If you assume for a second that there isn't an actual earth connection to the chassis... the currents can still return via the y-caps and therefore it's still not important that the earth is connected from a noise perspective, obviously necessary for safety. Same with the EDN article, I'm not disputing its statement, but the noise reduction may be more of a consequence of the live and neutral's y-capacitance to the chassis than the chassis' connection to earth.
...Thanks, well, yes...indeed, having the chassis connection to earth will indeed allow the Y caps to do a better job of EMC improvement….so I agree with you, but say that this is an example of a chassis earth connection improving the EMC situation.
In conducted emissions testing, as you know, we test at frequencies up to 30MHz, and that conducts down line and neutral, so why not down the earth connection aswell?….And indeed, as we know, higher frequencies than this are actually in there.
For any mains SMPS powered circuit, the actual full electric circuit goes back to the power station. Going right back to the power station, there are multiple earth connections of the neutral. As such, there’s always the possibility of emissions coupling out of the SMPS, such that the current coming in through the line, does not go back out of the neutral, and then we have common mode emissions...noise…..and its because the emissions couple to earth, and return via earth, rather than returning via neutral. So if we earth the chassis, then we “invite” the noise to couple to chassis (because it will be trying to couple to the “world outside” anyway)...then having the chassis connected to earth, means that the emissions will be channelled back down the mains earth wire….and in so doing, they will get attenuated by the common mode filtration in the SMPS.
So yes, earthing an enclosure is all about EMC improvement, as well as safety.
Now, if the chassis wasnt connected to earth.....(and by that i specifically mean a connection via the earth wire that comes into the product along with line and neutral), then you'd get emissions from the SMPS coupling to the chassis, and with no connection of chassis to incoming earth wire, then the emissions will couple to the "world around the smps, ie earth"...and couple back to the power station like that....and these emissions would not get so heavily attenuated by the common mode filter (Y Caps) in the smps, so would cause a big problem.....thats why earthing a chassis is definitely about EMC improvement.