I'm surprised the motherboard fried while behind a UPS.

No surprise at all...

 

The potential of a lightning strike is not what damages a motherboard (or any electronics)

 

It is difference of potential.  In other words, let's say you had a perfect UPS and its output socket stayed at absolute zero volts potential (or rather, no higher voltage than your normal line voltage)

 

Now, let's take a different input to your PC, and raise its potential to one million volts (easy to do in a lightning strike)  Let's do that to the LAN connector...

 

Ok, now we have 1,000,000 volts on the lan connector, and ground at the UPS, and a nice path through the motherboard for the 1,000,000 volts to get to ground. (and sensitive electronics in the middle to fry)

 

Did the UPS help prevent a surge from coming in on the line, yes.  Did it prevent the motherboard from being fried, no.

 

The ONLY way to prevent the server from being zapped it to keep the potential between ANY two wires connected to it from raising above their normal working voltages.  Many UPS have a set of LAN connectors to route the LAN through to the protected PC.  If your was of that type, then the LAN voltage would have been clamped to a safe voltage potential relative to the power line.  With just a little difference in potential, no damage would occur.

 

In some documents, this is called a ground reference window.  All connections must be "surge protected" to the same reference window.  Then, if the "window" is raised to 1,000,000 volts, and the entire PC chassis raised to that same level, no harm.  You see this every day as birds land on high-tension lines.  Their feet are at the same high voltage potential, so no harm to the bird.

 

In your case, it sounds as if the difference in potential, the path not through your UPS was probably through the cable modem and the connected coax.  The burn marks showed the arcing, and trust me, if the lightning strike can arc through a mile or so of air, the extra quarter inch or so from the cable modem chassis to the server chassis was easy. 

 

I'm really glad your disks were not damaged, (or your house) as their contents would be far more difficult to replace.

 

Joe L.

http://www.tripplite.com/en/products/model.cfm?txtModelID=151

Ideally I would place one of these at the network output of the cable modem, and another mounted directly to the server. The green wire should be connected to the house ground system, which should be at the same potential as the server chassis if everything is wired correctly. Theoretically one at the cable modem output should be enough, but if you have lots of cable runs inside the house you could potentially get a surge that existed inside your network. Many years ago I had a serial cable linking two devices 50ft apart, lightning struck a tree outside, and the ungrounded length of cable picked up enough EMP to fry the devices at both ends.

 

For what it's worth, my Ethernet did run from the cable modem, to the UPS, and then back out to the rest of the house (and server). The coaxial cable was similar (from outside, first to the UPS, and then to the rest of the house).

 

However, the lightning did not strike a power line/pole.  It struck a corner/very near a corner of my house, far from where the power & cable enter.  Your guess is as good as good (or better) as mine with regards to what path the surge took through my house to zap my stuff.  The items that were damaged were in seemingly random parts of the house.  Some near the strike, others almost as far way as you can get.  With many things in between undamaged. Most of the items were on different circuits from each other.

 

Anyway, I don't really think that there was anything that I could have done differently to prevent the damage.  I hope that it doesn't happen again soon. Now I'm nervous about every storm that comes through...

 

I agree that I was "lucky" that there was no real damage to the house or my data.

 

Thanks all

 

John