How To Measure Earth Fault Loop Impedance (Zs & Ze)

Earth fault loop impedance is one of the most critical measurements in electrical testing. Knowing how to measure earth fault loop impedance correctly confirms that a circuit's protective devices will disconnect fast enough to prevent electric shock during a fault. Get it wrong, and you risk signing off an installation that fails to protect the people using it.

At Electrical Testing London, our engineers carry out these measurements daily across domestic and commercial properties throughout London and the South East. It's a core part of every Electrical Installation Condition Report (EICR) we produce, and it's a test where precision matters. Understanding both Zs (external plus internal impedance) and Ze (external earth fault loop impedance) values, and how they relate to maximum permitted values in BS 7671, is essential for any electrician or inspector working on UK installations.

This guide walks you through the full process: what equipment you need, how to take accurate Ze and Zs readings, the formulas behind the numbers, and the safety standards that govern acceptable results. Whether you're preparing for certification or sharpening your testing knowledge on site, this is a practical, step-by-step breakdown built from real-world experience.

What earth fault loop impedance is and why it matters

When a live conductor touches an earthed metal part, current takes the path of least resistance back to the source. That path is called the earth fault loop, and its total resistance to the flow of current is the earth fault loop impedance. The lower that impedance, the higher the fault current, and the faster the protective device (a fuse or circuit breaker) operates to disconnect the supply. If the impedance is too high, the fault current stays too low, the device doesn't trip quickly enough, and exposed metalwork remains live long enough to cause a fatal shock.

The fault loop path explained

The earth fault loop is not a single cable. It's a series circuit made up of several physical sections that the fault current travels through from the point of fault back to the transformer supplying the installation. You need a clear picture of this path before you start learning how to measure earth fault loop impedance accurately.

The complete loop includes the following segments:

• The supply transformer winding (from neutral to phase)
• The phase conductor from the transformer to the meter
• The installation's phase conductor from the consumer unit to the point of fault
• The circuit protective conductor (CPC) from the point of fault back to the main earthing terminal
• The earthing conductor and the supply network's earth return path back to the transformer

Every one of those segments adds impedance to the total. Even a corroded earth clamp or an undersized CPC can push your measured value above the permitted maximum.

Zs and Ze: understanding the two values

These two values describe different portions of the same loop, and you'll measure both during a full test.

Ze tells you what the supply network contributes. Zs tells you the full picture for a specific circuit, including the wiring resistance inside the building. BS 7671 (the IET Wiring Regulations) sets maximum permitted Zs values for every type of protective device and rating. Your measured Zs must fall at or below those limits to confirm the device will disconnect within the required time under fault conditions.

A high Ze reading at the origin is a strong indicator of a supply network problem outside your control, and it will affect every circuit's Zs value throughout the installation.

Why impedance values directly affect disconnection time

Ohm's law drives everything here: fault current equals the supply voltage divided by the total loop impedance (I = V ÷ Z). A 230 V supply through a loop impedance of 0.8 ohms produces roughly 287 A of fault current. Drop that current and the circuit breaker's magnetic trip element may not activate at all, leaving you dependent on the much slower thermal element. BS 7671 Table 41.2 and the device manufacturer's time/current curves define the exact Zs limits for each protective device type and rating, and those figures are what your test results must be checked against.

Step 1. Prepare for testing and stay safe

Before you pick up your loop impedance tester, you need the right equipment assembled and a clear grasp of the hazards involved. Earth fault loop impedance testing is performed on live circuits, which means a single oversight can result in a serious electric shock or damage to the installation. Taking ten minutes to prepare properly protects both you and the integrity of your readings.

Equipment you need

To measure earth fault loop impedance accurately, you need a dedicated loop impedance tester that complies with BS EN 61557-3. Standard multimeters cannot perform this test. Your instrument must display results in ohms and carry a valid, in-date calibration certificate before you take it on site.

Gather the following before you start:

• Loop impedance tester (BS EN 61557-3 compliant)
• Calibrated test leads with shrouded, finger-safe probes (CAT III or CAT IV rated)
• Insulated gloves and safety glasses
• Flathead and crosshead screwdrivers for consumer unit access
• Test record sheets or a digital log to capture all readings as you go

Safety checks before you connect the meter

Before connecting anything, inspect your test leads for cuts, cracked insulation, or bent pins and replace them if there is any doubt about their condition. Confirm the instrument performs a self-check and reads within tolerance against a known reference value. Also verify that no sensitive equipment is connected on the circuit you are about to test, since a loop tester injects a brief current pulse that can affect variable speed drives, certain RCDs, and medical devices.

Switch off all loads on the circuit before connecting the tester, and confirm the supply voltage is within the expected range before you take any readings.

Keep a colleague informed whenever you are working alone on live equipment, and follow your organisation's safe system of work throughout. Solid preparation at this stage directly determines whether your subsequent Ze and Zs readings are both safe and accurate.

