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I don't agree with that, and I am involved in safety standards. Even if signal ground jumps to a high voltage due to a fault, the metal case is connected to chassis ground, which (in Class 1 equipment) is connected to the building safety ground. There is no electric shock hazard. If something has a metal case, it should be either Class 1 or Class 3 (powered by extra-low voltage). Class 2 ('double insulated') boxes should be non-conducting, even if they require an internal conductive coating or an internal foil shield for EMC reasons.

For example, typically on the
boards I design I use 0.2 mm spacing, but with chassis ground (when
present) I make the spacing 0.85 mm.?? That way, if the internal
circuitry suddenly finds itself at a high voltage relative to chassis
ground during a fault, anyone touching the metal casing at the time wont
have their socks blown off.

-- 
OOO - Own Opinions Only
Best Wishes
John Woodgate
Keep trying