Hallo Erik,

I agree with you:

7. It’s better if a ferrite is used to connect signal ground and enclosure ground.

is a very strange design rule, which seems to make no sense at all! Nevertheless this design rule can be found in many many similar lists of tricks 'How to deal with EMC', which are published in the net and text books.

This design rule is totally wrong of course, because an enclosure can no longer act as a shield, when isolating signal ground and enclosure by the help of inductivity!!! And that's the reason, why I don't like these lists of tricks very much: You will find good design rules there, which are based on physical thoughts, but also tricks, which were found by trial and error, which worked in CERTAIN cases. But this does these 'tricks' not necessarily make good design rules...

The idea behind this recommendation of isolating signal ground and enclosure is, that an inserted inductivity CAN help to prevent malfunction caused by an ESD event.

How that?
Assume your printed circuit board has cable which leave the board at the left. Further assume, that the enclosure is connected somewhere at the right, which is already a very very wrong decision!! Then, if an ESD event hits the enclosure, ESD current tries to propagate via the cables. But because these are sitting on the other side of board, any ESD current must travel ACROSS the whole board... THEN, AND ONLY THEN, inserting an inductivity between signal ground and enclosure COULD indeed have solved the ESD susceptibility of this VERY CERTAIN application. But to make this a design rule is most questionable, because it's totally wrong!!!!!

A much better approach is to connect the cable shield to the enclosure in such a way, that the formed Faraday cage is uninterrupted, means that any ESD event hitting the enclosure can directly flow to cable screens, not being able to penetrate enclosure and run across the whole board!! And if enclosure is connected to signal ground at this cable entry point, where the cable screens are connected to enclosure, then the enclosure can furthely work as shield for the electronics sitting on PCB.

It might be useful sometimes to isolate the enclosure to signal ground for mains frequency, and connect the enclosure to signal ground by the help of capacitor. But for high frequencies THERE MUST ALWAYS BE a low impedant connection between signal ground and enclosure. Just what a capacitor or a direct connection yield. But, NEVER insert an inductivity. Then enclosure can not act as a shield anymore. And if this insertion was made, then only to correct a former mistake, according the motto, or better hope, that two mistakes give something that is correct again....


Kai