perhaps it helps to revisit the big question is why we need earthing at all.
As I see it there are 2 interlinked reasons.
1) to make sure that the metal case of equipment is at more or less the same voltage as the surface the user is standing on in normal operation. - touch voltage if you will. for this you cannot beat TT outdoors, and indoors a regime of bonding to all services, heating etc.
Any TNx system is using the ground voltage from some distance away, and the offset voltage is at the mercy of faults (and just heavy loads in TNC-s) in other installations sharing the substation.
2) to allow pre-emptive disconnection of faults, ideally still meeting condition (1) while doing so. Since about 1970 the approach has been formalised as double fault to danger, in that at least 2 independent things must fail to reach a dangerous condition that does not automatically disconnect the supply. This requires a CPC, but it does not need to be exactly at earth potential all the time.
However a similar level of safety can be reached another way without earths at all - also in the early 1970s, flexible cables that were not already became the modern construction of insulated and sheathed, rather than just twisted singles, and the use of 'double insulated' construction became the norm for things like garden tools and hand drills. (A 1969 act of parliament had already passed to accept the use of double insulated tools in a workplace setting - as an alternative to the previous Factories Electrical Regulations act of 1908 requirement for all electrical equipment with exposed metal parts to be earthed. )
perhaps it helps to revisit the big question is why we need earthing at all.
As I see it there are 2 interlinked reasons.
1) to make sure that the metal case of equipment is at more or less the same voltage as the surface the user is standing on in normal operation. - touch voltage if you will. for this you cannot beat TT outdoors, and indoors a regime of bonding to all services, heating etc.
Any TNx system is using the ground voltage from some distance away, and the offset voltage is at the mercy of faults (and just heavy loads in TNC-s) in other installations sharing the substation.
2) to allow pre-emptive disconnection of faults, ideally still meeting condition (1) while doing so. Since about 1970 the approach has been formalised as double fault to danger, in that at least 2 independent things must fail to reach a dangerous condition that does not automatically disconnect the supply. This requires a CPC, but it does not need to be exactly at earth potential all the time.
However a similar level of safety can be reached another way without earths at all - also in the early 1970s, flexible cables that were not already became the modern construction of insulated and sheathed, rather than just twisted singles, and the use of 'double insulated' construction became the norm for things like garden tools and hand drills. (A 1969 act of parliament had already passed to accept the use of double insulated tools in a workplace setting - as an alternative to the previous Factories Electrical Regulations act of 1908 requirement for all electrical equipment with exposed metal parts to be earthed. )
