Dave Jones wrote:


12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which
would happen to an earthed cable sheath anyway.

I've no experience of supply cables to speak of; but it sounds more PME
(TN-C-S) than TN-S, simply because - going on your description - there'd
be no way for the installer to distinguish between the 12 cores used to
provide N and the 4 used for E. For all we know the cores you describe
are bound together where the final supply to your place tees off the
feeder in the road...

But Dave has it most right when he says,

If you're not sure phone the supplier up, they should tell you

The "should" here refers to the fact that they have a legal obligation
to tell you what earthing arrangement you have, what the maximum earth
loop impedance of the supply is, and what the maximum prospective
short-circuit fault current is. (Don't expect those last two figures to
be consistent with a 240 (or 230)V supply and Mr Ohm's most excellent
Law, btw: they're both worst-case figures from opposite ends of the same
spectrum, and suppliers allegedly most often say "0.35ohm" (for TN-C-S)
or "0.8ohm" (for TN-S) (which would imply L-to-E fault currents -
admittedly not exactly the same as a PSC, but damn close for PME at
least - of about 690A and 300A respectively), while simultaneously
quoting PSCs of 10kA or 16kA (the Conventional Maximum figures).

Of course, getting through to anyone in "engineering" at an electricity
supply company in these days of micro-privatised
sub-sub-outsource-contracted Competitive Electricity Supply is a
patience-stretching experience...

Stefek