Step 2. Measure external earth fault loop impedance Ze

Ze is the impedance contributed by the supply network alone, measured at the origin of the installation before any internal wiring is included. You take this reading with the main switch open, which disconnects the installation's internal conductors from the measurement and leaves only the network side of the loop in circuit. This is one of the most important steps in learning how to measure earth fault loop impedance accurately, because a high Ze value will raise every circuit's Zs reading across the entire installation.

Isolate and connect at the origin

Open the main switch at the consumer unit or distribution board to disconnect all internal wiring from the test. Your loop impedance tester connects to the line (L), neutral (N), and earth (PE) terminals at the incoming supply tails, just ahead of the main switch. Do not connect downstream of the switch, as any internal impedance will corrupt the Ze reading.

Follow this connection sequence:

1. Confirm the main switch is open and lock it off where possible
2. Connect the earth (PE) lead of your tester to the main earthing terminal
3. Connect the line (L) lead to the incoming line terminal
4. Connect the neutral (N) lead to the incoming neutral terminal
5. Activate the tester and wait for a stable reading to appear

Never take a Ze reading without first confirming the main switch is fully open, as any parallel path through the installation will produce a false, lower-than-actual result.

Record and evaluate the Ze result

Write down the Ze value in ohms immediately after the tester stabilises. If your reading exceeds 0.8 ohms, flag it as a concern before you proceed, since this will significantly constrain the Zs values achievable for individual circuits throughout the installation.

Step 3. Measure and calculate circuit earth fault loop impedance Zs

With Ze recorded, you now move to measuring Zs at each circuit's furthest point. This is where the internal wiring impedance joins the external loop to give you the total earth fault loop impedance for that circuit. You close the main switch, restore supply to the installation, and work through each circuit individually with the tester connected at the furthest socket, light fitting, or accessory from the consumer unit.

Connect at the furthest point of each circuit

Understanding how to measure earth fault loop impedance at circuit level means physically reaching the most remote point on the circuit, since that location carries the highest combined resistance of the phase and protective conductors. Connect your loop impedance tester between the line (L) and earth (PE) terminals at that point, with all loads switched off but the circuit energised.

Follow this sequence for each circuit:

1. Turn on the circuit's protective device at the consumer unit
2. Navigate to the furthest accessory on that circuit (final socket, light rose, or spur)
3. Connect tester leads to L and PE terminals
4. Activate the tester and record the stable Zs reading in ohms
5. Move to the next circuit and repeat

Never test Zs at an intermediate point and assume it represents the worst case for the circuit; always measure at the furthest point to capture the full conductor impedance.

Calculate Zs and verify with the formula

Your tester displays the Zs value directly, but you can also calculate it manually as a cross-check using the measured Ze and the known resistance of the phase conductor and CPC combined (R1 + R2):

Zs = Ze + (R1 + R2)

For example, if Ze is 0.25 ohms and R1 + R2 for the circuit measures 0.42 ohms, your calculated Zs is 0.67 ohms. Compare this against the directly measured figure. A close match confirms both your Ze measurement and your continuity test results are reliable and consistent with each other.

Step 4. Check results against BS 7671 and record them

Once you have your Zs readings for every circuit, the final step in knowing how to measure earth fault loop impedance correctly is verifying each value against the maximum permitted figures in BS 7671 and logging everything clearly. A measurement only becomes useful when it is checked against a standard and documented in a form that supports sign-off and future inspections.

Compare each Zs reading to BS 7671 maximum values

BS 7671 Table 41.2 sets the maximum permissible Zs for each type and rating of overcurrent protective device. Your measured Zs must sit at or below the relevant figure. In practice, many engineers apply a multiplier of 0.8 to the tabulated maximum to allow for the fact that conductor resistance increases with temperature during a fault condition and that measured values are taken at ambient temperature.

The table below shows common examples from BS 7671 for Type B circuit breakers to BS EN 60898:

If any measured Zs exceeds the 0.8 working limit for its protective device, that circuit fails the test and requires remedial work before it can be certified.

Record your measurements in a clear test schedule

Your test record sheet must capture the Ze value at the origin, the Zs reading at the furthest point of each circuit, the type and rating of the protective device, the BS 7671 maximum Zs for that device, and the pass or fail outcome. Use the following template row as a minimum for each circuit:

Accurate, complete records protect you professionally and give the property owner clear evidence of compliance at the time of the inspection.

Next steps after your loop tests

Completing your loop impedance measurements is not the end of the job. If every circuit passes against the BS 7671 limits, compile your test schedule into the relevant section of the EICR or inspection certificate and retain a copy for your records. The property owner needs a signed, dated document that clearly shows every Ze and Zs reading alongside the pass or fail outcome for each circuit.

If any circuit fails, raise it as a code C2 or C1 observation depending on the level of risk, and carry out or recommend the necessary remedial work before the installation can be certified. Once remediation is complete, re-test the affected circuits and confirm the corrected Zs values fall within the permitted limits before updating your records.

Knowing how to measure earth fault loop impedance correctly keeps installations safe and legally compliant. If you need professional loop testing or a full EICR in London, request a quote from Electrical Testing London today.