Electronic – Where to put a spark gap for ESD protection–to an isolated “GND” or to protective earth

Most differential RX circuits by their nature require that the amplifiers have CMFB (Common Mode Feedback) indeed, differential amplifiers are mostly useless w/o this. So the chip automatically compensates for this itself. Some chips will have the CMFB pin available for monitoring or external capacitance but that is not strictly necessary. Of course on startup and plug/ un-plug situation this is disturbed, which can mean that the signals hit against the ESD rails. But the higher level protocols of re-transmit deal with this situation.

Additionally, one some advanced differential signalling situations, there is active equalization upon startup, so this packet corruption at this level is understood and handled in this case.

If you really want ESD protection there is no half-baked setups. ESD protection is annoying and usually only enforced in industries where quality control is essential because failures are costly (areospace) or deadly (medical). NASA's JPL (Jet Propulsion Lab) has a great training document describing how to control ESD. ESD comes from several sources including 1) Humans, and clothing and chairs 2) Paper and other materials and surfaces that are susceptible to triboelectric charging

If you have a wrist strap, that's great, but your still going to build up quite a charge on clothing and still generate a few kV potential through the air. No your not shocking your boards by providing a path directly to ground through the board when people touch them, but you still have charging through the air. ESD lab coats are required to prevent this.

You also have materials that build up charge, paper is a nemesis and shouldn't be allowed near electronics. Wood is another bad material and there are lists to find out if the materials you have are bad for charging around electronics.

Wrist straps are a way to prevent ESD, but there are a few problems. If you don't know how much contact the wrist strap is making to the wrist, then it may not be doing anything, they make monitors that ensure the person wearing them is actually actively being 'grounded' (its not a 0 ohm ground, but enough to dissipate high voltages).

One way you could prevent ESD is by enclosing your electronics in a metal\conductive box of some kind. This would prevent electric fields from getting to the boards and keep people from touching them.

The biggest problem is personnel, if everybody doesn't buy in and the ESD rules aren't enforced, then you might as well not have them. I know there are places where you could get written up for not following the ESD rules. You don't need to be that strict, but try and incentivize good and deincentivize bad behavior.