POE switches and lightning

John Lee john at internetassociatesllc.com
Fri May 14 20:44:17 UTC 2010

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.


John (ISDN) Lee
From: Caleb Tennis [caleb.tennis at 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?

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