Stefek Zaba wrote:

But I've neither read deeply nor thought hard about this, so I'd welcome
anyone better-informed weighing in...

I haven't been following this thread that closely, but the recent
questions seem to be ...

Yea, but, when I earth my metal gas pipe and metal water pipe, like the
good little boy I am, does this then make my house installation partly TT,
thus potentally unsafe as you describe!
(This is why I asked :-) )

An installation can't be partly TT. If there's a metallic earth path
back to the substation transformer then it's TN, if there isn't then
it's TT. (IT doesn't apply here.)

Can any one shed any light on this question!?

(Another reason I ask this, is, if a generator is installed, this has to
have a rod (AIUI), but if this has a rod, connected to the house earth
supply, does this them break the earthing rules on my TN-S system?)

Transfer switches only seem to switch the L&N (Via an "all off" position)

A generator installation has to have its own means of earthing, in case
the connection to the supply distributor's mains earth has been severed.
The earths can be connected in parallel - there's no need for switching.

Here's a generator wiring diagram I've posted befo


L o----------------o
\ SW1a
Mains o---------- L to CU
from
meter +-------o
| SW1: DPCO changeover switch
| 100A break before make!
N o----------------o
| \ SW1b
| o---------- N to CU
|
| +---o
| |
| |
Supplier's E o-----------------------------o Main earth terminal
earth | | /
| | |
| | 16mm^2 |
| | earth |
------- |
| | |
| G.P. | | G.P. = Generator
| | | protection,
------- | fuse or CB
| | | plus 100mA RCD
| | |
-- | - | -- |
| G( | | |
| G( | | |
| G( | | | G = generator winding
| G( | | |
| G( | | |
| | | | |
Generator | +---.B | |
frame - | | | | B = bonding connection
-------.-- |
B|\ |
| \-------------+
|
|
|
-----
--- Your earth electrode(s)
- Re 200 ohm


NB 1. You are supposed to consult with your electricity supplier /
distribution network operator before installing switched standby
generating plant.

NB 2. Reg. 551-02-03 requires automatic load shedding arrangements if
the generator is not rated to supply the whole installation.

--
Andy