If we have no extraneous-conductive-parts in an installation, TN-S could be more dangerous than TN-C-S, 

That's an interesting statement. Is that just down to a broken PEN being "more obvious" to consumers than a broken PE, so will likely persist for a shorter duration, as it'll be called in to the DNO earlier? In terms of the risk up to that point, I suspect I'd rather discover the "tingle" of a broken PE (leakage currents) rather than the "belt" of a broken PEN (load currents) - not to mention the additional fire risks from a broken PEN.

   - Andy.

AJJewsbury said:

Is that just down to a broken PEN being "more obvious" to consumers than a broken PE, so will likely persist for a shorter duration,

Yes, the touch-current will just sit there waiting for a victim.

However, in the case of a TN-S system with no extraneous-conductive-parts, in the event of a PE break, a consumer's earth electrode connected to MET will help divert at least 83 % of the available touch-current (assuming body resistance of 1 kΩ, and earth electrode resistance at the 200 Ω maximum recommended for stability).. So, let's say we had a potentially lethal touch-current of 30 mA, this is reduced to a much less problematic 5 mA (at least in dry conditions).

There is a tacit assumption that we consider only un-monitored TNS as it used to be when introduced in the 1930s. We can of course have a modern and  much smarter TNS and actually log that leakage and the NE offsets, and either report it or ADS on it . You cannot do this as successfully on PME as there is no other conductor to compare voltages with and you cannot have any kind of EFR at the substation, as fault current and legit neutral current look the same.

Mike.

There is a tacit assumption that we consider only un-monitored TNS as it used to be when introduced in the 1930s. We can of course have a modern and  much smarter TNS and actually log that leakage and the NE offsets, and either report it or ADS on it . You cannot do this as successfully on PME as there is no other conductor to compare voltages with and you cannot have any kind of EFR at the substation, as fault current and legit neutral current look the same.

Mike.

mapj1 said:

We can of course have a modern and  much smarter TNS and actually log that leakage and the NE offsets, and either report it or ADS on it .

Food for thought ... although N-PE voltage is also affected by line currents (or, in a three-phase portion of the network, balance of line currents).

And, since we are talking about such small currents that might be indicated in an N-PE break, I'd not therefore immediately be a proponent of 'ADS' on it, as the impact of supply interruption could outweigh the benefits of monitoring. Quite possibly, this is possibly (from a correlation vs scientific "actuality of a fault") monitoring L-N (or L-PE) voltages for a single-phase supply to detect a PEN break.

But certainly 'we' (i.e. the distributor) could act  more quickly upon a repeated or unexpected N-PE voltage condition.

Indeed. I do not know how common these sort of monitors are in new substations, but it would  only be a modest extension of the design to include some extra channels of ADC to cover the sorts of thing that would be needed. I think 21st century power distribution will need to be quite a bit smarter than it was in the 20th.

M.