Eiron wrote:

Eeyore wrote:
Eiron wrote:
Don Pearce wrote:
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.

A (design ?) defect.


Modern laptop PSUs have three core mains leads

Nonsense.

so it probably doesn't happen on new kit.

Equally nonsense. The Y cap bridging the live and secondary sides is still there
(for EMC - LOL).

http://www.maplin.co.uk/module.aspx?...29240&doy=11m7
"An IEC-320 type C5 moulded ‘cloverleaf’ connector on 2m of 3-core cable
with a moulded 13A UK mains plug. Typically used on laptop computers."

A modern laptop in the UK has an earthed chassis.

More likely to have have an earthed PSU as opposed to chassis AFAIK. Would like to see
evidence for your claim.


A five year old laptop doesn't, and if you connect it to
a bunch of old unearthed kit such as an AV amp also connected to
a CRT TV, DVD player, satellite receiver and VCR, all legal and
working as designed, the combined leakage is quite enough to float
the common ground and exposed metal at 100v or so; good for a tingle
when you connect anything. :-)

That's certainly true.


New kit seems much better though I haven't measured any leakage currents
yet.

I simulate them ! Funny waveforms too. ;~)

Graham