8.1 Ze & Zs Measurement

The Earth Fault Loop Path

Every earth fault in a TN system closes a loop back to the source. To understand earth fault loop impedance you must first trace that loop. For a phase-to-earth fault on a final circuit, the fault current travels:

1. From the distribution transformer secondary winding
2. Along the phase conductor (L) from the origin to the fault point
3. Through the fault from the live conductor to the circuit protective conductor (CPC)
4. Back along the CPC (R2) to the main earthing terminal (MET)
5. From the MET through the main earthing conductor and the supply earthing path back to the transformer neutral/earthing point
6. Through the transformer winding to complete the loop

The total impedance of that loop is Zs. It has two parts: the external part (transformer winding + supply phase + supply earth return) which belongs to the distribution network operator (DNO), and the internal part (the installation's phase conductor R1 + CPC R2). The core relationship is:

Zs = Ze + (R1 + R2)

where Ze is the external earth fault loop impedance and (R1 + R2) is the sum of the phase conductor resistance and CPC resistance from the origin to the point of fault.

flowchart LR
    A[Transformer winding] --> B[Supply phase conductor]
    B --> C[Installation phase R1 to fault]
    C --> D[Fault L to CPC]
    D --> E[CPC R2 back to MET]
    E --> F[Main earthing conductor]
    F --> G[Supply earth path]
    G --> A

The loop must be low enough that the fault current is large enough to operate the protective device (fuse, MCB or RCD) within the prescribed disconnection time. If the loop impedance is too high, the fault current is too small, the device does not operate in time, and dangerous touch voltage persists on exposed-conductive-parts.

Ze — External Earth Fault Loop Impedance

Ze is measured at the origin of the installation (usually at the incoming supply position / consumer unit). It is a property of the supply, not of the installation — the installer cannot change it. Typical values are around 0.35 Ω for TN-C-S (PME) and up to 0.8 Ω for TN-S, but you must always measure, never assume.

GN3 preferred method (disconnected main earthing conductor)

The GN3 10th Edition (2026) preferred method is:

1. Isolate the installation (main switch off, locked off if necessary).
2. Disconnect the main earthing conductor from the MET. This is the critical step: with the earth conductor disconnected, the installation's R1+R2 cannot influence the reading, so the tester sees only the supply-side loop.
3. Connect a loop impedance tester between the phase conductor (at the incoming side, which remains live) and the disconnected main earthing conductor (or the MET earth bar that is now isolated from the installation earthing).
4. Record the reading — this is Ze.
5. Reconnect the main earthing conductor before re-energising the installation.

Safety warning: during this test the supply upstream of the main switch is live (you are measuring the external loop, which requires the DNO supply to be on). The installation downstream is isolated. Follow your safe isolation procedure and use properly rated test leads. Never leave the main earthing conductor disconnected once the test is complete.

No-trip (3-lead) method

Some modern testers offer a no-trip loop test that can measure Ze without disconnecting the main earthing conductor by using a 3-lead method that filters out RCD nuisance tripping. GN3 recognises this but stresses that the disconnected method gives the truest external value. If you use a no-trip method, be aware the reading may include a small installation contribution.

Zs — Total Earth Fault Loop Impedance

Zs is the loop impedance at a point on a circuit — the value that determines whether the protective device will disconnect in the required time. It is measured per circuit, at the furthest point of each circuit.

Why the furthest point?

Resistance increases with conductor length. The furthest accessory (last socket, end of the radial, last luminaire) has the longest run of both phase and CPC back to the origin, so it has the highest R1 + R2, and therefore the highest Zs. If Zs is satisfactory at the furthest point, it is satisfactory everywhere on that circuit. Testing closer in would give a falsely optimistic value.

Two ways to obtain Zs

Method 1 — Direct live measurement. With the circuit energised, place the loop tester between phase and earth at the furthest accessory. The instrument injects a test fault and measures the loop directly. This is a live test and carries arc-flash and shock risk.

Method 2 — Calculated (Ze + R1 + R2). Take Ze from the origin test, add the (R1 + R2) value obtained from the dead continuity test (sections 6.1 and 6.2). The sum is Zs. This avoids live work and is often the preferred method on site because the R1+R2 is already recorded from the continuity test.

GN3 note: both methods are valid. The calculated method is widely used because it removes the risk of live testing at the furthest point and reuses the continuity result. The direct method is useful for verification where R1+R2 is not available.

Recording Ze and Zs

• Ze is a supply characteristic — record it once on the schedule of test results (it applies to the whole installation fed from that origin). If the installation has multiple supplies, record Ze for each.
• Zs is recorded per circuit — each row of the schedule of test results has a Zs column. The value entered is either the measured-live value at the furthest point or the calculated Ze + (R1 + R2), clearly identified.

Checklist — measuring Ze and Zs

Chapter 8.1 summary

• Zs = Ze + (R1 + R2) — learn this relationship; it underpins every disconnection check.
• Ze is the DNO's external loop impedance, measured at the origin with the main earthing conductor disconnected.
• Zs is the total loop at a point, measured (or calculated) at the furthest point of each circuit.
• The calculated method (Ze + R1 + R2 from dead continuity) avoids live work and is often preferred.
• Record Ze once; record Zs per circuit.