For what does PELV require protective conductor? And also what is differences between PELV and FELV?

Our posts overlap. I am being a bit arm waving, and simplifying a bit, and Graham is more rigorous with the terminology , and yes it is not strictly called double insulation  for SELV - much as a mains lead is not called double insulated, it is insulated and sheathed, but both the sheath and the insulation on the wire inside have to fail, and they are contrasting colours so hopefully damage to the outer is visible before the copper is exposed to touch.

The supply to SLEV equipment has to be designed to have more things that would have to fail before the primary mains voltage gets to where it should not. PELV permits less insulation, in principle just basic insulation on the primary,  on the clear understanding that the earth connection will cause automatic disconnection if the mains breaks through.

Our posts overlap. I am being a bit arm waving, and simplifying a bit, and Graham is more rigorous with the terminology , and yes it is not strictly called double insulation  for SELV - much as a mains lead is not called double insulated, it is insulated and sheathed, but both the sheath and the insulation on the wire inside have to fail, and they are contrasting colours so hopefully damage to the outer is visible before the copper is exposed to touch.

The supply to SLEV equipment has to be designed to have more things that would have to fail before the primary mains voltage gets to where it should not. PELV permits less insulation, in principle just basic insulation on the primary,  on the clear understanding that the earth connection will cause automatic disconnection if the mains breaks through.