Another Supplementary bonding Question

[uberfreak]

The only circuit coming into my bathroom is a lighting circuit connecting to a pullcord on the ceiling in zone 3. Am i right in saying that my bathroom supplementary bonding should end at the earth in that pullcord switch?

When reading about supplementary bonding its made plain that you don't run to the CU earthing terminal but if your connecting to a circuits earth aren't you indirectly earthing to the CU anyway? So what's the difference?

 

[Spark123]

Sort of a mixture of different regs question - the 16th edn regs had a zone 3 but no longer exists in the 17th.
In the 16th edn regs you were required to connect together all extraneous and the CPC of the circuit supplying any class 1 or 2 apparatus within the zones.
Under the 17th this has been relaxed providing all circuits in the location are RCD protected, disconnection times are met (which should be nice and easy if they are RCD protected) and the pipework is effectively connected back to the MET - you would need to confirm this by measurement.
If the above requirements for the 17th edn regs aren't met with then supplementary bonding will be required - this needs to connect together all ECPs of the location.

Supplementary bonding is carried out local to the location, the idea is it holds all parts which can introduce a potential at or about the same potential. There is no need to connect it to the MET.
You only need to connect the CPC of each circuit once hence once at the light switch would be fine if supp bonding is reqd.