I am planning on using a tvs diode to clamp power peak to protect network switch; the Port is POE. The issue is that when I measure the voltage of ethernet line to ground, I get around 0.6V. When I use a POE measurement device, its shows the Pin 1,2,3,6 and voltage of 54V. Why can I not get 54V when measuring using Multimeter. Please help me understand.


1 Answer 1


POE source uses a low voltage/current to detect whether a POE sink is present and how much power it needs before giving power to it, so unless you fake a POE device you don't get voltage. And voltage is between pairs and most likely isolated from ground, so trying to measure against ground will just not work.

  • \$\begingroup\$ What clamping voltage between line to ground should I use then if I cannot measure the voltage ? \$\endgroup\$ Nov 5, 2019 at 5:59
  • \$\begingroup\$ Are you sure you even want a TVS between ground and Ethernet pins? That is usually for lightning protection. It depends a lot and might be too broad a question. Is your equipment grounded or not, and does it have any interfaces that could ground it? What is the installation class of the device and does it need to pass 1kV, 2kV or 4kV potential tests? What standards the device must comply with? TVS manufacturers and PHY manufacturers like TI and transformer manufacturers should have reference designs for PoE interface protection that might help you with this. \$\endgroup\$
    – Justme
    Nov 5, 2019 at 6:59
  • \$\begingroup\$ Thanks for asking questions. Yes, it is meant for lightning protection. The network Switch has a chassis ground. Network Switch complies with IEC 61000-4-5 but doesn't mention what class it complies it. I have looked into IEC 62000-4-5 and it does give an example using a tvs diode, but I was not sure about it. The Network Switch I am using is a Aruba 2540 JL356A. \$\endgroup\$ Nov 5, 2019 at 7:27
  • \$\begingroup\$ I just want to know how would you decide the line to ground tvs clamping voltage? \$\endgroup\$ Nov 5, 2019 at 20:50

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