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It seems that some users on another Stack have reported GFCI nuisance trips caused by mains leakage through twisted-pair Ethernet cables connected between computers on different branch circuits, or more specifically, between a computer with a Class I, chassis-mounted, supply conforming to IEC 60950 connected to a grounded receptacle with UL 943 Class A GFCI protection, and a switch that is a Class III appliance with a Class II power supply, connected to a grounded, but unprotected, receptacle on a different branch circuit.

While, conceptually speaking, the idea that there could be a leakage path through the data cable makes some sense, and I have seen Ethernet reference circuits that have termination RC networks from the port-side center-tap terminals in the magnetics to chassis ground as well as a 1nF capacitor between chassis and signal grounds, it seems to me that it would be very poor engineering for this leakage path to allow the mains leakage current to rise to a magnitude exceeding the IEC 60950 standards.

What is the magnitude of this Ethernet-connection-induced leakage current rise, what factors in the design of the equipment involved control this rise, and can someone describe to me the precise leakage loop involved?

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    \$\begingroup\$ I think the user on DIY is full of it. if the power supply in the PC is isolated, there shouldn't be any leakage that will trip a GFCI. Maybe he routes his ethernet cables by coiling them around the power cables? \$\endgroup\$ – The Photon Apr 3 at 3:00
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    \$\begingroup\$ @ThePhoton -- unfortunately, I'm not in a situation to test it (don't have the network setup or the sensitive leakage clampmeter needed for that) or else I would put this theory to the test! If anyone wishes to experiment with this, though, I'd love to hear about it! \$\endgroup\$ – ThreePhaseEel Apr 3 at 3:05
  • \$\begingroup\$ I've got a router, computer and laser printer networked together on a GFCI receptacle. Networked back to FIOS box and to another computer both not on this GFCI, and the GFCI hasn't tripped ever (~ 13 years). On the other hand, all the GFCI stuff is also going through a UPS (laser is on the surge-protection-only part) so that may mask any potential problem, though until ~ 6 years ago it wasn't on a UPS. But (as noted in DIY), I've never seen this problem anywhere and I have quite a few customers - I would think I'd come across the problem occasionally if was at all common. \$\endgroup\$ – manassehkatz Apr 3 at 3:10
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    \$\begingroup\$ One caveat to my above comment: It's entirely possible (in fact, it's certain) there are some shitty power supplies out there being used in PCs. \$\endgroup\$ – The Photon Apr 3 at 3:33
  • \$\begingroup\$ Related: Does an ATX power supply have any isolated outputs?. \$\endgroup\$ – The Photon Apr 3 at 3:45
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The ethernet connection leakage current should be negligable, with UTP. Every port has a array of transformers for high frequency, the leakage at 50 Hz common mode will be very low. enter image description here

However, if shielded cable is used, S-UTP or CAT7 cables, there will also be made a chassis connection between the two devices.
Then the power supply leakage enters the equation, and those may leak several milliamperes.

The simple act of reversing the plug might remove the nuisance tripping of the GFCI/RCD.

*(image source)

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  • \$\begingroup\$ Yep, shielded cable was my concern too. \$\endgroup\$ – Ale..chenski Apr 3 at 5:57
  • \$\begingroup\$ I've actually done high voltage insulation tests once, on an ethernet port and it's really negliable! \$\endgroup\$ – Anton Ingemarson Apr 3 at 7:23
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    \$\begingroup\$ What you mean by reversing the plug? \$\endgroup\$ – Joshua Apr 3 at 13:53
  • \$\begingroup\$ @Joshua swapping phase and neutral around on one device can reduce leakage between two devices. \$\endgroup\$ – Jeroen3 Apr 4 at 7:18
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So I'm the user on DIY.

I had some original experience at work where we coudn't get the new portable generator to power more than one computer even though the old one did. We eventually bisected it to the GFCI outlet in the new generator.

Later on, I had to track down why my AFCI breaker kept tripping. The electrician I called tested the AFCI breaker by bridging a resistor between power and ground. That tripped it. He said that AFCI breakers work by detecting ground faults. I originally said he was nuts, but it turns out it was true.

I got a copy of a circuit for RJ45 magnetics. The critical point is RXN and TXN are tied together by a pair of identical resistors R6 and R7 and capacitor C15 which is 10nF ties that bridge line to ground. In steady state, C15 would indeed conduct no current; however when sending a packet, the impedance of C15 1 / jωC = 1 / j(2·10⁹)(10·10⁻⁹) = 1/j20. This gives the resulting current flow of I = V/R = 3.3/2/49.9² + 1/20²)¹ᐟ² = .033 amps.

And that's just that one capacitor. I haven't been able to locate the indicator LEDs yet. I've noticed that the connection indicator LED on quite a few computers will light even when the board is unpowered but not when unplugged. Conclusion: that LED is tied between the Ethernet cable on one side and the ground on the other, and that ground is often the neutral wire rather than the house ground (two wire devices ...).

Now the electrician was in fact telling the truth. Old series AFCI breakers would trip at something like .1 amps of ground loop by specification. The gigabit switch I was using at the time was a two wire device (no dedicated ground) so all of that current had to go into the neutral wire. New AFCI breakers have since been fixed to work by other means than ground fault detection and replacing the AFCI breaker was the solution.

GFCI outlets are documented to trip at .004 amps. Guess what happens when you run Ethernet cables between devices on different circuits where one of them doesn't have a ground wire. And I'm pretty sure from the bisection that most of these cheaper power supplies were tying the motherboard ground to the neutral wire not the ground wire despite the ground wire being available.

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    \$\begingroup\$ When you say "that ground is often the neutral wire", if that's true then your PC vendor hasn't provided a properly isolated power supply and is violating safety regulations in any first-world country. \$\endgroup\$ – The Photon Apr 3 at 3:50
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    \$\begingroup\$ The ground on this schematic would be the negative power rail for the entire circuit. Every part of the computer will send current to the same ground that's on this schematic. In electronics, ground usually just means zero volts, not "real ground". \$\endgroup\$ – immibis Apr 3 at 3:53
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    \$\begingroup\$ @Joshua One goes down to 0V at the same time the other goes up to 3.3V and vice versa - that's how differential signals work. The capacitor voltage will be around 1.65V at any time. \$\endgroup\$ – immibis Apr 3 at 3:58
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    \$\begingroup\$ @BenVoigt, There will be a capacitive coupling across the isolation barrier. This should only pass current signals at well above the mains frequency. Meanwhile the GFCI response time is measured in milliseconds. \$\endgroup\$ – The Photon Apr 3 at 4:33
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    \$\begingroup\$ @BenVoigt, but the mutual inductance can only produce a balanced current on the other side, equal current flowing in one terminal and out the other, which doesn't trip a GFCI. \$\endgroup\$ – The Photon Apr 3 at 4:47
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Tripping a GFCI usually occurs when there is a missmatch between the current going in and the current going out. While there could exist mutual inductance between cables, I don't think it would generate enough current because properly designed Ethernet ports have megaohms of impedance at DC. I'll find some impedance graphs for the chokes tomorrow, but if I remember right there is high attenuation for lower frequencies through chokes, and highly unlikely to pass much current 60Hz through DC.

If the cable was improperly built there could be a pathway there.

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  • \$\begingroup\$ See the answer by Nick A in the question I linked in a comment above. \$\endgroup\$ – The Photon Apr 3 at 23:19

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