I think we are in agreement about some of the pitfalls of spark gaps that are simply added in the copper layout.
Regarding "spark gap" components, the devices you cite (also known as gas tubes) typically fire at just a few hundred volts. Adding these to enhance the surge tolerance of an air gap can have unintended consequences. My philosophy for lightning protection is that the designer has only two options:
1) Block the surge current using adequate insulation
2) Direct the surge current to a known, safe path
My preference is to use Option 1 whenever possible. As I noted in my earlier posting, the probability of experiencing a given surge drops off dramatically with increasing voltage. So, if a product is designed with spacings that break down at 10 KV, the probability of a surge exceeding that threshold is fairly low. If you then add a 400 volt gas tube across the barrier, the probability of a surge exceeding that threshold is far higher. The result is that the gas tube "invites" surge current that otherwise would not have flowed.
This may be okay *if* the gas tube is reliable and *if* the surge path that it creates is also reliable and robust. The key thing to be aware of is that with 400 volt gas tubes installed across the barrier, surge currents will flow across the barrier on a fairly regular basis. If the system has a 10,000 volt breakdown without the gas tubes installed, surge currents flowing across the barrier will be far less common without the gas tubes (probably by a factor of 1000 or more).
Another unintended consequence of using conventional gas tubes is that they create a very steep current rise time when they trigger. This generates an electromagnetic pulse that can propagate through the system and upset sensitive electronics. In the IEEE PSES Telecom TAC we have been discussing this problem for a while now. Interestingly, some Ethernet ports have higher failure rates with gas tubes installed than without the gas tubes installed.
For the above reasons, I prefer to use Option 1 (block the surge current) whenever possible. If I had a system that could withstand 10 KV but I wanted to add a spark gap for the (rare) cases where the surge exceeds 10 KV, I would try to set the spark gap breakdown as high as possible, such as 9 KV.
I am not aware a conventional gas tube with a 9 KV threshold, but perhaps such devices exist. One alternative that I have often wished someone would make is a simple air gap component with tungsten electrodes that could withstand multiple surges. Such a device could be made quite inexpensively and would be more robust than a simple gap in a copper layout. If the nominal trigger voltage was very high (say, 9 KV), the device could be expected to trigger very rarely, if ever, in actual use.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
Brian,
I am personally opposed to such an idea. The first time I saw this was circa 1982 on an electronic security system; designed to tie into the public telephone network. The manufacturer wanted additional protection above the outdoor surge protector. The design involved two parallel zig-zag traces, one ground and the other telephone line with alternating points in close proximity.
The problem with this is if the gaps you created ever activate, they always leave a permanent carbon track in the surface of the PCB. Given adequate time and humidity exposure, these tracks become conductive and leakage current can begin to increase dramatically. Eventually, this circuits will become permanently shorted. This the main reason for evaluating comparative tracking index (CTI) of insulating materials along with creepage evaluation.
Best to simply use a glass or ceramic spark gaps which are inorganic and cannot produce carbon when arced (http://www.globalsources.com/manufacturers/Glass-Switching-Spark-Gap.html).
thanks, –doug
Douglas E Powell
doug...@gmail.com
http://www.linkedin.com/in/dougp01
From: Kunde, Brian
Sent: Friday, September 06, 2013 9:56 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Spark Gap PCB Layout on AC Mains
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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Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
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