[sdiy] Re: Leaking line filter

[Ingo Debus]

Magnus Danielson wrote:
> 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.

As I understand it, this is not correct. In the frequency range where
common mode is important, "EMC earth" and protective earth are two very
different things. A few inches of wire do not make a connection for
these frequencies.
Concerning EMC, the earth wire (if there is one) in the mains cord has
to be treated like the other two wires too: as a potential path for HF
to get into (and out of) the device. So it has to be connected directly
to the chassis where the mains cord enters the device. For obvious
reasons the other two wires cannot be connected directly to the chassis,
hence the Y caps.
If there is no protective earth wire in the mains cord, the Y caps will
still do their job maintaining a high frequency connection between the
mains wires and the surface of the device's enclosure.

> 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.

I disagree here. I wouldn't go so far and say a safety earth switch is
always a life saver, but they often make accidents less harmful. Using a
safety belt in a car doesn't *guarantee* that you don't die in an
accident, but often keeps you from getting hurt.
We have a 30 mA safety earth switch here in our apartment, it has never
tripped erraneously yet. The only time it tripped was when I painted a
ceiling and touched the wires for the ceiling lamp with the wet paintbrush.

Ingo

> 
> 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...