RE: POE switches and lightning

[John Lee <john () internetassociatesllc com>]

IMHO,
Long runs of UTP  (unshielded twisted pair) make wonderful antenna systems for EMI and EMP which is why they are 
matched to differential drivers and receivers to reject as much common noise as they are designed to. Older and larger 
Ethernet interfaces have drivers separated from the logic components that can handle higher over currents and voltages 
that are induced on the cable. Newer, smaller integral designs cannot usually handle as much power as the older 
designs. 
UTP systems in industrial environments require higher performance drivers that handle higher currents and voltages 
since they are induced by large motors, HVAC and florescent lighting systems.
EMP (lightening) is a very wide band, high number of frequency bands signal with large amounts of power being induced 
into the systems they are impinging very rapidly. ITGOD (In the Good Old Days) we used to run everything through 
conduits which when properly grounded protected both power and signal circuits against lightning very well.

There is a very large wifi network in multiple mile long structures connected by underground tunnels, in the most 
active thunder storm zone in the country, some of the issues were:

• There were a number of power and grounding zones in the buildings
• There were local grounds at each of the wiring closets that all equipment in that zone was tied to. (Have the earth 
ground checked and if it they are corroded or no longer working, have a new earth ground field dug or tie to the 
“building steel” if it has one or more grounding points.)
• All inter zone runs were fiber
• Both PoE switches and local PoE bricks were used to power the remote access points to keep power drops over the utp 
to within design parameters.
• Some zones had switches with both fiber and PoE ports with the PoE ports handling local access points and the fiber 
ports running to smaller remote switches with PoE from there to the edge devices.
• If the power runs were too long and their was no local power available, custom cables were manufactured to increase 
power conductor sizes to lessen voltage drops
• All outside runs were in conduit and would preferably be fiber.

When I installed my first Ethernet with RG-9, I had to ground the cable at the center of the run and tape each end of 
the cable since it had almost lethal voltage at the either end of the Ethernet cable.

My analog circuits professor said forget this digital design stuff, it as an analog signal in a transmission medi(a)um.

Regards,

John (ISDN) Lee
________________________________________
From: Caleb Tennis [caleb.tennis () gmail com]
Sent: Thursday, May 13, 2010 11:36 AM
To: North American Network Operators Group
Subject: POE switches and lightning

We had a lightning strike nearby yesterday that looks to have come inside our facility via a feeder circuit that goes 
outdoors underground to our facility's gate.

What's interesting is that various POE switches throughout the entire building seemed to be affected in that some of 
their ports they just shut down/off.  Rebooting these switches brought everything back to life.  It didn't impact 
anything non-POE, and even then, only impacted some devices.  But it was spread across the whole building, across 
multiple switches.

I was just curious if anyone had seen anything similar to this before?  Our incoming electrical power has surge 
suppression, and the power to the switches is all through double conversion UPS, so I'm not quite sure why any of them 
would have been impacted at all.  I'm guessing that the strike had some impact on the electrical ground, but I don't 
know what we can do to prevent future strikes from causing the same issues.  Thoughts?