Question 1

What is earth fault loop impedance (Zs)?

Accepted Answer

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.

Question 2

What are the maximum permitted Zs values?

Accepted Answer

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 1.37 ohms for 0.4 second disconnection. This 1.37 ohm figure reflects the Cmin = 0.95 voltage factor, which was introduced by the 17th Edition Amendment 3 (effective 1 July 2015) and reduced the previous value of 1.44 ohms; the maximum Zs tables have been unchanged from the 17th Edition through the 18th Edition and its amendments A2:2022 and A4:2026. Final circuits up to 63A supplying socket outlets require 0.4 second disconnection; distribution circuits may use 5 second values.

Question 3

Why do I need to correct Zs for temperature?

Accepted Answer

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 rises with temperature, so the measured value must be corrected upwards. This calculator uses the BS 7671 On-Site Guide convention: multiply R1+R2 by (230 + operating temp) / (230 + ambient temp). For a 70 degrees Celsius thermoplastic conductor measured at 20 degrees Celsius this gives a factor of 1.20 (90 degrees Celsius thermosetting/XLPE gives 1.28). This corrects the measured R1+R2 up to operating temperature so the resulting Zs can be compared directly against the BS 7671 Tables 41.2 and 41.3 limits. Note this is a different correction from the on-site 0.8 rule (BS 7671 Appendix 14): there, instead of correcting the measurement, you compare a cold measured Zs against 80 percent of the tabulated limit, and Table B8 of the On-Site Guide / Table A6 of Guidance Note 3 then applies a small ambient factor (about 1.06 at 20 degrees Celsius) to that maximum measured value. The two methods are alternatives for verification but the factors are not interchangeable.

Question 4

What is the difference between Ze and Zs?

Accepted Answer

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).

Question 5

What happens if Zs is too high?

Accepted Answer

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

How We Calculate This

The calculation

Disconnection times (TN systems, BS 7671 Regulation 411.3.2.2)

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