[sdiy] ot: shocking notebook experience

[Czech Martin]

Beeing an EE, people often ask me to repair their { }
(replace { } by ovens, hairdryers, radios, etc.).
It is amusing for me to tell them that EEing has almost nothing to do
with the things that electricians do and that I have no license
for manipulating mains power devices.

Anyway, a relative observed the following interesting :
she used her notebook in the garden, powered by cord
out of the cellar (the notebooks battery is almost useless now,
she always has to use the DC converter box).
Now standing on the grass, she got little electric shocks when
touching the notebook. This was never observed under other circumstances,
ie. in the living room.

Ok, I took a look at it. The alu chassis inside the (plastic) coating
is connected to the outer ring of the dc coax plug. This
in turn is wired to protective earth of the dc box power inlet.
Makes sense. But: how can a plastic encapsuled alu chassis
lead to electric shocks, when it is properly grounded?
Even if it is not grounded and on a dangerous potential
difference towards earth, wouldn't the plastic isolate?

So far I can only assume that the power outlet in the cellar 
has maybe some failure, or that protective earth has a potential
difference to earth, this could happen if the house has no
proper grounding "ancor". Perhaps grounding was done via the 
water pipe, which is now changed mostly to plastic.

Any ideas?

m.c.