I'm not sure where it is going, either, but I would again ask.... if, although it "has an earth" it has no exposed metal, what relevant thing(s) is/are actually being 'earthed' by 'the earth'?
Lets say I'm designing an isolated SMPSU with a floating output. Lets further say that the output will be connected either directly or via non safety rated components to touchable metalwork. This is nearly always the case with AV and IT equipment. Even if the equipment itself has plastic boxes, there are plenty of connectors with exposed metal used on AV and IT equipment.
SMPSUs are noisy, to meet EMC standards I need to create a high frequency path between the input and output sides of the transformer. Otherwise the interaction of the parasitics in the transformer with the switching action of the converter will result in unacceptable RF emissions.
I could just put a capacitor between input and output sides of the circuit, many smaller SMPSUs do, but if I do that then.
1. The capacitor has to be rated to safety standards equivilent to "double or reinforced insulation". IIRC this means a "Y2" capacitor.
2. The capacitor value has to be small enough that the touch current is acceptable. Multiple items of equipment may be interconnected, so the total touch current may be several times the touch current from any one power supply.
Alternatively I could take two capacitors. Connect one between the input and mains earth, and the other between the output and mains earth. Now my capacitor selection becomes much easier.
1. The capacitor between the input and mains earth only needs to provide protection equivilent to "basic insulation". IIRC this means a "Y1" capacitor.
2. The capacitor between the output and mains earth doesn't need to be safety rated at all.
3. The capacitor value can be larger, because currents in the capacitor will give rise to earth leakage currents rather than touch currents.
The "relavent thing being earthed" is then the point where the input-earth capacitor and the output-earth capacitor meet. In an ideal world one would test earth continuity from the earth pin on the plug to that point.
In the real world of course that isn't possible, so one just has to do the best that is reasonablly practicable. For example for a power supply with a detatchable lead testing earth continuity to the end of said detachable lead.
