Electrical Resistance

[rustyruss]

Good Morning,

Wonder if you could help.

I am designing an earth mounting that needs to withstand 10kV. I need to work out the minimum gap between two metallic fixings mounted in a polymer (DMC) to prevent voltage leak / arc.

Is there a calculation that can be done?

Many thanks

 

[neiltechspec]

I think you are on the wrong forum for that sort of question.

 

[PhilHornby]

Google didn't help, so I asked ChatGPT

It says:

Paschen's Law provides a relationship between the breakdown voltage, the gas pressure, and the distance between two electrodes. Although Paschen's Law was initially formulated for gas-filled gaps, it can provide an estimation for the minimum gap distance in other insulating materials, such as polymers.
Here is the general formula for Paschen's Law:

V = B × d × p × ln(pd/a)

Where:

• V is the breakdown voltage
• B is a constant that depends on the gas or material being used
• d is the gap distance
• p is the pressure of the gas or material
• a is a constant representing the size of the gas molecules or material atoms

Since you are working with a polymer (DMC), the specific values of the constants B and a would be different compared to gas-filled gaps. Unfortunately, I don't have access to the specific values for DMC. However, you may be able to find these values from manufacturer specifications or by conducting experiments.
Once you have the specific values for B and a for DMC, you can rearrange the equation and solve for the minimum gap distance (d) that will prevent arcing at 10kV. This would involve rearranging the equation as follows:

d = (V) / (B × p × ln(p × d / a))

Keep in mind that this equation assumes a uniform electric field distribution and does not take into account factors such as surface irregularities, moisture, or contamination, which can affect the breakdown voltage. It's always a good idea to consult with a qualified engineer or conduct testing to ensure the design meets safety standards and requirements.

Paschen's Law is described here.

 

[hippy]

It's always a good idea to consult with a qualified engineer or conduct testing to ensure the design meets safety standards and requirements.

I would say the recommended path would be to consult with those who have actual experience in such things and imagine testing would be essential, not just a good idea.

 

[Buzby]

This is another forum I use, and there are lots of television guys on there who could answer your question, probably in much greater detail than you would ever need !.

Old CRT tellies could have upto 27kV inside them, and they were in your living room.

https://www.vintage-radio.net/forum/index.php?

 

[AllyCat]

Hi,

You might get a rather "cold" response on that forum now:
https://www.vintage-radio.net/forum/showthread.php?t=200626

However, as an "ex TV guy" I will try to answer your question, but as usual some "background" information would be useful. For example, is this a "safety critical" application (either for personnel, or just other equipment), what is the "polymer", what is the power level and what does your "Withstand" mean? For our TV development we tried to ensure that the 27 kV (at up to 40 Watts !) stayed where it was supposed to be! Slightly safer, my very first project in industry was the design of a 10 kV d.c. power supply module for an oscilloscope!

Firstly, I'm assuming that this 10kV is principally "d.c.", because high voltage (and high frequency) a.c. brings along many further reliability and safety issues. Also, the basic "voltage breakdown" characteristic (e.g. kV/mm or whatever) of the bulk material may be only a small part of the overall analysis. That value might be quite high, perhaps 20 kV/mm, so you can afford to be quite "generous" in the basic design calculations.

But the breakdown voltage of air is much lower (perhaps 10 kV/cm*), so you may need to ensure that all the electrical connections are very well covered with insulation. * The quoted "Paschen" value for air seems to be much higher than I recall as being usable, but that test appears to use parallel flat plates whilst "sparks" jump much more freely between (or from) any pointed conductors (e.g. an automobile sparking plug).

However, IMHO the most serious issue is likely to be "surface tracking" between any exposed conductors. (Conductive) "Dirt" may build up on any exposed surface (often attracted by the "static electrostatic field"), even before you consider condensation (water) from the air in higher humidity conditions. So we really need much more information about the environment and requirements of your application. And in passing, I noted that "Electrical Breakdown current does not Obey Ohm's Law".

Cheers, Alan.