Zs Values & Earth Fault Loop Impedance Explained

Earth fault loop impedance is the total impedance of the path that fault current takes when a line conductor comes into contact with earth. This path starts at the supply transformer, travels through the line conductor to the point of the fault, through the fault itself, back through the earth path (the circuit protective conductor and main earthing terminal), and returns to the transformer neutral. The impedance of this complete loop is called Zs — the impedance of the earth fault loop.

Zs determines how much fault current will flow when an earth fault occurs. A lower Zs means higher fault current, which causes the protective device (MCB, fuse, or RCBO) to operate more quickly. A higher Zs means lower fault current, which may result in the protective device taking too long to disconnect — or not disconnecting at all. This is why BS 7671 sets maximum permitted Zs values: to ensure that the protective device disconnects within the required time to prevent electric shock.

For socket outlet circuits and circuits supplying portable equipment, the maximum disconnection time is 0.4 seconds. For distribution circuits (those supplying other distribution boards rather than final circuits), the maximum disconnection time is 5 seconds. These times are specified in BS 7671 Chapter 41.

The earth fault loop impedance is calculated using a straightforward formula:

Zs = Ze + (R1 + R2)

Each component of this formula represents a different part of the fault loop path:

Ze — External Earth Fault Loop Impedance

Ze is the impedance of the earth fault loop path from the supply transformer to the origin of the installation (typically the main switch in the consumer unit). This includes the impedance of the supply transformer winding, the line conductor from the transformer to the property, and the earth return path back to the transformer. Ze is determined by the electricity supply and the earthing system — it is outside the control of the electrician.

R1 — Line Conductor Resistance

R1 is the resistance of the line (phase) conductor from the distribution board to the point of the fault. This depends on the conductor material (copper or aluminium), the cross-sectional area of the cable, and the length of the circuit. Larger cross-section cables and shorter circuit lengths give lower R1 values.

R2 — Circuit Protective Conductor (CPC) Resistance

R2 is the resistance of the circuit protective conductor (earth conductor) from the distribution board to the point of the fault. In twin-and-earth cables, the CPC is often a smaller cross-section than the line conductor, meaning R2 may be higher than R1. For example, in a 2.5mm² twin-and-earth cable, the CPC is typically 1.5mm².

The Ze value depends on the earthing system of the supply. There are three main earthing systems used in the UK, each with characteristic Ze values. These typical values are published by the Distribution Network Operators (DNOs) and are used during the design stage of an installation.

TN-S (Separate Earth — Cable Sheath Return)

In a TN-S system, the earth return path uses the metal sheath (or armour) of the supply cable. The earth and neutral are separate conductors throughout. Typical Ze values are 0.35 ohms or less, with a maximum design value of 0.8 ohms. TN-S supplies are common in older properties with lead-sheathed or steel-wire-armoured supply cables.

TN-C-S (PME — Protective Multiple Earthing)

In a TN-C-S system, the supply neutral also serves as the earth conductor (a combined PEN conductor) within the supply network. At the origin of the installation, the neutral and earth are separated. Because the neutral conductor provides a low-impedance return path, TN-C-S supplies typically have the lowest Ze values: 0.2 ohms or less, with a maximum design value of 0.35 ohms. This is the most common earthing system in modern UK installations.

TT (Earth Rod)

In a TT system, the installation has its own earth electrode (usually a driven earth rod) with no direct metallic connection to the supply earth. The earth return path goes through the general mass of earth, which has relatively high impedance. Ze values on TT systems are typically 20 ohms or more and can be much higher depending on soil conditions. Because of this high impedance, fault currents on TT systems are too low to operate MCBs or fuses within the required time — which is why RCD protection is essential on all circuits in TT installations.

BS 7671 specifies maximum earth fault loop impedance values to ensure that protective devices disconnect within the required time. These values are found in Chapter 41:

• ✓Table 41.2 — Maximum Zs for circuits protected by BS 88 fuses (HRC fuses)
• ✓Table 41.3 — Maximum Zs for circuits protected by BS EN 60898 Type B MCBs
• ✓Table 41.4 — Maximum Zs for circuits protected by BS EN 60898 Type C MCBs
• ✓Table 41.6 — Maximum Zs for circuits protected by BS EN 60898 Type D MCBs

The maximum permitted Zs value varies depending on the type of protective device and its current rating. The principle is straightforward: the protective device must receive enough fault current to trip within 0.4 seconds (for final circuits) or 5 seconds (for distribution circuits). Since fault current equals voltage divided by impedance (I = V / Zs), a lower Zs produces a higher fault current, ensuring faster disconnection.

