An installation design must start with the supply characteristics including the earthing arrangements. In the UK and most parts of Europe this information is available from the Electricity distributor.All other details are to be provided by the designer. Earthing arrangements fall broadly into 3 types:
- TN-S Supply has a seperate earth (typically the cable sheathing).
- TN-C-S (ProtectiveMultipleEarthing) the supply has a combined neutral and earth.
- TT. No earth provided by distributor, earthing achieved with earth rods locally.
Declared supply characteristics in the UK (More or less universal in 50 Hz regions)
Earth fault loop impedances:-
- 0.35Ω for PME supplies TN-C-S sytems.
- 0.8Ω for separate earth supplies TN-S systems.
- 21Ω where no earth is provided TT systems.
Prospective Fault Current I pf
Will normally be given at 16kA and is sometimes higher in inner city areas.