How to Reduce Zs: Design Changes That Lower the Reading

A high Zs reading is not something to “fix” by changing the test or picking a more convenient limit. If the result has been verified, there are two different questions: can the impedance of the fault loop be reduced, or does the installation need a different, compliant fault-protection design? Those are not the same thing.

First separate the reading from the limit

The earth-fault loop includes the external supply contribution, Ze, and the circuit contribution, R1 + R2. A change to the circuit can affect its contribution, but it cannot correct an unexpectedly high value originating on the supply side. If you need the diagnostic sequence first, see what to check when a Zs reading is too high.

Also confirm the comparison basis before choosing any design response. The applicable maximum depends on the protective arrangement and required disconnection time; measured values and design values are not interchangeable. The maximum Zs reference explains the common comparison traps.

What can actually lower the loop impedance?

Once the source of the value is established, a designer can assess changes to the part of the loop that is contributing to it. For a circuit-side contribution, conductor resistance depends on factors including cable length, conductor characteristics and temperature. IET guidance treats circuit length and cable cross-sectional area as design choices constrained by more than Zs alone: fault disconnection, protection against thermal damage, and voltage drop all have to be considered.

That means a shorter route or a different conductor selection may change the circuit contribution, but neither is a universal remedy. Any alteration has to be designed for the actual installation and checked against all applicable requirements. A lower Zs figure on its own does not prove the circuit is correctly designed.

Temperature matters too. Conductor resistance rises as conductors heat under load, which is one reason the design comparison and a measured value taken at ambient temperature may differ. Use the correct guidance for the device and verification context rather than trying to make the measured number resemble a design figure.

When the supply side is the issue

If the verified concern is an excessively high external loop impedance, do not try to alter sealed supply equipment. Electrical Safety First’s Wiring Regulations guidance says an excessively high external earth-loop impedance value should be brought to the attention of the Distribution Network Operator (DNO). Record the evidence and raise it with the network operator responsible for the supply.

A different protective design is not the same as reducing Zs

In some designs, fault protection may be provided by an RCD or the RCD element of an RCBO, in which case BS 7671 uses a different maximum-Zs table from the one used for overcurrent-device fault protection. Electrical Safety First’s guidance also notes that an overcurrent protective device is still needed for overload and/or short-circuit protection.

This is a design decision, not a shortcut for making a high reading pass. Changing the protective arrangement does not physically reduce the impedance of the existing loop. The designer has to check the complete protective measures, device coordination, disconnection requirements and installation conditions. Do not swap protective devices or add equipment solely to improve a certificate value.

The safe decision path

Verify the reading and compare it with the correct device-specific limit and disconnection-time basis.
Establish whether the significant contribution is on the supply side or within the circuit. See earth-loop impedance test values and comparison methods for the measurement context.
If a circuit design change is being considered, assess Zs alongside current-carrying capacity, thermal protection and voltage drop. Use the current BS 7671 and manufacturer guidance for the installation.
If the external supply value is excessively high, refer it to the DNO rather than interfering with sealed equipment.
Re-verify and record the completed design and test results using the applicable procedure.

There is no generic “make Zs lower” modification that is safe for every installation. The right outcome is a verified installation with the correct protective design, not simply the smallest number on a tester.

Sources and further reading

IET Wiring Matters, Earth fault loop impedance and circuit design constraints.
IET Wiring Matters, Why maximum earth fault loop impedance values differ.
IET Wiring Matters, Determining maximum earth fault loop impedance for protective devices.
Electrical Safety First, Wiring Regulations Help.