Earth Fault Loop Impedance (Zs)
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Full course section in Plus
The earth fault loop is the path a line-to-earth fault takes: out through the line conductor, through the fault, back through the protective conductor and the supply's earth return to the transformer. Its 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.
Free interactive sample
Choose an option, check it and retry here. No account, payment or energy needed. These public samples are separate from the full course and practice papers; progress is not saved.
Original public revision material · A4:2026. For revision only; do not use samples to guide work on site.
Community draft — not yet reviewedWhat this means
This question has not yet been reviewed by a qualified electrician. It is a community draft: check anything you plan to act on against BS 7671 and your own supervisor before you rely on it. Completing it does not make you competent, certified or registered.
Common questions
- Can I try Earth Fault Loop Impedance (Zs) free on this page?
- Yes. The public sample below accepts answers and gives feedback and retries without an account, payment or energy. It is a short revision sample, not the complete course or a full mock paper. The full course section requires Plus.
- Where does Earth Fault Loop Impedance (Zs) sit in the City & Guilds scheme?
- (Zs) maps to 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 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 of the loop external to the installation, measured at the origin. Zs 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 limits fault , 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 Zs can break it.
Explore key terms
Open a term to read its meaning here, then close it or explore another. Free to use without an account.
ZsEarth 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 full Zs glossary entryZeExternal 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 full Ze glossary entryADSAutomatic 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 full ADS glossary 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 full R1+R2 glossary entry