Question about ground and potential PE

[KKK]

Hello Everyone,

I have some questions about wiring the boards of a pcb. I have 3 different pcb's as per the diagram attached.

1. The pcb named "slave 1" has a USB connector and Ethernet. For instance, the USB connector has a SHIELD pin. Should this be connected to a chassis and then connected to the potential PE? Should the SHIELD be connected to GND (common to all pcbs)?
2. Should the USB port on my pcb have ESD protection?
3. In which case is it better to connect the pcb ground together with the PE potential or leave the pcb ground floating?

[BennoG]

in most applications (industrial) you see the GND connected to PE as close as possible by the PSU.

Benno

[selcuk]

1. If your board doesn't have a special requirement about isolation (medical device etc.), your USB connector shield will touch the chassis. The chassis should be connected to PE at some point for human protection. And you will connect USB connector shield pins on your board to DC ground return plane. So all of them will be a short on the schematics. But the exact shorting points in the layout need some experience.
2. Yes. USB, ethernet and other connectors as well. You need to connect ESD protectors close to the connectors, and their ground pins close to the shield. So that the ESD transient path will be very short to the chassis.
3. Generally you can connect them. Be careful about the isolation requirements if any. If you have a desktop computer, you can check the ground and PE connections with a multimeter (when the PC is not energized) and apply the same rules if you are not sure what to do. Computer hardware usually complies with many international standards.

[Terry Bites]

Although the circuit "ground" or 0V and PE perform separate functions its normal to connect them together near to the PSU common.
In general, all exposed metal parts need to be tied or "bonded" together.
The enclosure also connected to PE if its made from metal.

There are situations in which the circuit 0V is left floating with respect to PE. This is often seen in high end instrumentation for measuring very low level signals.