Q: Why can't we unplug the PoE cable of the ORS ?

Technical Analysis of Surge Events and Ethernet PHY Damage in Passive PoE Systems

Executive Summary

Unplugging an RJ45 connector while a passive Power‑over‑Ethernet (PoE) supply is active can generate destructive electrical surges. These surges arise from the sudden interruption of current flowing through the cable’s inductance, producing voltage spikes capable of exceeding the breakdown limits of Ethernet magnetics, ESD protection components, and the Ethernet PHY silicon itself. This document explains the mechanisms behind these surges, quantifies them for typical cable lengths, and outlines why Ethernet ports—especially on PCs—are easily damaged. It concludes with recommended design practices to prevent such failures.

1. Technical Background

1.1 Passive PoE vs IEEE 802.3af/at

Passive PoE injects a fixed DC voltage onto twisted pairs without negotiation, inrush control, or surge suppression. Passive PoE power exposes Ethernet hardware to electrical conditions it was never designed for if you do not follow power-on and power-off procedures.

IEEE 802.3af/at is a specification to allow increase of current and voltage step by step in the PoE cable.

Due to the length of the cable between the ORS and its power injector (10, 20 or 30 meters), PoE power compliant with the IEEE standard would not improve much the hot-unplug protection.

1.2 Ethernet PHY Vulnerability

Ethernet PHYs are designed for low‑voltage differential signals and short ESD events. They are not designed for sustained DC voltage, high‑energy inductive surges, or common‑mode spikes of tens or hundreds of volts.

2. Surge Mechanisms During Hot‑Unplugging

2.1 Cable Inductance

Ethernet cable typically has 0.5–0.7 µH/m of inductance. When current is flowing, this inductance stores energy:

E =LxI²/2

Interrupting the current forces the voltage to rise sharply.

2.2 Arc Formation

As RJ45 contacts separate, a microscopic arc forms, briefly maintaining current before collapsing and producing a high‑frequency, high‑voltage spike.

2.3 Hot‑Unplugging

Micro-arcs from PoE power can also damage RJ45 contacts, especially if the connectors are handled while live or if the termination is incorrect. Careful installation and handling, without active PoE power, significantly reduces this risk.

2.4 Why the Surge Reaches the PHY

High‑frequency spikes pass through inter‑winding capacitance, overstressing ESD diodes and PHY input transistors.

3. Surge Calculations for Typical Cable Lengths

Assumptions: 50 V passive PoE, 0.8 A load, 0.6 µH/m cable inductance.

3.1 Stored Energy

• 5 m: 3 µH → 0.96 mJ

• 10 m: 6 µH → 1.92 mJ

• 20 m: 12 µH → 3.84 mJ

• 50 m: 30 µH → 9.6 mJ

Even a few millijoules can destroy silicon when released in microseconds.

3.2 Resulting Voltage Spike

For a 20 m cable:

V = L · (di/dt) = 12 µH · (0.8 A / 50 ns) ≈ 192 V

Real‑world measurements often show 200–300 V spikes.

4. Failure Mechanisms in Ethernet PHYs

4.1 ESD Diode Breakdown

TVS and ESD diodes clamp at 70–100 V. A 200 V spike forces them into avalanche, causing failure.

4.2 Transformer Saturation

High common‑mode voltage saturates the magnetics, allowing the surge to couple directly into the PHY.

4.3 Silicon Overstress

The PHY input stages are designed to accept power levels on the order of milliwatts. A single surge can damage differential amplifiers.

5. Current power design in ORS

For our PoE design, we are using SM51625EL (from Bourns) PoE transformer with 75 Ohm resistor and 1nF capacitor between the input pairs and the ground. See above schematic. You can also find the full schematic at https://lab.nexedi.com/nexedi/ors-hardware.

6. Possible solutions and why we don't use them

6.1 Use IEEE 802.3af/at

This is not a real solution as IEEE 802.3af/at is protecting the powered device during current establishment but it cannot protect the powered device when you unplug it as the transformer doesn't have time to react.

6.2 Prevent totally the unplugging at the ORS side

We could simply have a RJ45 going out of the ORS with no connector at all. In this case, you can only unplug at the power injector side. This solution is not implemented because it prevents choosing a different cable length.

6.3 Prevent the unplugging at the ORS side when online

If we could block the unplugging of the ORS while it is online, it would be a good solution. For example with a magnetic connector. But we didn't find such connector.

6.4 Add more capacitors after the PoE transformer in our PCB

This would effectively protect the RJ45 interface of the PC but it would degrade the RJ45 signal quality and therefore it would prevent using long PoE cable at the input of the ORS. This would also require a bigger SDR PCB.

6.5 Use only 2 pairs for the power and 2 pairs for the networking

This would also work but the ethernet traffic would be limited to 100Mbps. And also using only one pair for the current would be a bit near the limit for the power consumption of the ORS (since the ORS is consuming 35W and we can have only 1A in a single pair).

6.6 Add resistors in serial in the PoE input

We could add 200 mOhm in each copper wire of the PoE input but this would consume 2W constantly.

6.7 Add more transient-voltage-suppression (TVS) diode

This solution involves TVS diodes adding capacitance to the line which can break the ethernet signal. The caracteritics of TVS diode suitable for PoE are really strict and we didn't find it yet. 

7. Final Summary

Unplugging an RJ45 connector while passive PoE is active generates high‑energy voltage spikes due to cable inductance and arc collapse. These spikes can exceed 200 V and couple directly into the Ethernet PHY, destroying ESD protection and damaging the silicon. We didn't find yet a good solution to circumvent this without major downside.