10.1 Earth Fault Loop Impedance
Key Takeaways
- Earth fault loop impedance (Zs / EFLI) must be low enough for the protective device to disconnect within the required time (ADS)
- The fault loop path includes source, active conductors, fault at the equipment, protective earthing, MEN link, and neutral return
- Measure Zs live when supply is available; calculate or estimate from design when supply is not yet available
- Live EFLI testing requires the MEN link in place and follows dead tests in the Section 8 sequence
- High Zs often means poor connections, long/undersized runs, or missing/loose bonds—not “acceptable because an RCD is fitted”
Purpose of Earth Fault Loop Impedance Testing
After dead tests (visual, continuity, insulation resistance, polarity, correct connections), AS/NZS 3000 Section 8 moves into live verification. Earth fault loop impedance (EFLI; often written Zs at the circuit or point of test) answers one safety question:
Under an earth fault at this point, is the impedance of the complete return path low enough that fault current will operate the protective device within the required disconnection time?
If Zs is too high, fault current is too low. The MCB, fuse, or other overcurrent device may not clear in time (or at all under some conditions). Exposed conductive parts can remain at hazardous touch voltage while the fault persists. That is a failure of automatic disconnection of supply (ADS)—the core of fault protection in MEN installations.
Classic teaching pairs EFLI with the disconnection-time regime from earlier chapters: typically 0.4 s for many final subcircuits and 5 s for distribution/submains contexts (confirm exact application in your permitted AS/NZS 3000 edition). RCD residual protection is additional protection in many cases; it does not replace the need for a competent earth-fault loop for ADS design and verification where the rules require it.
Path Components of the Earth Fault Loop
Mentally walk the loop for a single-phase final circuit fault to earth:
| Segment | What contributes impedance |
|---|---|
| Source / distribution network | Utility transformer and upstream supply impedance |
| Active path | Consumer mains, submains, and final-circuit active conductors to the fault point |
| Fault | Active-to-exposed-conductive-part (or earth) at the equipment |
| Protective earthing path | Circuit PEC, earth bar, main earthing conductor arrangement |
| MEN link | Neutral–earth link at the main earthing point |
| Neutral return | Neutral conductors back toward the source |
Zs is the sum of these contributions as seen from the test point. A sound PE conductor can still yield high Zs if the MEN link is missing, a neutral joint is poor, or the run is long and undersized. Continuity of earthing (dead test) proves the metal path is continuous; EFLI proves the whole live loop is low enough for protective operation under actual supply conditions.
AS/NZS 3017 is the methods companion for instrument technique, probe points, and interpretation aligned with Section 8 outcomes. On the open-book exam, treat 3000 §8 as “what must be verified” and 3017 as “how the test is performed safely and consistently.”
Measurement vs Calculation
When supply is available (live measurement)
- Use a suitable multifunction installation tester on the energised installation after dead tests pass and the installation is safe to liven for verification
- Measure Zs (or related loop/impedance values per instrument method) at relevant points—commonly final-circuit extremities, socket outlets, fixed equipment earth points, and as required at distribution points
- Compare results against maximum Zs limits for the protective device rating/type and the applicable disconnection time (tables and rules in your permitted edition / manufacturer data / AS/NZS guidance)
When supply is not yet available (design / calculation)
- Calculate or estimate loop impedance from cable lengths, conductor sizes, and known or assumed external source impedance (Ze / external supply contribution)
- Design selection of cable CSA and protective devices must leave headroom so that measured Zs after construction still meets disconnection requirements
- Calculation is not a permanent substitute for live verification once the installation can be safely energised—exam stems often expect both: design competence and post-energisation measurement where required
Relationship to Disconnection Times and Trip Curves
Protective devices clear on current vs time characteristics:
- Higher fault current (lower Zs at a given voltage) → faster operation on the device curve
- Maximum Zs tables encode the impedance that still produces enough current for the device to operate within 0.4 s or 5 s as applicable
- Using the wrong device type/rating, or comparing Zs to the wrong table, produces false “pass” confidence
Exam habit: link high Zs → low If → slow or failed disconnection, not “high ohms means good insulation.” EFLI ohms are loop impedance under fault, unrelated to megohm IR values.
Live Test Precautions — MEN Must Be in Place
EFLI is a live test (≈8.3.9). Prerequisites:
- Dead tests complete and satisfactory (visual through correct connections as applicable)
- Installation in a safe construction state for energisation
- MEN link restored and confirmed at the main earthing point—live EFLI testing with MEN open is incorrect practice for a normal MEN installation verification story
- Competent prove-live / instrument connection; PPE and isolation awareness for nearby live parts
Never “remove the MEN to see what happens” as a routine test shortcut. The MEN is part of the designed fault-return path. Continuity and visual stages already checked its presence; live testing assumes it is in circuit.
High Zs Causes Candidates Must Diagnose
| Common cause | Why Zs rises |
|---|---|
| Poor connections | High-resistance joints on active, neutral, earth bar, MEN, or equipment terminals |
| Long runs / undersized conductors | Excess conductor impedance on long finals or submains |
| Missing or loose bonds / PECs | Fault current diverted poorly or path incomplete |
| Open or high-R neutral joints | Neutral return path degraded |
| Wrong test point / method | Measuring upstream of a series impedance or on a different circuit |
Remediate and retest—do not certify a circuit that fails maximum Zs limits for its protective device and disconnection requirement. An RCD may still trip on residual current in some fault modes, but RCD presence does not excuse an EFLI fail when the verification requirement is ADS by overcurrent within time for that circuit arrangement.
Exam traps
- Confusing Zs (low ohms good) with IR (high MΩ good)
- Skipping EFLI because “all circuits have 30 mA RCDs”
- Live testing before dead tests or with MEN removed
- Accepting high Zs as “old installation character” without remediation
Study note
Sketch the MEN fault loop once from socket earth pin → earth bar → MEN → neutral → supply → active → fault. Flag 8.3.9 and maximum Zs / disconnection-time tables in your permitted AS/NZS 3000. Know that low Zs supports ADS; high Zs is a fail until fixed.
What is the primary purpose of measuring earth fault loop impedance (Zs / EFLI) during AS/NZS 3000 Section 8 verification?
For live earth fault loop impedance testing on a normal MEN installation, which condition is required?
Which set best explains a high measured Zs on a final subcircuit?