Re: ESD protection to chassis gnd or isolated gnd?

On Jun 7, 11:38 pm, jma...@xxxxxxxxx wrote:
The protection should go to the NON-ISOLATED ground, because the ISOLATED
one doesn't have any current path to real ground to drain off the charge.

Still not clear - what if the circuit was powered from a battery

If one applies shunt-type overvoltage protection to an isolated
ground, the problem isn't solved- your isolation is usually good
to a few hundred volts, so ISOLATION WILL FAIL. So, the
recommendation to shunt to chassis ground makes sense.

In the case of a battery powered circuit, isolation can be in the
high kV range, so maybe that would be OK. The useful properties of
DC/DC converter isolation are generally in the line-voltage range,
and that's fine for disabling ground-loop problems.

So if there are two wires coming in with a signal, one being pseudo-
ground, one should shunt from pseudo-ground to chassis, and from
signal to pseudo-ground, to protect both from common mode and
differential spikes. If neither is pseudo-ground, usual practice
(look at Ethernet) is to use a transformer and apply the shunt
from the center tap of the transformer winding to chassis ground.

.