No disrespect to the other guys but the reg book quotes "supplementary bonding is not required to metal parts supplied by plastic pipes, such as metal hot and cold water taps supplied from plastic pipes. A metal bath not connected to extraneous-conductive-parts (such as structural steel work)...
undermounted sinks to do it properly u have to fill in the top part of the cabinet with chipboard or something along those lines with screws all round or just put extra supports in .
then mark the top out and cut a hole so the sink sits flush with with the top of the cabinet .
after that u...
there is no need to bond the copper stems if they are fed by plastic because there is no path to earth hope this helps but some old skool sparkys go bonding mad.
ps i,m nic eic registered as well as part p qualified oh and corgi reg :lol: