grounding questions...

[Ingo Debus]

René Schmitz wrote:
> 
> At 15:25 10.05.00 +0200, Ingo Debus wrote:
> >
> >
> >Tony Allgood wrote:
> >>It must be said that in the UK all metal cases/parts that are
> >> touchable by the user have to be directly connected to earth in any
> >> mains powered unit.
> >
> >What about the tip of a phone jack (when the other end of the cable is
> >plugged into a mains powered unit)?
> >
> >;-)
> 
> In essence this is connected to ground via the output of the circuit.
> I think there needs only be a conductive path to mains GND

This is no 'direct' connection, there are electronic parts in between.
If there's an accidental connection between the live line and some point
in the circuit no-one can guarantee that these parts are fried before
the circuit breaker trips.

> , so that no
> parts can (e.g. capacitively) charge up to dangerous voltages.

Well this would need a fairly large capacitance to become dangerous (or
a really high voltage as in CRTs). In EMI mains filters there are
Y-capacitors between live and chassis of several nF.

Ingo