Connecting COM and mains earth SMPS 3D printer

[mawood]

Hi, I am working on a pretty large scale 3D printer and I have a 24v switchmode supply, the typical kind from china and I would like to know if it is safe to connect the terminals labeled com to the mains ground and the chassis of my printer.
 I would like to do this so the chassis can be used as gnd and keep the metal chassis safe (electrically).
 I'll attach some pics of the power supply and printer frame for people who may be interested.

[MosherIV]

I would like to know if it is safe to connect the terminals labeled com to the mains ground and the chassis of my printer.

Have you checked is the metal chasis of the PSU is grounded?

It should be!
A) for safety and
B) to help with EMC

You need to also check if the 'Common' is isolated (with power off, check with DMM is there is continuity between GND to Common and Neutral to Common) SMPS usually are isolated.

Whether you can ground the common (or 0V) is dependant on the SMPS type or topology - so you need to find out.

[mawood]

The com has no continuity to mains earth, the PSU chassis is mains earthed. So I should be OK to connect the COM to the chassis and use it as the -VE connection?

[MosherIV]

Whether you can ground the common (or 0V) is dependant on the SMPS type or topology - so you need to find out.

[mawood]

Heres a photo of the inside of the PSU, can you take a look for me please?
Terminals are, from bottom to top, L, N, E, COM, COM, COM, +V, +V, +V

[MosherIV]

Showing a photo is still pretty useless. Are there any instructions? Have you asked the seller?

Try joining earth to Common with a small value (eg 250mA) fast blow fuse. If it is ok, the fuse will not blow. If it is not ok, it will blow and hopefully the small value will stop any damage.

[jwm_]

I have a similar setup and hooked ground to the chassis but did not connect it to negative. The reason is that will cause your USB and other ports (like uC programming ports) to be referenced to earth ground, which can be problematic.

[Brumby]

I have a similar setup and hooked ground to the chassis but did not connect it to negative. The reason is that will cause your USB and other ports (like uC programming ports) to be referenced to earth ground, which can be problematic.

This is a very important point.

Earthing is not just an issue for the PSU and your chassis - but for everything that is connected to either.