I think it is a good idea to increase the spacings if you have room available. The statistical relationship between peak surge voltage and probability of occurrence strongly favors larger spacings. Doubling the spacings may reduce the probability of breakdown by a factor of ten.
Regarding spark gaps, there are at least two known limitations for spark gaps that are simply added in the copper layout:
*The actual firing voltage is highly variable
* Copper traces are poor electrodes for high current surges. If the gap fires for a lightning surge, you will likely see considerable erosion of the copper "electrodes" and some carbon tracking across the gap.
The first limitation may not be a major issue for you in this application. The firing voltage can be made less variable using pointed electrodes to produce an inhomogeneous field in the gap.
The second limitation should be considered very carefully with respect to your goals. If your view is that you "know" that breakdown will occur in rare circumstances and you simply want to control where it occurs, adjusting the layout accordingly might be okay. However, any spark gap that you create in the copper layout will be damaged by the very first surge that breaks down over the spark gap, and the board may need to be replaced anyway.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements.-
But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap.
I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions??
Thanks to all for your help.
The Other Brian
LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you.
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