> Beware also that the isolated ones often have a high leakage current
> from AC ground to the output. .. if you measure the available AC current =
between
> this source and line ground, you may get as much as a few milliamps.
>  I have definitely had ones which I could feel if I touched the output
> and was also touching a metal frame which was grounded.

> This is why they always stress that medical grade isolation (uses an
> electrostatic shield between transformer pri and sec) is needed in
> equipment which is meant to be in direct electrical contact with a
> person.

A shield probably  won't help you in the above cases :-)
In most such cases it is probably due to a capacitor purposefully
connected from input to output. I understand that this is done to meet
certain regulatory requirements. I once had a printer destroy the
parallel port on a PC due to such an arrangement.
A 0.01 uF will give the better part of 1 mA at 50 Hz, 230 VAC and  0.1
uF about 7 mA.

I am displeased by the "bite" that I get from such supplies.
On reflection, it may be possible to reduce or almost eliminate it by
swapping phase and neutral connections. Out power plugs are all
polarised but plugpack with cord supplies can be connected either way
depending on cord termination. Manufacturers who have a phase/neutral
mindset will probably connect such a capacitor to neutral. Those who
don't may connect it randomly.

Off thread, somewhat related ...

An interesting and usually less painful effect can be observed with an
electric blanket.
Target person lies under blanket which is turned on.  A person
standing nearby and electrically isolated will feel a "burring" at
point of contact if finger tips or back of hand is moved in contact
with target. The effect is caused by capacitive coupling from blanket
element to target body.

Capacitive coupling between a popup toaster  element and toaster body
is easily measurable but, as the body should be and  usually is
grounded, this coupling is not usually noticed. Some years ago there
were toasters here that in some cases had a stuck contact from one
mains lead to the element. The toaster would turn off but the element
was connected to phase or neutral if a contact was randomly stuck on.
I found I could reliably detect this state by shorting phase and
neutral pins at the plug and measuring capacitance from there  to
ground pin. This allowed the toasters to be tested in their boxes with
only the plug accessed, if desired. I did this to allow me to bulk
measure supermarket stock of these as Zip, the toaster suppliers, were
not keen to see if the original problem I reported was an isolated
one.  I demonstrated that a significant % of the toasters had the
problem and a product recall was initiated. Not overly related to dual
 output power supplies :-).

 Russell
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