[sdiy] Leaking line filter (was: Grounding & computers)

[Ingo Debus]

Synthusiast wrote:
> 
> I was suspecting the line filter of the computer too. Now why are those caps
> in the line filter directly connected to the earth ground plug? Isn't it a
> little odd to leak current to the safety earth when this has nothing to do
> with safety measurements? If those caps are at their max limit of leaking
> it's very possible (especially in areas where you have 230V) that they set
> off the earth safety switch. And if people then stop grounding their
> computer to keep it from triggering the safety earth switch, they create a
> very unsafe situation in their home with 110-115V on their computer case.
> All because of these 2 caps in the line filter. Why?

The line filter is needed to meet EMC (electro-magnetic compatibility)
requirements. Simply spoken, these two caps (called Y caps) keep (very)
high frequency stuff that comes along the mains wires out of the device.
At the same time they prevent high frequency stuff from Xtal oscillators
or switching power supplies escaping from the box via the mains wires.
For this task they have to be connected to the chassis directly where
the mains cord (or plug) is mounted. They are just a short for the high
frequencies, they can't get in or out anymore (Faraday cage). For their
HF rejection purpose, it's not required that these caps are connected to
protective earth, but if there is a protective earth then for safety
reasons it's connected to the chassis too. I have several devices
without protective earth but with Y caps. When I connect the audio cable
of my Roland sampler to a grounded device, I can even sometimes watch a
little spark.
Nevertheless, the leaking current through a Y cap is quite low. It takes
quite a lot of them in parallel to make a ground fault interupter trip.
I never saw this happen yet. There's no real hazard caused by this caps
when protective earth is interrupted. Yes, if you measure with a high
impedance AC voltmeter between chassis and ground it'll show about half
the mains voltage, but as soon as you touch the chassis the measured
voltage will go down significantly.
It is not allowed to use "any" cap as a Y cap. There are some special
safety requirements for these caps (for instance, if they break down,
they must never create a short), and they are more expensive than
standard caps.

In a line filter there's usually a X cap (across the two mains wires)
too. This one is responsible for the zap you sometimes get when you
touch a mains plug immediately after unplugging it from the wall outlet.
The X cap is much larger than the Y cap, because it's against
differential mode HF stuff which is in a lower frequency range than the
common mode HF stuff the Y caps are supposed to suppress.

Ingo