USB Shield. To ground or not to ground?

Best Practice

Firstly (as a bit of a cop out) personally, in designs I always ground through a 0R resistor so that the decision can be changed. This goes for pretty much any shield (Ethernet, USB etc)

The main problem that can arise is when the shield is grounded at either end, and the two ends don't agree on what 0V is. This can cause damage to either end, by currents flowing where they shouldn't (if the shield path is 0.2ohms, and the voltage difference 1V, that's 5A going where it shouldn't)

You might think why would this ever happen? But think of the situation where a laptop is connected to a piece of mains powered equipment over USB. The laptop could be on battery only (no true earth reference), but the equipment is connected to mains and thus may have a true 0V earth reference.

So the solution is to connect at only one end, but have some agreement on which end.

Generally, a USB host device will be expected to provide the power and the slave device is quite often entirely bus powered and has no connections to anything else in the outside world (think USB memory stick, WiFi dongle etc). In general, the USB host should connect the shield to ground (and earth, if possible). This is why the host side is typically expected to tie the shield to ground or earth.

The fact that there are so many conflicting comments from people and different experiences shows clearly that it is far from safe to assume this is always adhered to, so as I mentioned firstly - add the option to change it easily.

In This Situation

After discussing this in a chat, the proposed solution is different. Since this is a question about ESD, it's messy and complicated and involves many aspects of the design (electrical, mechanical, system). The chat is available for all to see, but there important bits:

• This datalogger has no other connections, apart from the USB connection to a PC/laptop
• The datalogger has a metal chassis, that is bonded to the PCB board ground.
• When the USB shield is not directly connected to PCB board ground (for example connected by R||C or HiZ), the datalogger fails (loses memory contents).
• In the ESD test, the USB cable is not attached (or is floating at the other end).
• The OP is not the design author, and has very limited scope for making design changes to solve this problem.

I surmise the problem is most likely PCB layout related. The ESD surge is taking a path from the shield, past sensitive electronics and finally reaching the chassis. By directly connected the shield to the chassis with a wire, ESD surge path reaches the chassis without going near the PCB so avoids the problem.

In this situation, as the datalogger has no other connections to any other devices; the potential issues (pun intended) cannot occur. So I would suggest connecting the shield to the chassis. Either by a wire, or a more production friendly approach is an ESD gasket around the connector which is a spongey conductive material that gives a connection without manual soldering and doesn't permeantly attach the chassis to the board.

In a more ideal world, I would respin the board so the chassis is isolated from the PCB board ground and the chassis is connected to the shield. That means that its not possible for ESD surges to reach the sensitive electronics at all. Except if you poke the datapins on the USB connector for fun - in which case, ESD diodes on the datalines that give a path to chassis ground, not PCB board ground.

You need to examine the high-current path across your design, and the design must provide a separate shield net to avoid the ESD discharge to go over signal ground, which will create "ground bounce" and disrupt functionality. This is not an easy matter. By making a simple solid connect between signal ground and shield, you might run into EMI issues and fail EMI certifications. For more details, you might want to review this topic on how to balance two contradictory requirements for USB shields.

Considering what you have told us about the device:

• Battery powered
• Not normally connected to USB
• Does not have connections to external sensors or devices during measurements
• Does not have any accessible metal parts apart from chassis and USB
  shield.

Just connect the chassis to USB shield and be done with it.

Previous answer pointed out issues with loop currents (two different GND paths in circuit to mains) but since you have floating battery-powered device, this is a non-issue.

If you want to experiment, you may try removing resistor/capacitor between the shield and the GND. Also you may want to use smaller NP0 C0G ESD capacitor, 100nF capacitor has X7R dielectric which is not well suited to this kind of task.

The GND-to-Shield connection is apparently rather weak and not near the USB connector. So shorting shield to GND makes the transient travel through your PCB until it hits the chassis tab.

I think the problem here is that the original designer put USB shield under the signal traces. Zapping the ESD gun makes the shield "jump" which couples capacitively with the traces and components nearby. Now signal and VBUS traces are crowbarred to GND so they're protected. However, these traces then go to have CMC and ferrite while the GND is directly coupled - So probably these suppress the transient in those wires while the GND transient continues unabated.

NB this is just speculation.