## calculating minimum air gap

## calculating minimum air gap

(OP)

Mechanical engineer here, so sorry if this is a an easy question, or in the wrong forum. I need to determine the minimum air gap between two voltage carrying terminals, in order to prevent arcing. The terminals are parallel, with no protruding shapes in the direction of the other terminal. The circuit voltage is 14v dc with very low current Assume the nominal air condition is low humidity and high temperature (>130 degrees C). Hope this is enough information. Thanks

## RE: calculating minimum air gap

Is this on a board?

Why are there no design standards for you to use?

Can't imagine being able to get terminals close enough to cause arcing without raising other concerns, e.g., the substrate bending and causing the terminals to short out altogether.

TTFN

## RE: calculating minimum air gap

## RE: calculating minimum air gap

TTFN

## RE: calculating minimum air gap

## RE: calculating minimum air gap

TTFN

## RE: calculating minimum air gap

Calculate what has to be calculated,

Don't calculate what don't have to be --or can't be --

calculated

and know the difference

--- OR ---

spend unlimited money and time.

<nbucska@pcperipherals DOT com> subj: eng-tips

read FAQ240-1032

## RE: calculating minimum air gap

you specify >130C but not the highest temp. IRstuff's rule is not valid for much higher (e.g. 500 C )...

To prove the previous rule of thumb:

A yound engineer once specified a $1500. -2V precision pover supply -- I changed the design to use a 10K 1/4W resistor to -15V which is less than a dime.

<nbucska@pcperipherals DOT com> subj: eng-tips

read FAQ240-1032

## RE: calculating minimum air gap

----------------------------------

One day my ship will come in.

But with my luck, I'll be at the airport!

## RE: calculating minimum air gap

When simply considering likelihood of failure, there are significantly more and more probable possibilities than worrying about the 14V delta causing an arc across a relatively huge gap.

TTFN

## RE: calculating minimum air gap

## RE: calculating minimum air gap

Does this coil ever switch? If so (in absence of surge supression) there may be a voltage transient may be orders of magnitude higher than 14v.

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## RE: calculating minimum air gap

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## RE: calculating minimum air gap

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## RE: calculating minimum air gap

identifies the minimum "sparking" potential for air is 327 volts.

Once again what it is saying is that at your voltage 14V (or any voltage below 320volts) in theory you could put your terminals one Angstrom apart and there will be no arcing.

Revised calculation 14v / [75kv/inch] is even more

outrageous 0.0002 inches.

Once again I am not proposing to use any of the above "theory" as a guide. Exactly the opposite.

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## RE: calculating minimum air gap

I think if the gap is VERY small, the current can just tunnel through. There is a gap between the Cu atoms,

too...

<nbucska@pcperipherals DOT com> subj: eng-tips

read FAQ240-1032

## RE: calculating minimum air gap

This is especially true for breakdown voltages.

A rule of thumb is if the voltage is doubled, the gap is tripled.

Reference Data for Radio Engineers fifth edition page41-1 has Spark-gap breakdown voltages.

The lowest chart goes is 1000 volt peak For needle gap it is 0.025 inches at 25C, 29.9 inches of mercury.

The multiplication factor for 60C is 0.50

So breakdown for 0.025 inch gap at 60C is 500 volt peak.

Page 41-2 states "The breakdown voltage is approximately proportional to pressure and inversely proportional to absolute (degrees Kelvin) temperature"

If you are actually operating at 120C, I would suggest making some tests.

## RE: calculating minimum air gap

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## RE: calculating minimum air gap

TTFN

## RE: calculating minimum air gap

But since it is the subject of this thread, why don't you tell us the factors that contributed to your arcing?

Eng-tips forums: The best place on the web for engineering discussions.

## RE: calculating minimum air gap

The gap can be narrowed by contamination. Was this a new

PCB or an old one ?

<nbucska@pcperipherals DOT com> subj: eng-tips

read FAQ240-1032

## RE: calculating minimum air gap

TTFN

## RE: calculating minimum air gap

Do you have an explanation for the arcing? Why did you call it badly designed?

Eng-tips forums: The best place on the web for engineering discussions.

## RE: calculating minimum air gap

Bad design for a number of reasons.

TTFN