Some years ago I was working at in a wireline telecomms equipment
maker. Robust protection was the order of the day; the outside physical
plant was just *waiting* for lightning.
We protected each circuit to a level it could withstand, and worked back
to the line inputs, where (IIRC) we had to withstand 2 KV Oc or 500 Amp
short circuit lightning transients. It helped that everything in the
Central Office had a common-point ground, even if it was 150 feet below,
in the basement, as this made backdoor entry less likely.
In a later incarnation, I found aviation customers who wanted to test
transient protection without opening the equipment, which is another
story -- and on these, we had to inject transients on the CASE.
I suggested some commercially available modules for AC power protection
here because one does NOT want to try to protect the AC power network,
only what he builds, and just throwing in spark gaps, gas tubes,
Tranzorbs(tm) or MOV's might be asking for trouble. Know the threat, and
protect against THAT.
Cortland Richmond
On 9/9/2013 1130, Kunde, Brian wrote:
What are the safety considerations using gas tubes on the AC mains? Do
you have to fuse them or are they not likely to fail shorted? Can you
use them between line and PE? Do you have to use multiple parts in
series? I often see them in series with MOVs in a “T” configuration to
protect against line to line and line to PE surges.
A few year back we had a product that had several surge suppression
circuits located on different PC boards within (some assemblies were
very expensive and we wanted to protect them). Well, at our customer
site they experienced some kind of huge surge, transient or
overvoltage (we do not know what exactly happened). Of all the
equipment that was on-site including many of our competitors
equipment, only our instrument was damaged. Our surge suppressors were
blown up, charred, and/or vaporized. The warranty repair cost was
$10,000US but the hit to our reputation was probably worst. We
believed that our equipment probably protected all the other equipment
on-site but it is hard to get your customers to believe you. So now we
want to better control our surge protection and if we see a huge surge
we hope it to destroy something much less expensive to replace or at
least minimize the damage.
What we are currently thinking is to use over the counter Surge
Suppressor modules, but they are only good to about 3KV – 4KV. Then we
thought we would add a spark-gap in the board that would only kick in
if our surge suppressors failed. Maybe we can add some very high
voltage Gas Tubes also or instead of the spark-gap. I’m not sure what
more we can do. Many of the circuits/assemblies we are trying to
protect are buy/sell components where we do not control spacings.
Any comments?
Thanks to all.
The Other Brian
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