What is earth fault loop impedance (Zs)?

Zs is the total impedance of the earth fault loop path. It consists of Ze (the external earth fault loop impedance from the supply transformer to your installation) plus R1+R2 (the circuit conductor impedances — R1 being the line conductor and R2 the circuit protective conductor). Zs must be low enough to allow sufficient fault current to flow to operate the protective device within the required disconnection time.

What are the maximum permitted Zs values?

Maximum Zs values are specified in BS 7671 Tables 41.2, 41.3 and 41.4. They depend on the protective device type and rating. For example, a 32A Type B MCB has a maximum Zs of 1.44 ohms for 0.4 second disconnection. Final circuits supplying socket outlets and portable equipment require 0.4 second disconnection; distribution circuits may use 5 second values.

Why do I need to correct Zs for temperature?

When you measure R1+R2 with a low-resistance ohmmeter, the conductors are at ambient temperature (typically 20 degrees Celsius). Under fault conditions, the conductors heat up to their maximum operating temperature (typically 70 degrees Celsius for thermoplastic insulation). Conductor resistance increases with temperature, so the measured value must be corrected upwards using the factor (230 + operating temp) / (230 + ambient temp) to give the worst-case Zs.

What is the difference between Ze and Zs?

Ze is the external earth fault loop impedance measured at the origin of the installation (before any of your circuits). It depends on your supply type: TN-S typically gives Ze around 0.35 to 0.8 ohms, TN-C-S (PME) around 0.2 to 0.35 ohms, and TT systems may be much higher. Zs is the total loop impedance at any point in the circuit and equals Ze plus the impedances of the line and earth conductors of the circuit (R1+R2).

What happens if Zs is too high?

If Zs exceeds the maximum permitted value, the protective device may not operate quickly enough to disconnect the supply during an earth fault. This creates a risk of electric shock. Options to remedy this include: using a larger cable cross-section to reduce R1+R2, shortening the circuit length, installing an RCD (which can protect circuits with higher Zs values), or changing to a protective device with a higher maximum permitted Zs.

Earth Fault Loop Impedance Calculator — BS 7671

Calculate total earth fault loop impedance (Zs) from Ze and R1+R2 measurements. Check compliance against BS 7671 18th Edition maximum values for automatic disconnection of supply.

External Loop Impedance (Ze)

Measured at the origin of the installation

R1+R2 (Measured)

Combined line and CPC resistance

Circuit Type

Select the circuit and its protective device rating

Protective Device Type

Type of overcurrent protective device

Earthing System

Supply earthing arrangement

How We Calculate This

This calculator determines whether the earth fault loop impedance at a point in a circuit is low enough for the protective device to disconnect within the required time specified in BS 7671 18th Edition.

The calculation

Step 1: Total earth fault loop impedance is calculated as Zs = Ze + (R1+R2), where Ze is the external earth fault loop impedance and R1+R2 is the combined resistance of the line conductor (R1) and circuit protective conductor (R2).

Step 2: The measured R1+R2 value is corrected for conductor operating temperature using the correction factor (230 + operating temp) / (230 + ambient temp). For thermoplastic cables at 70 degrees Celsius measured at 20 degrees Celsius, the factor is 1.20.

Step 3: The corrected Zs is compared against the maximum permitted value from BS 7671 Tables 41.2, 41.3 or 41.4 for the specific protective device type and rating.

Disconnection times

0.4 seconds: Required for final circuits up to 32A supplying socket outlets or portable equipment
5 seconds: Permitted for distribution circuits and fixed equipment circuits above 32A