Earth Fault Loop Impedance (Zs)
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Premium section
The earth fault loop is the path a line-to-earth fault current takes: out through the line conductor, through the fault, back through the protective conductor and the supply's earth return to the transformer. Its impedance sets the fault current, the fault current sets how fast the protective device opens, and that time is what stands between a fault and an electrocution.
is the external part, measured at the origin with the installation's conductor disconnected — an operation with obvious implications, which is why no-trip test methods exist. is the whole loop, and on a circuit it is simply Ze plus . That relationship is why the dead continuity test earns its place first: it lets you predict Zs before energising anything.
Measured values are compared against a maximum for the device, and the common working practice is to allow for conductors being warmer in service than they were when tested — which is why a measured value comfortably under the tabulated maximum is what you want, not one that just squeaks in. On a TT installation the loop runs through the ground and is high by nature, so an , not the device, provides disconnection.
Common questions
- Is there a free Earth Fault Loop Impedance (Zs) quiz?
- (Zs) sits in the Testing & Inspection section, which is part of TradeTraining Premium — but the app is free to start, sample questions are open to everyone, and the free sections cover the fundamentals first.
- Where does Earth Fault Loop Impedance (Zs) sit in the City & Guilds scheme?
- (Zs) maps to C&G 2365 Unit 304 / 2391. On TradeTraining it lives in the Testing & Inspection section as bite-size lessons with practice questions.
- Do I need Earth Fault Loop Impedance (Zs) for the 18th Edition exam?
- Yes — the C&G 2382-26 exam is set against the current BS 7671, and this topic is part of that ground. The exam is open book, so practising fast navigation to the right regulation matters as much as recall.
- What is the difference between Ze and Zs?
- is the impedance of the loop external to the installation, measured at the origin. is the whole loop for a particular circuit, and equals Ze plus the circuit's own R1+R2.
- Why does a high Zs matter?
- A high loop impedance limits fault current, and a limited fault current means the protective device takes longer to open. Disconnection time is the safety requirement — 0.4 s for most final circuits on a TN system, 5 s for distribution circuits — so a high can break it.