http://www.iee.org/Publish/WireRegs/...asticPipes.pdf
P.S.
The thrust of the document is to encourage the use of plastic pipework. The
IEE believes that plastic pipework and the subsequent lack of earthing
potentials in the room is a safer environment than an bonded earthed metal
one. If there have to be earthed potentials in the room, it is best that
they are the same, however, which is what supplementary bonding is about.
Christian.