[Elecraft] Negative rail grounding

[Josh Fiden]

There's no safety requirement that DC return be connected to chassis.

SS30 is an SMPS so a different beast than the prior discussed linear 
supplies. It's possible that the DC return to chassis connection is 
needed for EMC compliance. That's providing a low impedance return path 
for noise generated by switch elements. Without the connection, noise 
will take an unwanted path and come out as common mode noise. A frequent 
source of the noise is capacitive coupling from MOSFETs to chassis, 
sometimes via a heat sink.

I would be hesitant to simply insulate the DC neg from chassis. One 
possibility would be to cut the copper around PCB mounting screws 
(carefully!), and put a cap between them so you maintain an AC 
connection but block the unwanted DC path. You want the capacitor value 
to be much greater than the parasitic capacitance.

73,
Josh W6XU

On 4/18/2018 11:10 AM, Michael Blake wrote:
> I though I would separate this from the other discussion. It seems that my newest Astron SS30 power supply does not allow for the negative post to be isolated from the chassis. The green wire can be disconnected from the chassis but the negative rail of the DC output can not be isolated from the chassis.  The negative rail is firmly connected to the chassis via two of the circuit board mounting screws. Separation would only be possible by using insulating washers and nylon screws.
>
> This is not true in one of my older supplies. Has there been an NEC or UL change that requires that one side of the output of a DC supply be grounded?