[sdiy] Re: Leaking line filter

[Magnus Danielson]

From: "Synthusiast" <synthusiast at gmx.net>
Subject: Leaking line filter (was: Grounding & computers)
Date: Fri, 28 Dec 2001 02:02:41 +0100

> 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 thing is, the earth connector is no longer only a protection
measure, it is also explicitly used in EMC. The EMC tests relating to
the power-line treats the 0 and line connectors in relation to the
earth. If you read carefully on most computer's manuals you will find
that they explicitly say that you should use an earthed outlet. If you
ask me we should clean the house of all unearthed outlets! (Just to
show the halting logic, the computer I type this on is eventually
hooked to an unearthed outlet, but I don't like it never the less).

The current should not be enougth to trigger the safety earth
switch. Also, please note that it may not be such a good idea to have
such protection switch all over the please. I rarely see them used
generally for a installation. They are good to have for heavy duty
machinery, but for many household appliances I am more doubtfull about
the need or usefullness.

As for computers and EMC regulations, you are in practice forced to
use these line filters and you are bound to have those caps there,
allways on the live line. If you have the box hooked to an unearthed
outlet you are using the box incorrectly. The EMC and safety
regulations is not in totally collision with themselfs, but it may not
match the full reality of old electrical installations.

The best description of how a line filter actually works I found in
"Introduction to Electromagnetic Compatibility" by Clayton Paul. This
book is a bit over-theoretic on most things, but did explain the
different functions of the line filter separately and it does more
than you first imagine. It is actually a quite clever design.

So, the safety regulations say there should be an 3rd connection, the
"protection earth".
The EMC regulations requires both differential (between 0 and line)
and common mode (between 0 + line and earth) requirements.
Both must be followed to be allowed to be sold (overstrict
interpretation for the general case, but is for many devices a correct
interpretation) and in Europe for instance you usually has to go under
the LVD (Low Voltage Directive) and the EMC directive and meet the
specified specs for your type of device to get the CE mark. In US
there is the FCC requirements on EMC and UL 1950 (Underwriters
Laboratory variant of ISO 60950, also known by its old reference as
ISO 950, which BTW just was released in a revised version) for safety.

I can add that there is also very simple reasons for the line filter
NOT to be after the mains switch, unless possibly they are built into
a common box.

I also think that this cutting of safety earths in instruments is an
unwise thing. Very few actually understand the full implications of
this. Most of these cases it is really using the incorrect probing
methods (diff-probes should have been used).

My conclusion is that you really *should* allways use earthed
connectors if you want to avoid this problem. The added personal
safety is just one of the benefits. I also realize that this is not
matching the real world in relation to current electrical
installations, and this was also understood by the people making these
standards. They do however assume that over a period of time will most
electrical installations have been replaced. The requirement to have
the earthed outlet actually is there to force the need for upgrading
old installations.

So, tell me what is right and wrong in this?
Shall we not put standards up that eventually improves the state
personal safety and EMC regulations even if this means that existing
installations eventually has to be upgraded?
Shall we not repair old misstakes by fading out old faulty solutions?

This is indeed a much bigger issue than a pair of caps "in the wrong
place" (or so it seems). This also shows that it is incorrect to call
it "Protection Earth" since it has to do with more than safety issues.
It is just "Earth" as a separation from "Line" (or "Phase") and "0"
(the intended return conductor of current supplied by the "Line" in a
single-phase setup).

Cheers,
Magnus - have actually worked with these issues a little...