When to grounding to Earth(PE).

In article , Simon wrote: [....]

First, I'd look for a floating pin in the design based on what you've said.

You may want there to be a connection to the earth ground but you don't want the ground currents of other devices flowing through the sensitive parts of the PCB. You need to expand your thinking to enclose the entire system.

For safety reasons, the external metal has to be well grounded. For EMI reasons, you are likely to want to enclose your completed product in a conductive housing. This may mean metal or metal coated plastic.

The equipment you are connecting to may have its own ground connections. The cables may force the issue somewhat. You want to first figure out where the low frequency and DC currents will flow. This will determine some of the direct connections. In other places, you may want to connect RF currents, there you have the choice between a direct connection and a capacitor.

One thing you want to avoid is making flaky connections for RF. Two bits of sheet metal touching lightly can cause trouble. A little bit of, lets say 100MHz, leaking into some analog circuits may not cause any problems. If that 100MHz gets strongly AM modulated by the fan vibrations and a couple of bits of metal, it is far more likely to cause trouble.