TN-S
An system where the supply itself provides a separate earth conductor back to the source. Traditionally that conductor is the lead sheath or armour of the distributor's cable. Neutral and earth stay separate throughout, which keeps the earth path clean but depends entirely on the integrity of that supply conductor.
Where TN-S fits in the learning path
The Installation section covers these units:
- Wiring Systems & Enclosures
- Cable Selection & Circuit Design
- Earthing, Bonding & Consumer Units
Curriculum mapping: C&G 2365 Units 203/204/305.
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- Earthing & BondingInstallation
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The most common modern UK supply arrangement: the distributor combines neutral and earth in a single conductor (the PEN) up to the service head, where it splits into separate neutral and earth for the installation. Its weakness is a broken PEN conductor, which can put the whole installation's earthed metalwork at a dangerous voltage — the reason special rules apply to EV charging and other outdoor equipment on PME supplies.
TN-C-S
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TN-C-S — Protective Multiple Earthing (PME)
The most common modern UK supply arrangement: the distributor combines neutral and earth in a single conductor (the PEN) up to the service head, where it splits into separate neutral and earth for the installation. Its weakness is a broken PEN conductor, which can put the whole installation's earthed metalwork at a dangerous voltage — the reason special rules apply to EV charging and other outdoor equipment on PME supplies.
Read the TN-C-S entryTT system
An earthing system where the installation provides its own earth via an electrode in the ground. Common on rural and overhead supplies, it takes no earth from the distributor at all. The soil path makes the earth fault loop impedance high, so an RCD, not the overcurrent device, is what disconnects earth faults on a TT installation.
Read the TT system entryEarthing
Connecting an installation's exposed metalwork to earth and back to the supply source. That gives fault current a deliberate low-impedance path home, turning a live casing into a large, brief fault current that the protective device can detect and disconnect — without it, the casing simply stays live.
Read the Earthing entryZe — External Earth Fault Loop Impedance
The portion of the earth fault loop that lies outside the installation — the supply transformer winding, the distributor's cables and the earth return path, measured at the origin with the installation's own parallel paths disconnected. Ze depends on the earthing system: a TT installation relying on an electrode in soil sees a far higher value than a TN supply with a metallic return.
Read the Ze entryA definition is a start. Using it in a real question is where it sticks.