Zs
Earth Fault Loop Impedance
The total opposition to 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 . It is the supply's own part () plus the circuit's R1 + R2, the 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.
Where to look it up: BS 7671 Chapter 41.
Where Zs 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 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.
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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 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 entryR1+R2
The combined resistance of a circuit's line conductor (R1) and its circuit protective conductor (R2), measured end-to-end during dead testing. It proves the protective conductor is continuous, and added to Ze it predicts the circuit's earth fault loop impedance before the supply is ever switched on.
Read the R1+R2 entryADS — Automatic Disconnection of Supply
The standard protective measure in UK installations: earthing, protective bonding and a protective device working together so that a fault to earth causes disconnection within a prescribed time. If any leg fails — poor earth path, missing bond, wrong device — the fault can sit live on touchable metalwork instead of clearing.
Read the ADS entryPFC — Prospective Fault Current
The highest current that could flow at a given point if a fault of negligible impedance occurred there — whether between live conductors or from live to earth. Every protective device must be able to break the prospective fault current at its position without failing, which is why PFC is measured or calculated at the origin of every installation.
Read the PFC entryImpedance
Impedance is the combined opposition that resistance and reactance present to alternating current, measured in ohms (Ω). Reactance comes from inductance and capacitance, so impedance is not always the same as a resistance reading. Earth fault loop impedance, Zs, describes the whole fault-current path; R1+R2 describes the resistance of two circuit conductors.
Read the Impedance entryA definition is a start. Using it in a real question is where it sticks.