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# gap between conductors

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#### bigjoe

##### Member level 2
hello there!

just want to ask, Is there a formula (theorerically) or a way to determine the minimum gap between conductors for a certain voltage between them?

say for example, a 500 volts potential difference is to be applied between two points, can the minimum safe spacing between those points be calculated?

thanks

Re: gap between conductos

What is the dielectric constant of the material between them?

Re: gap between conductos

If you have a dielectric between them (say two traces on different layers of a PCB), then the voltage breakdown/handling of the substrate will give you that number (read it from spec sheet).

For two conductors on the same layer, exposed to air... that gets tricky. Paschen's Law gives you an equation for determining voltage breakdown of a gas, given the gas pressure, separation distance, and gas composition. Read more about it here.
Paschen's law - Wikipedia, the free encyclopedia

The two most useful equations are in the section Pachen Curve; V= and pd=.

That should give you a conservative estimate for low frequency signals. When you get into RF, those distances get smaller. At altitude (and extremely high voltages), you can also run into issues with corona and multipaction.

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Re: gap between conductos

Aside from calculating what the medium can physically withstand you should also look for "creepages and clearances" in any relevant standards that you may wish to, or have to adhere to

Re: gap between conductos

say the two conductors are just in free air

you have not given us sufficient information to answer your question. Are these two points on a circuit board, on a conformally coated circuit board, in a wet environment, at sea level or elevated altitude? Are the points separated only by air? Separated by dielectric? Are these points smoothly rounded, or are there sharp/jagged edges? Is the envirnment dirty/dusty? Humidity level? Condensing moisture or rain? Is the voltage DC, AC, RF? etc etc.

@biff44.. I didn't know that there is so much to consider when determining the minimum safe clearance between conductors. do you now a site that discusses all what is written in your post?

for example, a 13,800 volts overhead distribution power lines, how do you determine the safe distance between them, between one conductor and the ground wire, and between them and the cross arm? considering the weather, constantly changes, example a sunny then rainy weather.

thank you

On the outside of local network switching centre there is a sign that says " beware 33Kv 18 feet working clearance required" {AC 50HZ}
Frank

Well, you would best consult a power engineer. The power companies have established REAL WORLD spacings to provide a factor of safety for arc over.

In air, you have to worry about the dielectric breakdown voltage of air. That voltage varies somewhat with altitude, humidity, etc.

If you are now trying to attach to this cable...other factors come in. You can take a ceramic cylindrical post and use it to support the cable. But what happens if the outside of the cylinder gets dirty (rain, dust, bird droppings). That dirt my start to burn (carbonize), and form a path for current to flow. More current flows, and then more carbonization occurs, and then booom, an arc over happens (even if the cylinder is theoretically long enough to not arc over). They solve problems like this by making the shape of the cylinder more bulbous.

Also, if there are any sharp metal objects, the elctric field gets concentrated at these points, possibly 10X higher than in free space. So anywhere there is a sharp or discontinuous metal object, you have to space things much further apart then you might think.

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