@Njk:
Thanks for the explanation. Interesting.
The FY3224S does not actually have a chassis in that sense.
The BNCs are soldered directly to the PCB and the housing is made of plastic.
I put in a three core wire because otherwise the BNC have 94VAC against earth, which made me a bit worried.
But as I said, I installed a switch so that it can be disconnected again.