Hello,

I'm working in a project which involves mains. It's a board fixed in a metal chassis, where this chassis serves as heat sink for a power mosfet, but the MOSFET's drain is isolated from it. What should I connect to the chassis? Main neutral/ground? Digital ground? I've seen some boards that uses a low resistor path from digital ground to chassis. I have also seen other boards where the main ground is connected to another system ground through a RC network to avoid ground loops (I think). Also, I've seen situations where the main ground is not simply connected to anything. What are your opinions?

 

Never connect the Neutral to anything other than the power connection.
It is generally a good idea to connect the mains ground to the case.
Connecting the digital ground to the case is another question. Some times it is worth while, other times no.

 

My question comes because where I live, I have the second configuration as shown below (Bootleg ground)

 

I don't know about the "bootleg" ground. Here (in Australia), the neutral is connected to the ground at the power board. But NEVER do that in your device. Just imagine what the results would be if you have an extension lead wired incorrectly, or use a 2 pin power plug inserted the other way. Then ACTIVE is connected to the case!
I've come across a similar thing here . A new molded lead had neutral and ground swapped. Then a power lead with neutral and active was used. Fortunately, it just blew up a PC and a monitor. It could have well killed someone.

Having the connections like you show in the wall socket is one thing. NEVER in your device plugged into it!!!!

But the bootleg connection could defeat earth leakage protection devices. Not a good idea.

 

I rented an apartment about 10 years ago that had no grounded outlets and I was constantly getting a nasty tingle from my computer. The solution was to have a worker come in and run an earth wire from the breaker box to the buildng's steel frame. Maybe the electrical code doesn't speak to that but at least my equipment was at the same potential as my floor (barefoot on ceramic tile floor).

I think that would be a much better solution than the bootleg ground since you do not really know where the bootleg ground is with respect to "your earth" mainly because of the reasons given by dendad and any IR drop that might develop in the Neutral circuit.