If a malicious user gain physical access to your network, a high-voltage attack is the least of your worries. Network sniffers and other tools can quickly own your entire network doing far more monetary damage then some fried networking equipment.
The thing is that this kind of attack can be done quickly and un-detectably. Some access-layer switch hardware has hundreds of ports. Ten seconds unobserved in a cubicle would be all you need to plug a cable with a mains plug at one end, and RJ45 at the other into the relevant ports, pull it back out, and stuff it back in your laptop bag. $1k per work area x 400 switch ports (plus the expensive switch) is a lot of money to go poof with 10 seconds of effort, not to mention the 400 people that will now be sitting around doing nothing for several weeks while their stuff is fixed/replaced, and the data from their devices that wasn't backed up.
Even places that x-ray or search bags probably wouldn't find that kind of device in the usual tangled mess of cables in a computer bag.