Why on a TN-S earthing system is the maximum Ze given as 0.8 ohms by the supply authority when it’s often protected by a BS 88-3 100A fuse which has a maximum Zs of 0.38 ohms?
This maximum value for Ze of 0.8 ohms for a TN-S earthing system is a value that is quoted by the distributor and is commonly used for calculating likely Zs values within an installation at the design stage.
The main reason why you don’t need to change the BS 88-3 fuse within the supply cut-out when you have a measurement above the maximum value of 0.38 ohms for a 100A BS 88-3 fuse (T41.4) is because the Zs values given in the tables 41.2 to 41.4 BS 7671: 2018 are for protective devices providing fault protection (automatic disconnection in case of a fault).
The main fuse at the cut-out position is only providing overload protection for the incoming supply conductors. Fault protection is provided by the individual circuit breakers, fuses etc within the consumer unit which also provide individual overload protection for their own circuits.
The second reason why you shouldn’t change the main fuse at the cut-out is that it doesn’t belong the building / installation owner and is the property of the supply authority and is usually sealed to prevent removal. If there are any problems with this fuse, the supply or the cut-out then the supply authority should be informed.
This could also apply to a TT earthing system