First let me say that I understand that this question is best answered by someone familiar with relevant standards (whatever those may be).
Let me give some background. When I'm designing a board at work I usually separate my circuits into several classes. Two of those classes are "signals" and "AC". Nets in the signals class are, e.g., 3.3 or 5 VDC level and may be present on things like user interfaces (the end user doesn't come into direct contact with these nets, but will come closer than to nets in any other class). The AC class contains line voltage nets like 120 or 240 VAC.
For safety we set design rules that say there must be, e.g., 0.250" between nets in the signals class and nets in the AC class. Let's pretend that this clearance is called for by some standard that we would like our products to meet (I don't know if that is true). The nets in the signals class are isolated from nets in AC class by some kind of transformer that can withstand a few kV.
Usually I include the protective earth net in the class AC. This means that signals nets can't come within 0.250" of board mounting points that will be connected to PE.
Now my question. Is this kind of isolation clearance usually required between protective earth and these low level nets? Protective earth is, by definition, accessible to the end user and is considered safe so I see no reason that I would require this large physical isolation distance.