I'm sure I should know - but what is PME ?

It stands for Protective Multiple Earth and is an obsolete term for
describing what is now known as TN-C-S. These letters, which are based on
French indicate that:

T - earth is related to ground potential. (alternative being I- isolated)
N - earth is provided by the supplier
C - the supplier's installation combines the earth and neutral into a single
conductor.
S - the user's installation has the earth and neutral in separate
conductors.

The other commonly used systems a
TT - earth is provided by a local earth rod. (Should probably be called
TT-S, but isn't).

TN-S - earth is provided by the supplier using their own specific earthing
conductor, not sharing with neutral.

Other systems are possible, but are not usually legally permitted on fixed
installations.

It is good policy to assume that, even what may be a standard acronym
to the author, is not known by the reader and should always be spelled
out once fully in a document.

Unfortunately, in a technical discussion, it is not possible to avoid the
use of jargon and acronyms without disrupting the argument. Many online
resources are available for those wishing to get up to speed.

Christian.