Why can you not use TN-C-S in special locations that are outdoors?

[ringer]

Looking in BS7671 at Part 7, Special Locations, in those involving circuits that are typically run outdoors, TN-C-S is specifically excluded in preference of TN-S or TT. So why is TN-C-S specifically excluded? I know the ESQCR is quoted, but want to understand why the ESQCR makes this prohibition. Regulation 717.411.4 may hold the key to this, but I still cannot figure out why.

Relevant regulations are below (note the variation in wording, but all essentially saying the same thing)

704 Construction and Demolition Site Installations
704.411.3.1 A TN-C-S system shall not be used for the supply to a construction site, except for the supply to a fixed building of the construction site.
NOTE: The ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction

705 Agricultural and Horticultural Premises
705.411.4 A TN-C system shall not be used in the installation.....

708 Electrical Installations in Caravan / Camping Parks and Similar Locations
708.411.4 In the UK where the installation is supplied from a TN system, only a TN-S installation shall be installed.....
NOTE: The ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction

709 Marinas and Similar Locations
709.411.4 In the UK for a TN system, the final circuits for the supply to pleasurecraft or houseboats shall not include a PEN conductor.
NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to boat (sic) or similar construction

711 Exhibitions, Shows and Stands
711.411.4 In the UK where the system type of earthing is TN, the installation shall be TN-S
NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction

717 Mobile or Transportable Units
717.411.4 In the UK a TN-C-S system shall not be used to supply a mobile or transportable unit except:
(i) where the installation is continuously under the supervision of a skilled or instructed person, and
(ii) the suitability and effectiveness of the means of earthing has been confirmed before the connection is made

721 Electrical Installations in Caravans and Motor Caravans
721.411.4 NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to a caravan

 

[Deleted member 26818]

There's a number of reasons, mostly it is down to the reliability of earthing in these locations, and diverted neutral currents.
With construction sites, you could have a part that is reliably earthed, and another that is not. Thus introducing an earth potential difference, that diverted neutral currents may find appealing.
With caravans and transportable units, there is the possibility that the metal clading could become live in the event of a supply neutral failure. If the caravan or unit is not reliably connected to earth, then a person might become the neutral return path for all the installations affected by the supply neutral fault.
With agricultural installations, livestock are more prone to electric shock, due to the distance between their front and rear feet.
Stray diverted neutral currents can pop up anywhere, where there is a reliable connection to earth.
Etc. etc.

 

[MarkieSparkie]

Spins covered it pretty well, but just one point I'd like to make:
Even in the no fault condition there exists a measurable potential difference between the PME earth terminal and true earth due to the impedance of the PEN conductor. As Spin alluded animals are more susceptible to the induced leakage current due the potential between front and hind legs. So say, a milking parlour on an agricultural installation was fed from a TN-C-S supply with all the exposed metal work bonded to the PME earth terminal there could be an potential impressed on that metalwork (railings, pens and cattle grids etc) with respect to true earth such that the cows feel it and become uncomfortable enough that their milk yield reduces or they are reluctant to even enter the parlour. Even though the level is not enough to cause injury.

 

[Chr!s]

Looking in BS7671 at Part 7, Special Locations, in those involving circuits that are typically run outdoors, TN-C-S is specifically excluded in preference of TN-S or TT. So why is TN-C-S specifically excluded? I know the ESQCR is quoted, but want to understand why the ESQCR makes this prohibition. Regulation 717.411.4 may hold the key to this, but I still cannot figure out why.

Relevant regulations are below (note the variation in wording, but all essentially saying the same thing)

704 Construction and Demolition Site Installations
704.411.3.1 A TN-C-S system shall not be used for the supply to a construction site, except for the supply to a fixed building of the construction site.
NOTE: The ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction

705 Agricultural and Horticultural Premises
705.411.4 A TN-C system shall not be used in the installation.....

708 Electrical Installations in Caravan / Camping Parks and Similar Locations
708.411.4 In the UK where the installation is supplied from a TN system, only a TN-S installation shall be installed.....
NOTE: The ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction

709 Marinas and Similar Locations
709.411.4 In the UK for a TN system, the final circuits for the supply to pleasurecraft or houseboats shall not include a PEN conductor.
NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to boat (sic) or similar construction

711 Exhibitions, Shows and Stands
711.411.4 In the UK where the system type of earthing is TN, the installation shall be TN-S
NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction

717 Mobile or Transportable Units
717.411.4 In the UK a TN-C-S system shall not be used to supply a mobile or transportable unit except:
(i) where the installation is continuously under the supervision of a skilled or instructed person, and
(ii) the suitability and effectiveness of the means of earthing has been confirmed before the connection is made

721 Electrical Installations in Caravans and Motor Caravans
721.411.4 NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to a caravan

Pme will be an issue anywhere where its difficult to maintain an equipotential zone, if you look at your list you can see why that would be difficult to achieve, and the risk that may pose as already mentioned.