4.1 Earth Fault Loop Impedance (Ze & Zs) Testing

Introduction to Earth Fault Loop Impedance

Earth Fault Loop Impedance is the total impedance presented to an earth fault current flowing from the point of fault back to the supply transformer. In UK electrical installations designed for Automatic Disconnection of Supply (ADS) under BS 7671, the earth fault loop impedance dictates whether overcurrent protective devices (miniature circuit breakers or fuses) will trip rapidly enough during a short circuit to earth to prevent fatal electric shocks and electrical fires.

The complete earth fault loop path consists of the following components in series:

1. The line conductor ($R_1$) from the transformer to the point of fault.
2. The earth fault itself (assumed to have zero impedance).
3. The circuit protective conductor ($R_2$) from the fault back to the Main Earthing Terminal (MET).
4. The installation earthing conductor connecting the MET to the main earth electrode or supply earth terminal.
5. The external earth return path back to the supply transformer neutral point.
6. The supply transformer secondary winding.

The total impedance of this entire loop is designated as $Z_s$, while the external portion of the loop (outside the consumer's installation) is designated as $Z_e$.

Measuring External Earth Fault Loop Impedance ($Z_e$)

External earth fault loop impedance ($Z_e$) represents the supply network's impedance up to the origin of the installation. It is a critical baseline measurement performed at the main intake position.

The Strict $Z_e$ Measurement Procedure

To measure pure $Z_e$, the inspector must isolate the installation and physically disconnect the main earthing conductor from the Main Earthing Terminal (MET). This step is non-negotiable. If the earthing conductor is left connected to the MET during testing, the test current will flow not only through the supply earth path but also through all parallel earth paths created by main protective bonding connected to metallic water pipes, gas pipes, and structural steelwork. These parallel paths would artificially lower the measured impedance reading, presenting a dangerously false impression of a sound supply earth.

The exact step-by-step procedure for measuring $Z_e$:

1. Isolate the main switch of the installation following safe isolation procedures.
2. Disconnect the main earthing conductor from the MET.
3. Select the loop impedance function on a calibrated test meter complying with BS EN 61557.
4. Connect test leads using GS38 compliant probes to the incoming Line terminal of the main switch and the disconnected earthing conductor.
5. Execute the test and record the $Z_e$ value.
6. De-energize, safely reconnect the earthing conductor back to the MET, and torque securely.

Typical Supply System $Z_e$ Maximums

Distribution Network Operators (DNOs) in the UK state standard maximum design $Z_e$ values:

• TN-S System (separate earth sheath on supply cable): Maximum $Z_e = 0.80,\Omega$.
• TN-C-S / PME System (protective multiple earthing, combined neutral/earth supply): Maximum $Z_e = 0.35,\Omega$.
• TT System (installation relies on local earth electrode): $Z_e$ depends on earth rod resistance ($R_A$), typically ranging from $20,\Omega$ to $200,\Omega$.

Total Earth Fault Loop Impedance ($Z_s$): Direct Test vs Calculation

Total loop impedance $Z_s$ can be determined by direct measurement or by calculation using the fundamental formula:

$Z_s = Z_e + (R_1 + R_2)$

Advantages of Calculation over Direct Measurement

BS 7671 strongly encourages calculating $Z_s$ using measured $Z_e$ and previously measured dead $(R_1 + R_2)$ values rather than performing live $Z_s$ tests at every outlet. Calculation offers major safety and technical advantages:

• Safety: Eliminates live testing at accessories throughout the building, drastically reducing shock and short-circuit risks.
• Accuracy: Direct live testing can be skewed by parallel earth paths (e.g., metal conduits or pipework touched by accessories), showing a low $Z_s$ even if the circuit protective conductor ($R_2$) is severed. Calculation relies on the verified, isolated $(R_1 + R_2)$ value.
• RCD Nuisance Tripping: Direct testing on RCD-protected circuits can trip RCDs unless expensive "no-trip" high-resolution instruments are utilized.

BS 7671 Disconnection Times

For Automatic Disconnection of Supply (ADS) under Regulation 411.3.2, maximum disconnection times for final circuits not exceeding 32A are:

• 0.4 seconds for TN systems at 230V to earth.
• 0.2 seconds for TT systems at 230V to earth.

For distribution circuits and final circuits exceeding 32A, maximum disconnection time in TN systems is 5.0 seconds.

Temperature Adjustment: The 80% Rule

Tabulated maximum $Z_s$ values in BS 7671 Appendix 3 and Part 4 (e.g., $1.10,\Omega$ for a 32A Type B MCB) assume conductors are operating at their maximum permitted operating temperature (70°C for thermoplastic PVC).

However, testing is conducted at ambient room temperature (approx 20°C) when conductors are "cold." Because copper resistance increases with temperature (coefficient of 0.004 per °C), a cold conductor will have roughly 20% lower resistance than when carrying full fault current.

To account for this thermal increase, inspectors must apply the 80% Rule:

$\text{Measured Cold } Z_s \le 0.8 \times \text{Tabulated Maximum } Z_s$

Alternatively, multiply the cold measured $Z_s$ by 1.25 and compare against 100% of the tabulated value. If the cold measured value complies with the 80% limit, the circuit is guaranteed to disconnect safely within statutory time limits when cables run hot under fault conditions.