Yep, the Main protective bonding conductors will protect in this example, however I am thinking also where someone has bodged a copper pipe installation with short pieces of plastic pipe / fittings which is the way things seem to be going.
In the example above you'd be relying on the CPC of the immersion heater circuit for disconnection, if to the 16th edition with a fuse up this can be up to 5s where the CPC can rise above 50v.
