown - because they do. The factors mentioned (environment, field strength, geometry, surface roughness, surface cleanliness, pulse length) 100% affect breakdown in vacuum.

The physics of the actual mechanism can range from pair production (unlikely without radiation or extremely high fields), to field emission, to field emission that causes localized heating on the anode or cathode which will then perpetuate the arc, or there is the clump hypothesis from Cranberg. There is plenty of peer reviewed research and experimental evidence out there for this.

I am not referring to an unrealistic pure vacuum here, but real vacuums obtained in chambers ranging from 10^-5 to 10^-10 torr. Vacuum arcs are frequent causes of failure in devices ranging from x-ray tubes to linacs.

I agree


- using any external source of energy violates Paschen's Law and significantly alters the plasma arc . e.g background gamma, cathode heater, etc.

1 Torr = 1000 micron improves the PD and BDV compared to 1 atm. 1e-10 torr is pretty serious and expensive pump. 10 micron are reasonably priced <$500

The initial plasma discharge follows inverse square law, then becomes a linear function of distance.

Math.

.https://www.fxsolver.com/browse/?like=1679

 

[FvM]

The discussion has moved pretty much from the original topic (not to say run off topic). Not sure if it should be continued this way.
I stumbled upon a statement about explosion level which isn't right. L.E.L. actually depends on O2 level, total pressure and other conditions. H2 value of 4% is valid for ambient air only.

 

[D.A.(Tony)Stewart]

There is some sensitivity on LEL for H2 & O2 concentrations vs pressure, but not much. Reducing pressure 6 orders of magnitude might raise LEL to 10%. and using varying O2 concentrations over several orders of magnitude, may raise H2 LEL by a few %, so my point is still valid.

Again, the best protection is to consult factory, but Again, I suspect, the most cost effective solution is a resistor to limit rise time and current and thus peak voltage applied to suspect DUT. The resistor /ESR ratio of DUT and also the capacitance ratio of same determines the % rise in voltage from the step voltage difference. This is basic Thevenin divider and Capacitive transformer divider action.

Thanks For your comments. The OEM engineers ought to know best if your complete setup is compliant or over-stressed.

It is important to also note that ignition energy of plasma in a vacuum rises significantly, so it appears to be safer, but then due to the lack of viscosity from low density vacuum, the energy of an initial Townsend discharge arc also increases significantly due to lack of friction and dampening effects, there is much higher impact velocity of charge ions, thus more kinetic energy converted.

It would be nice if the OP defined a range of desired specs , layout and more details for their application. I am sure this has been done before. Even a simple Analysis of Q=CV, ESR & ESL and capacitance ratIos between each connection would be useful at each non-ideal voltage source.is useful to do failure analysis and improved design.