How to test earth fault loop impedance (Zs)

Helpful video reference. We use Proactive Technical Training's video "How to Carry Out Earth Fault Loop Impedance Test - Measuring Zs | PTT" as the visual reference here. PTT (Proactive Technical Training) is a UK electrical training provider whose videos are aimed at electricians working towards their 18th Edition and inspection and testing qualifications. The video demonstrates direct live Zs measurement procedure clearly, covering test lead connections, RCD considerations and how to read the displayed result.

1. What earth fault loop impedance actually measures

When a line conductor accidentally contacts an earthed metal part (a fault to earth), current flows around a loop: from the source transformer, along the line conductor to the fault point, then back via the protective conductor, main earthing terminal, earthing electrode or supply neutral, and the source transformer secondary winding. The total resistance and reactance of this path is the earth fault loop impedance, Zs.

Zs directly determines how much fault current flows and therefore how fast the protective device (MCB or fuse) operates. BS 7671 sets maximum permitted values for Zs in Table 41.2 (for TN systems, which covers most UK domestic properties with PME earthing) and Table 41.4 (for TT systems, common in rural areas). Exceeding the maximum means the device may not disconnect quickly enough to prevent a dangerous shock or fire.

The formula is simple: Zs = Ze + (R1 + R2), where Ze is the external loop impedance (measured at the consumer unit origin) and R1 + R2 is the sum of the line and protective conductor resistances for the circuit.

2. Confirm tester calibration and null the leads

Check the MFT calibration label before starting. In most commercial and inspection work, a calibration interval of 12 months is standard. An out-of-date instrument means results cannot be relied upon and the test schedule is not valid.

Null the test leads before measuring. Short the test lead probes together, run the Zs test function, and zero out the lead resistance on the instrument. This removes lead resistance from the readings, which matters when you are comparing against maximum values with a resolution of 0.01 ohms.

3. Measure Ze at the consumer unit

The external loop impedance Ze is measured at the origin of the installation, usually at the consumer unit, with the main switch open or with the tails temporarily disconnected from the main switch. The MFT connects between Line and Earth at the tails.

Where possible, remove parallel earth paths (such as the main protective bonding conductors to gas and water pipes) before measuring Ze. Leaving them in place gives a lower reading than the true Ze, which could cause you to underestimate circuit Zs values later. Once measured, reconnect all bonding before restoring power.

4. Choose live or dead Zs testing

There are two practical approaches:

• Live Zs test (direct measurement). The MFT connects at the socket, light fitting or accessory with the circuit live and measures Zs directly. This is the quickest method and gives the actual figure the protective device will see. Modern MFTs offer a high-impedance (non-tripping) mode that avoids operating RCDs during the test.
• Dead R1+R2 test. With the installation isolated, the MFT measures the resistance of the line and protective conductors from the consumer unit to the furthest point. Ze is then added manually: Zs = Ze + (R1 + R2). Safer on isolated circuits but requires accurate Ze measurement and good record-keeping.

For most domestic EICR and initial verification work, live Zs testing is the standard approach. Use the non-tripping mode on RCD-protected circuits. If any reading is borderline or the circuit is known to have a high resistance connection, cross-check with a dead R1+R2 measurement.

5. Test at the furthest point of each circuit

For socket circuits, the furthest socket from the consumer unit gives the highest Zs because the conductor run is longest. For lighting circuits, the furthest light fitting or switch position is the test point. For radial circuits feeding fixed appliances, test at the appliance connection point.

On ring final circuits, you can test at any socket and the ring topology keeps Zs relatively low. A high Zs on a ring often indicates a broken ring or a poor connection rather than excessive cable length.

Connect the MFT test leads to Line (L), Neutral (N) and Earth (E) at the socket or accessory. Select the live Zs function. The instrument runs a brief test current and displays the result in ohms.

6. Compare results against BS 7671 maximum values

The maximum permitted Zs depends on the protective device type, rating and the required disconnection time:

• For socket circuits in TN systems, the disconnection time is 0.4 seconds. A 32A Type B MCB has a maximum Zs of 1.37 ohms from Table 41.2.
• For final circuits supplying fixed equipment only, a 5-second disconnection time applies, giving a higher permitted Zs.
• For TT installations (earth rod earthing, common in rural east Kent), the Zs limits are much lower because the earth rod resistance is high. RCD protection is mandatory for all circuits on TT systems.

In practice, many electricians apply an 80% advisory limit to the tabulated values. This accounts for the fact that conductor resistance rises with temperature, and the BS 7671 values are based on the conductor at its maximum operating temperature. Testing at ambient temperature gives lower readings, and the 80% limit provides a margin so that results remain within the maximum when the circuit warms under load. For example, for a 32A Type B MCB with a maximum Zs of 1.37 ohms, the 80% advisory limit is 1.09 ohms.

7. Record results and deal with failures

All Zs results go into the schedule of test results alongside the circuit details (cable size, protective device type and rating, length, etc.). The schedule must be completed even for circuits that pass without issue, as it forms a legal document under the EICR or installation certificate.

Common causes of excessive Zs include: a corroded earthing electrode or poor earth rod connection, a corroded main earthing terminal or bonding clamp, a break in the circuit protective conductor, a poor connection at a socket or junction, undersized CPC (circuit protective conductor) in older wiring, or a high external Ze from a degraded supply.

Each of these needs locating and fixing before the circuit can be certified. The R1+R2 dead test is useful for locating faults: if the dead measurement is fine but the live Zs is high, the problem is in Ze; if both are high, it is in the circuit conductors themselves.

When to call us

EICR testing, including full Zs testing on every circuit, is part of the standard inspection Richard carries out across Sandwich and east Kent. If an existing EICR has flagged a C2 or C3 observation related to earth fault loop impedance, or if you have had nuisance tripping that might indicate a wiring fault, call to discuss what testing would reveal.