We are developing a device that contains an isolated AC/DC power supply, from mains voltage to 3.3V. A colleague wants to connect the protective earth (chassis ground) to the secondary 3.3V side ground via the screw holes in the pcb, allegedly for EMI reasons. The 3.3V is used for the internal electronics, but we will also have a terminal block type I/O connector connected to the secondary side, that people will be able to connect cables to. The chassis itself will be made out of plastic. I have very limited experience with isolated AC/DC power design, but this grounding scheeme makes me a bit nervous. I have never encountered an isolated AC/DC power supply where the primary and secondary have the same earth reference. Are there any potential problems in terms of performance or in terms of safety or other compliance testing that we will need to go through that is overlooked with this grounding scheme?
Your standard desktop PCs do it and have done for some 40 years now, among many other devices.
Just be sure what you really say or need when you say you need to have an isolated supply.
Sure, it provides safety and isolation from live, but it does not have a floating output.
So be sure if you just want a supply and it can be non-floating isolated supply, or do you require a floating isolated supply.
Power supplies you buy come with data sheets and manuals how to install and use them. If this power supply even allows to connect mains input earth and 3.3V output ground together with screw holes, it may even be the intended mandatory operation of the power supply.
Some supplies require mains earth and some don't.
The ones that don't require it may still need EMI filtering circuits and referencing the output to mains side, one way or another.
So any safety and compliance issues won't happen if power supply is connected and wired according to manufacturer instructions.
In itself, connecting mains earth to power supply output ground is not a safety or compliance issue, it may even help with safety and compliance.
It's just that connecting mains earth to supply output ground may be annoying depending on situation, especially if you want to avoid ground loops, as then the supply does not float as it has a reference to earth.
I have never encountered an isolated AC/DC power supply where the primary and secondary have the same earth reference.
You must have encountered a tower or a mini-style PC, or a point-of-sale (POS) system. They all have it set up this way.
OTOH, it's not only unnecessary but sometimes impossible in double-insulated systems. There, there's only a capacitive coupling between the primary and the secondary.