Ze
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 system: a TT installation relying on an electrode in soil sees a far higher value than a TN supply with a metallic return.
Where Ze fits in the learning path
The Testing & Inspection section covers these units:
- Initial Verification & Dead Testing
- Live Testing
- Inspection & Certification
Curriculum mapping: C&G 2365 Unit 304 / 2391.
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The total opposition to current around the loop a fault current would travel. That loop runs from the supply, along the line conductor to the fault, and back through the protective conductor and the earthing. It is the supply's own part (Ze) plus the circuit's R1 + R2, the resistance of its line and protective conductors. A low Zs lets a big fault current flow, which trips the protective device quickly, so electricians measure it and compare it with the maximum allowed for that device.
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Zs — Earth Fault Loop Impedance
The total opposition to current around the loop a fault current would travel. That loop runs from the supply, along the line conductor to the fault, and back through the protective conductor and the earthing. It is the supply's own part (Ze) plus the circuit's R1 + R2, the resistance of its line and protective conductors. A low Zs lets a big fault current flow, which trips the protective device quickly, so electricians measure it and compare it with the maximum allowed for that device.
Read the Zs entryTN-S
An earthing 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.
Read the TN-S entryTN-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 entryA definition is a start. Using it in a real question is where it sticks.