I'm looking at AC-DC PSUs for a system where all of the system needs to be protectively earthed (I'll call this PGND).

When browsing among suitable devices it is unclear to me what possible output ground configurations they can have. Let's take these two as examples:



As with most other AC-DC PSUs of class I, they have PGND at input, and isolated + and - outputs. But, is there a simple way to determine if the negtive output of such AC-DC PSU can be connected to the same PGND as input?

Do applications (let's say desktop computers) connect the PSU output to PGND as described above or is it more common to split the grounds and have a local floating ground surrounded by a enclosure connected to PGND? In that case - how are external connections managed? An external connection could have completely different GND / PGND reference.


1 Answer 1


Yes simple method is to measure if output common is connected to input earth. If there is no earth on inlet, it is definitely not earthed. Also if multimeter shows no connection from inlet earth to common then it is not connected.

In your case, you can just read the data sheet. Both supplies withstand kilovolts between output and input. So outputs cannot have a DC path to inlet earth, live, or neutral.

And yes a typical computer ATX supply has output common ground connected to inlet earth inside the supply.

It might be just as common to have an earthed metal chassis and isolated power supply output, so you can define yourself where and how you want to connect the supply common to earthed metal chassis, or leave unconnected.

  • \$\begingroup\$ If I understand correctly, connecting the negative output terminal to PGND is acceptable and frequent practice? As I see it, it should not have any negative consequences apart from: 1. If PGND is connected incorrectly, voltage might be present on secondary. 2. Output is no longer isolated, and it is possible to get an electric chock between the DC voltage and PGND. \$\endgroup\$ Commented Feb 1, 2023 at 8:56

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