a well instrumentation system , comprising : a power and data supply ; and a plurality of functional units attached to the power and data supply and distributed throughout the well , characterised in that the power and data supply comprises first and second substantially identical cables , and in that each unit comprises a first power supply channel and a first data channel connected to the first cable , a second power supply channel and a second data channel connected to the second cable , and a functional module which draws power from the first power supply channel or the second power channel module , and data from the first data channel or the second data channel . the power and data supply can comprise a surface unit that can be selectably connected to either the first or second cable . the selection of connection of the surface unit to one or other cable is effective to select the corresponding power and data channels in the functional units .