Power over Ethernet #3

"Lighting Protection and POE."

During a recent upgrade I encountered a very interesting problem. After we finished the preliminaries we typically test the ethernet circuit by just connecting the our POE board at the door. This tests the Switch, cabling and the board. When we moved the cabling from an ordinary port to a POE port the switch showed a POE fault. We proceeded with our normal troubleshooting. Our LAN test device showed no problem with the cabling. Just to be sure we remade the connections on both ends with the same result, LAN tester showed no problem but the Switch showed a POE fault. Then we used the techniques outlined on this site for testing the circuit with a Multimeter. Basic ethernet troubleshooting with a VOM.

This test showed no problems either with a small exception, a very high resistance across all four pairs. Measuring between say the blue/white and green white showed a complete open circuit, however between the blue-white and the white-blue several mega-ohms could be observed. At this point we decided that the wiring must be bad and began our preparations for pulling a new cable.

It was then we discovered that our cable was routed through a gray box six inches on a side. When we examined it closely we found it was a lightning protection device rated for 14 to 16 volts! Our test equipment used low voltage for all the tests but the POE failed the circuit during the handshaking phase as the lightning protection shunted what it saw as a surge to ground. As this circuit was entirely within the same building we don't understand the yet to be found installer's reasoning for the addition of this protection device.

We hope that you might profit from our adventure. Basic test procedures will sometimes show problems that were not considered by the designers of the more advanced test instruments.