Why Is My Zs Reading Too High? Causes and What to Check

Why Is My Zs Reading Too High? Causes and What to Check

A high Zs reading tells you the earth-fault loop impedance is higher than expected. It does not tell you why.

Before chasing a fault, be clear what “too high” means. Is the result higher than you expected, different from a previous reading, or above the correct comparison value for the circuit? Those are different problems.

The useful starting point is to check the comparison basis, then separate the external part of the loop from the circuit contribution. That gives you a direction to investigate without pretending one reading proves a cause.

First check what “too high” means

Zs is commonly understood as the external loop impedance plus the line and circuit protective conductor contribution:

Zs = Ze + (R1 + R2)

A result can look high because the wrong maximum value or test basis has been used. The relevant comparison depends on the protective device, its rating, the earthing arrangement and the required disconnection time. Measured and calculated values can also differ because the methods and test conditions are not identical.

So first pin down what you are comparing: the measured value, a calculated value, or a result against the permitted Zs for that circuit. For the device-specific comparison, use the maximum Zs reference or Max Zs calculator. Neither replaces checking the installation and the applicable current guidance.

Split the loop: Ze and R1+R2

Once the comparison basis is clear, look at the components rather than treating Zs as one unexplained number.

• Ze is the external part of the loop, measured at the origin.
• R1+R2 is the line and circuit protective conductor contribution for the circuit.

A high Ze can contribute to high Zs across multiple circuits. A high circuit contribution may instead point you toward the particular circuit. This is a reasoning aid, not proof: readings, test points and installation conditions need to support the conclusion.

If you need the wider explanation of how the measured and calculated routes relate, see Earth Loop Impedance Test Values: What to Compare and Why. For the TN-S supply-side reference specifically, see Maximum Ze for TN-S.

If the result is high across several circuits

When several circuits show a similar unexpected pattern, compare their results with the Ze measurement and the relevant records. If the external reading itself appears unexpectedly high, check that the earthing arrangement and measurement basis have been identified correctly.

Do not jump straight to “the DNO supply is faulty”. A previous certificate, one measurement or a set of similar circuit readings is not enough on its own to establish the cause. Verify the result using the applicable procedure and your instrument guidance. If an external supply concern remains, record the evidence and raise it with the relevant network operator. Do not interfere with sealed equipment.

If one circuit is the outlier

If one circuit stands apart while the origin and other circuit readings are consistent, focus the investigation on what is specific to that circuit. Compare the measured Zs with its calculated value and R1+R2 result, where those are available and valid for the method used.

A mismatch can help narrow the next check, but it does not identify a loose connection, damaged conductor or other defect by itself. Treat it as a clue. Further investigation needs the right test method, safe isolation and competent judgement for the installation.

Check the measurement before diagnosing the installation

Before deciding that the installation has a fault, rule out a result that has been misunderstood or poorly established. Check that:

• the correct earthing arrangement and measurement point are being used;
• the instrument, leads, settings and connections are suitable and in serviceable condition;
• the same measurement method is being used when comparing readings;
• the result is considered alongside Ze, R1+R2 and any relevant earlier records.

A live loop test and a calculated result from component measurements do not necessarily answer the same question. If they disagree, keep the difference visible and investigate it using the appropriate procedure.

Testing and fault-finding must be carried out by a competent person using the appropriate safe-isolation and test procedures. This article is a reasoning aid, not a step-by-step live-work method.

What to record and when to escalate

Keep enough detail for another electrician to understand how you reached the result:

• the measured Zs and where it was taken;
• the earthing arrangement and protective device;
• the test method and relevant Ze and R1+R2 readings;
• the comparison basis used;
• any limitation, uncertainty or follow-up action.

If the result remains unexplained, affects the assessment of the circuit or suggests an external supply issue, do not bury it in a single number. Record what you know, what remains uncertain and who needs to investigate next. A high reading is a reason to check the evidence, not a diagnosis by itself.

The short version

Check the comparison basis first. Then separate the external contribution from the circuit contribution, verify the test method and record the evidence. Only then decide what needs further investigation.

Use the Max Zs calculator for the lookup step. Use the measured results and installation-specific evidence for the judgement.

Sources and further reading

• IET Wiring Matters, Why are the values of maximum earth fault loop impedance different? — design and measured-value context.
• IET Wiring Matters, Determining the maximum earth fault loop impedance for protective devices — protective-device and disconnection-time basis.