For example, a Type B MCB requires a fault current of 5 times its rated current to trip instantaneously. A Type C MCB requires 10 times its rated current. This means that for the same rated current, a Type C MCB has a lower maximum permitted Zs than a Type B MCB — it needs more fault current, so the loop impedance must be lower.

When designing circuits, electricians must ensure that the calculated Zs (Ze + R1 + R2) does not exceed the maximum value for the protective device selected. If it does, either the cable size must be increased (reducing R1 + R2), the circuit length must be shortened, or a different type of protective device must be used.

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18th Edition (2382)

Understanding maximum Zs values and disconnection times is fundamental knowledge covered in the 18th Edition course.

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Earth fault loop impedance is measured as part of standard electrical testing procedures using a dedicated loop impedance tester. The instrument is connected at the point of use — typically at a socket outlet or at the terminals of a fixed appliance. It passes a brief test current through the earth fault loop and measures the impedance.

Where to Test

Zs readings should be taken at the furthest point from the distribution board on each circuit. This is where the R1 + R2 component is at its highest, and therefore where Zs will be at its maximum. If the reading passes at the furthest point, it will pass at every other point on the circuit.

Comparing the Reading

The measured Zs value must be compared against the maximum permitted value from the appropriate BS 7671 table for the protective device on that circuit. However, the measured value must first be adjusted using the temperature correction factor (Cr).

Testing is carried out at ambient temperature, but under fault conditions the conductor temperature will be much higher, increasing resistance. The correction factor accounts for this difference. The rule of thumb is that the measured Zs value should not exceed 80% of the maximum tabulated value — this provides a safety margin equivalent to the Cr factor for most common cable types and ambient temperatures.

Alternative Verification Method

Where live loop impedance testing is not practical or not safe, Zs can be determined by calculation: measure Ze at the origin of the installation, measure R1 + R2 for each circuit using a dead test (low-resistance ohmmeter), and add the two values together. This calculated Zs value should then be compared against the corrected maximum tabulated value. This method avoids the need for live testing at every point.

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Loop impedance testing and verification methods are core practical skills assessed in the 2391 qualification.

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If the measured or calculated Zs exceeds the maximum value for the protective device, the circuit does not comply with BS 7671. The protective device may not disconnect quickly enough to prevent electric shock. Several remedial options are available:

Increase the CPC Size

Using a larger cross-section CPC reduces R2, which lowers the overall Zs. For example, upgrading from a 1.5mm² CPC to a 2.5mm² CPC will reduce R2 significantly. In some cases, running a separate CPC alongside the existing cable is practical.

Run a Parallel CPC

Installing a supplementary bonding conductor or parallel CPC between the distribution board and the circuit provides an additional earth path, reducing the effective R2. This is sometimes used where re-cabling the entire circuit is not practical.

Reduce Circuit Length

Both R1 and R2 are proportional to circuit length. If the circuit route can be shortened (for example, by relocating the distribution board or re-routing the cable), the overall Zs will decrease. This is primarily a consideration at the design stage rather than a remedial measure.

Use a Larger Cable

Increasing the cross-sectional area of both the line conductor and CPC reduces R1 + R2. For example, upgrading from 2.5mm² to 4mm² twin-and-earth cable will reduce the combined resistance significantly, especially on longer circuits.

Add RCD Protection

Where reducing Zs is not practical, adding an RCD provides supplementary fault protection. When the Zs values are too high, this may be recorded on an EICR as a deficiency requiring remedial action. An RCD will detect earth fault current as low as 30mA and disconnect within 40ms — far faster than any overcurrent protective device. However, an RCD does not reduce Zs; it provides an additional layer of protection. BS 7671 requires RCD protection in many situations regardless of Zs values.