Generator power transfer switch with TN-S earth

[bobjob]

Hi can anyone give me a pointer on this one, I have been asked to install a 10kva gen circuit to a property that has a TN-S earth, I have installed befor but to properties that have been TT earth.

The regulations state that both phases must be isolated from the Utility companies and never allowed in parallel operation, this I did in my last install with a purpose designed swicth (no probs). however this install being a
TN-S will make things very diffcult to isolate the neutral ??. As when I connect up to the main earth bonding it will effectively put the generators neutral in parralel operation of the utility companies, and yes I will be tying neutral of the generator to a Ground refernce rod.

Even worse what is this were a PME ?

Am i miss understanding our regs, anyone assist here

Rob

 

[jbartos]

Suggestion:
Naming Electricity System Earthing Arrangements
The information on this document is based on UK DIY FAQ part written by Andrew Gabriel.

European naming conventions
Mains electricity systems are categorised in the many European countries (Finland, UK, etc.) according to how the earthing is implemented. The common ones are TN-S, TN-C-S and TT.

Note that in these descriptions, 'system' includes both the supply and the installation, and 'live parts' includes the neutral conductor.

Description of letters
First letter:
T The live parts in the system have one or more direct
connections to earth.
I The live parts in the system have no connection to earth,
or are connected only through a high impedance.

Second letter:
T All exposed conductive parts are connected via your earth
conductors to a local ground connection.
N All exposed conductive parts are connected via your earth
conductors to the earth provided by the supplier.

Remaining letter(s):
C Combined neutral and protective earth functions (same conductor).
S Separate neutral and protective earth functions (separate conductors).

Valid system types in the 16th Edition IEE regs:
TN-C No separate earth conductors anywhere - neutral used
as earth throughout supply and installation (never seen this).
TN-S Probably most common, with supplier providing a separate
earth conductor back to the substation.
TN-C-S [Protective Multiple Earthing] Supply combines neutral
and earth, but they are separated out in the installation.
TT No earth provided by supplier; installation requires own
earth rod (common with overhead supply lines).
IT Supply is e.g. portable generator with no earth connection,
installation supplies own earth rod.

Ways to provide grounding
Inside or nearby your consumer unit (fuse box) will be your main earthing terminal where all the earth conductors from your final subcircuits and service bonding are joined. This is then connected via the 'earthing conductor' to a real earth somehow. The following earthing conventions are used in UK:

TN-S The earthing conductor is connected to separate
earth provided by the electricity supplier. This
is most commonly done by having an earthing clamp
connected to the sheath of the supply cable.

TN-C-S The earthing conductor is connected to the supplier's
neutral. This shows up as the earthing conductor
going onto the connection block with the neutral
conductor of the supplier's meter tails. Often you
will see a label warning about "Protective Multiple
Earthing Installation - Do Not Interfere with Earth
Connections" but this is not always present.

TT The earthing conductor goes to (one or more) earth
rods, one of them possibly via an old Voltage Operated
ELCB (which are no longer used on new supplies).

There are probably other arrangements for these systems too. Also, a system may have been converted, e.g. an old TT system might have been converted to TN-S or TN-C-S but the old earth rod was not disconnected.