A TN-S system has the neutral of the source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source and the consumer’s earthing terminal is typically connected to the metallic sheath or armour of the distributor’s service cable into the premises or to a separate protective conductor of, for instance, an overhead supply.
Large consumers may have one or more HV/LV transformers dedicated to their installation and
installed adjacent to or within their premises. In such situations the usual form of system earthing is TN-S.

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References

• IEE Wiring Matters