Earth fault loop impedance is one of the most important verification checks in any BS 7671 installation. If the impedance of the fault loop is too high, a protective device will not operate within the required disconnection time during a line-to-earth fault — leaving exposed metalwork at a dangerous voltage for long enough to cause a fatal electric shock. Understanding Zs, how to calculate it, and what the limits are is essential for any electrician working to the 18th Edition.
What earth fault loop impedance actually is
When a line conductor comes into contact with exposed metalwork (a fault to earth), fault current flows in a loop from the source, through the line conductor, through the fault, back through the protective conductor (CPC), through the main earthing terminal, and back to the source via the earth return path.
The total impedance of this loop is called the earth fault loop impedance (Zs). It has two components:
- Ze— the external impedance: the part of the loop outside the installation, from the source to the installation's main earthing terminal. This includes the supply transformer winding, the supply cables, and the earth return path back to the transformer neutral.
- (R1 + R2)— the internal impedance: the resistance of the line conductor (R1) from the distribution board to the fault point, plus the resistance of the CPC (R2) from the fault point back to the main earthing terminal.
The formula is: Zs = Ze + (R1 + R2)
Why Zs must not exceed the maximum permitted value
A protective device (MCB, fuse, or RCD) must disconnect a circuit within a defined time when a fault occurs. That disconnection time depends on the fault current — and fault current depends on Zs via Ohm's law: If = Uo / Zs, where Uo is the nominal line-to-earth voltage (230 V in the UK).
If Zs is high, fault current is low. Low fault current means the protective device trips slowly — or may not trip at all within the required time. BS 7671 specifies maximum disconnection times of 0.4 seconds for final circuits up to 32 A in TN systems, and 5 seconds for distribution circuits and final circuits rated above 32 A. Each maximum disconnection time corresponds to a minimum fault current, which in turn corresponds to a maximum Zs.
Maximum Zs values from BS 7671 Appendix 3
BS 7671 Appendix 3 provides maximum Zs tables for common protective devices at their rated current. These are the values measured at the far end of the circuit (at the point of use) at the reference temperature of 70°C for standard thermoplastic cables. Some commonly used values for type B MCBs:
- 6 A type B MCB: max Zs = 7.67 Ω
- 10 A type B MCB: max Zs = 4.60 Ω
- 16 A type B MCB: max Zs = 2.87 Ω
- 20 A type B MCB: max Zs = 2.30 Ω
- 32 A type B MCB: max Zs = 1.44 Ω
Type C MCBs have a higher tripping threshold and therefore require lower Zs values (roughly half those of type B). Type D MCBs require lower still. Always use the correct Appendix 3 table for your specific device type and rating.
The temperature correction factor
BS 7671 Appendix 3 values are given at 70°C conductor temperature — maximum operating temperature. But you measure Zs on site at ambient temperature (typically 10–20°C). Cold conductors have lower resistance than hot ones.
To confirm compliance when measuring at ambient temperature, apply the correction factor: multiply the measured value by 1.24(the standard factor for copper conductors between approximately 15°C and 70°C). If the corrected value is below the Appendix 3 maximum, the circuit complies.
Alternatively, some testers apply this correction automatically. If your instrument does not, always multiply your reading by 1.24 before comparing to the table.
How to measure Zs on site
Zs is measured using a loop impedance tester connected between the line and earth conductors at the furthest point of the circuit (the last socket, light fitting, or piece of equipment on that run).
The test momentarily applies a low-impedance load to the circuit, measures the resulting voltage drop, and calculates impedance from the known test current. This requires the circuit to be live.
A no-trip (high-current) test gives the most accurate reading, but will trip RCDs rated at 30 mA or below. A low-current (non-trip) test avoids tripping RCDs but is slightly less accurate. For circuits protected by an RCD, use the non-trip method or temporarily bypass the RCD (following a safe system of work).
Calculating Zs without measuring: Ze + (R1 + R2)
Where direct measurement is not possible (during design, or before the supply is connected), Zs can be calculated:
- Obtain Ze from the DNO (Distribution Network Operator) or measure it at the incoming terminals. Typical TN-S Ze values are 0.35 Ω or below; TN-C-S (PME) systems are typically 0.35 Ω or below at the point of connection.
- Calculate R1 + R2 from the cable length and cross-section. Use the resistance values from BS 7671 Appendix 9 (or cable manufacturer data): R1 + R2 = (cable length × resistance per metre for line conductor + resistance per metre for CPC).
- Add: Zs = Ze + (R1 + R2). Apply the 1.24 temperature factor if calculating at ambient.
Use the Voltix cable sizing calculator to run R1 + R2 calculations automatically for any cable size and run length, with Zs verification built in.
TT systems: different rules apply
In a TT system (where the installation earth electrode is independent of the supply earth), Zs is typically very high — often tens or hundreds of ohms. Protective devices alone cannot guarantee disconnection within the required time. This is why BS 7671 requires all final circuits in TT systems to be protected by a 30 mA RCD. The RCD operates on earth fault current regardless of the loop impedance, providing the required protection where high Zs would prevent the protective device from tripping in time.
Common reasons for a high Zs reading
- CPC undersized relative to the line conductor, increasing R2
- Long circuit run, particularly on small cross-section cables
- Poor connections or corroded joints along the CPC path
- Broken or missing CPC — test will show a very high (or infinity) reading
- High Ze from a poor DNO supply or long service cable
When Zs exceeds the permitted maximum, the remedies are: increase cable cross-section (reducing R1 + R2), shorten the circuit run, install an RCD (which removes the Zs constraint for shock protection, though the Appendix 3 check may still apply for fault protection), or investigate and repair poor connections.