In article , Don
Pearce

Doesn't matter. You only connect to earth for safety.

That's not quite true. You sometimes connect to earth to eliminate
hum. I had a setup that wasn't earthed and when someone approached the
microphone it hummed; when they touched a chassis point it stopped
humming. That wasn't an earth loop but it was capacitative pickup of
mains interference as the ground of all the double-insulated kit was
floating at a hundred volts or so. Earthing the amp cured it. Modern
laptop PSUs have three core mains leads so it probably doesn't happen
on new kit.


That was some other problem, that just happened to be cured by grounding.

Depends on what you mean. A problem can arise if a system with no explicit
grounding link has a leakage path though a relatively large impedance that
then generates an 'error' signal injected into a signal-sensitive path.
(e.g. being the coupling between windings of an unscreened PSU transformer,
particularly if the effective impedances for the two polarities on each
side differ.) Depends on the details of the system, etc.

The behaviour can then be as described. Mains hum until the system has a
decent ground - which then provides a lower impedance path for the induced
hum currents, so suppresses the effect.

FWIW Many years ago I spent time working on ultra-sensitive signal
detection systems for astrophysics[1]. Often used in places with lousy
power and dubious grounding. I encountered a large variety of situations
that caused hum. Often in ways which the standard descriptions ignore or
assume won't happen. :-)

Slainte,

Jim

[1] Worst for this were bolometric (heat) detectors whose resistance
changed a tiny amount when warmed by the radiation from a distant galaxy.
Thus needing you to sense minute changes in current or voltage when such an
element was biassed. At least with heterodyne mixers you could do
everything at RF and avoid mains. :-)

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