Ethernet PCB Layout without Chassis Ground

[ransonjd]

Can anyone point to a reference for the appropriate way to layout an ethernet connector (UTP, no metal shell) with discrete magnetics without a chassis ground connection? I can't change the lack of chassis ground, so I would like to know best practices under the circumstances.

[nctnico]

Connect any metal part of the connector to the board's ground. With or without chassis, that is better to do anyway. Just make sure the signals have enough clearance to meet the isolation voltage requirement and don't have any copper pours under or over the ethernet signals at the connector side of the ethernet transformer.

For the Smith (or how it is called) termination: just connect the resistors together but don't connect to ground.

From an EMC perspective it is better to tie all grounds together as good and as much as possible in a device.

[m k]

Is it different if connected connector and cable are shielded and lightning is striking?

[nctnico]

The use of shielded ethernet cables is not wise between two seperate buildings because you can have differences in the ground potential which can cause high currents to flow; especially in industrial environments. And inside buildings both sides should be grounded. Recently I heard an interesting anecdote: A company used shielded ethernet cables exclusively in their network. With grounded desktops this wasn't a problem but with a couple of hundred laptops (each with capacitors connected between device ground and half of mains), the rack with the ethernet switches became dangerous (lethal) to touch due to the voltage present on it.

[razvan784]

With grounded desktops this wasn't a problem but with a couple of hundred laptops (each with capacitors connected between device ground and half of mains), the rack with the ethernet switches became dangerous (lethal) to touch due to the voltage present on it.

Well, then the rack metal parts weren't properly earthed according to relevant standards, right?

[nctnico]

With grounded desktops this wasn't a problem but with a couple of hundred laptops (each with capacitors connected between device ground and half of mains), the rack with the ethernet switches became dangerous (lethal) to touch due to the voltage present on it.

Well, then the rack metal parts weren't properly earthed according to relevant standards, right?

You are turning the problem around; without shielded ethernet cables a potentially dangerous situation can not occur. It just goes to show that using shielded cables is not a magic solution; quite the opposite, use of shielded cables requires planning the grounding carefully.

[thm_w]

I tested some DIN rail gear, the RJ45 shield is connected to the DIN rail, which you'd normally ground.
Same with some rack gear, the shields connect to chassis as well as mains earth plug ground.

I would be relatively surprised if there are many industrial products that don't connect back to mains earth.
All I can think of is cheaper 12V gear with isolated plugpacks would not be able to be earthed.

[ve7xen]

With grounded desktops this wasn't a problem but with a couple of hundred laptops (each with capacitors connected between device ground and half of mains), the rack with the ethernet switches became dangerous (lethal) to touch due to the voltage present on it.

Well, then the rack metal parts weren't properly earthed according to relevant standards, right?

You are turning the problem around; without shielded ethernet cables a potentially dangerous situation can not occur. It just goes to show that using shielded cables is not a magic solution; quite the opposite, use of shielded cables requires planning the grounding carefully.

Well the 'problem' was indeed that the installation of the network equipment was incorrect, not the laptops. Of course, I agree, just don't use shielded twisted pair. People think it's better, but it's not better. It just costs more and adds complications (if you do it right) or hazards (if you don't).

In the industry this is referred to as 'bonding', as in permanent bonding to earth. In a typical installation, all the structured (permanent) cable must be bonded. There's no need to ground (temporary) the attached equipment, though it commonly will be.

As far as PCB design, it mostly comes down to EMC concerns. If you don't have a chassis ground, the ground plane is likely the best thing to connect it to, probably through a 1nF-ish capacitor.

[m k]

Back in the day things were easier.
Signal ground was part of the cabling but connectors were shielded manually on the field.

If you now start wondering where all spark gaps are somebody will get a fire extinguisher.