240volts and ground hum problems.

[Martin Czech]

> Australian mains is the same deal. Though there seems to be a great
> discrepancy about what we're allowed to manufacture and use on our main and
> what we're allowed to import and use on our mains. I've told this story
> before but we've had lots of AKAI gear. All of it  had this curious mains
> filter as you'd expect. A couple of chokes and a few caps to ground. Or in
> this case, direct to the metal chassis of the gear. The thing is that this
> gear was technically speaking, double insulated. IE: has only the 2 pins.
> Neutral and Active. Now from my understanding, this is illegal. If it's a
> metal chassis then it must be earthed. 

The mains filter must be grounded to protective earth. It would be useless
and dangerous otherwise. 
> 
> 
> So my understanding of double insulated gear is that it must be housed in
> an all plastic case and no metal parts with any possible electrical
> connection between the chassis and the user may be exposed. But whilst this
> akai gear was listed as "double insulated" it seemed to not only break all
> the rules but also be actually dangerous. Since then I've noticed lots of
> gear which only have active and neutral connections and yet are in metal
> and some times even 19" rack enclosures.

There are severall classifications. I've forgotten the numbers.
Class so and so could mean isolated from chassis. Ie. wiring and transformer
is "safe", so the chasis can be floating. This is legal.

Another class requieres chasiss connected to protective earth.

> life chasing humm loops. And people wonder why I like digital audio!
> 
Yes, but you'll allways have some analog stuff also, ie. hybrid. In this
case cheap digital lines (SPIDF without transformer isolation) will give
you hum plus all kinds of hf artefacts, now you will have to hunt not
only hum but mirrored squeeking digital noise. This is because earth
will be earth only for dc, but for a couple of kHz wiring inductance
will prevent shorting this noise to ground.

That's why pro equipment has AES/EBU transformer isolated digital busses.

m.c.