" Tim (Remove NOSPAM." wrote in message
...

Our national grid system appears to be a TT one- where the earth
conductor is connected back to the neutral wire at the service fuse
where it enters the house, *and* there is an earth rod too, connected
to same.

That's not TT, that's TN-C-S (PME) with an additional earth electrode.

I am wanting to install a generator change over unit, which will
isolate N and L from the grid and allow the gen to feed the house.
(Honda 6kva)

Do I need to earth stake the genny from its earth terminal, and / or
connect the generator plug's N and E together to bring the N line of
the generator down to zero potential to create a safe system???

Or what?!

Yes to both questions, for a permanant installation. Here's a wiring
diagram that I've posted a couple of times in the last few months, and which
should be fairly self-explanatory. Your exisiting earth electrode, provided
that it's up to scratch and securely connected to the main earth terminal,
should be OK (unless it's the supplier's, in which case you should really
provide your own, connected in parallel).

Since the impedance of the supply from a small genny will be higher than
from the mains, earth fault loop impedance (Zs) values might be too high to
give proper protection. For this reason a 100 or 300 mA RCD should be used
in the generator feed as an aditional precaution.


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.

HTH
--
Andy