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Test Results Compliance Checker

Verify test results against BS 7671 limits — Zs, insulation resistance, and RCD trip times

Reference Info & Formulas
Pass Criteria

Zs: Must be ≤80% of tabulated max

IR: ≥0.5MΩ (≥2MΩ recommended)

RCD: ≤300ms at IΔn

80% Rule

Measured Zs values are taken at ambient temperature. At conductor operating temperature, Zs increases.

The 80% rule accounts for this — test values must be ≤80% of the tabulated maximum.

Circuit Test Results
Enter measured values for each circuit

Data: BS 7671:2018+A2:2022 — Tables 41.2–41.4, Table 61

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

This tool compares your measured test results against the pass/fail criteria in BS 7671:2018+A2:2022, applying the correct correction factors and tolerances.

Electrical testing produces three key measurements that determine whether a circuit is safe and compliant: earth fault loop impedance (Zs), insulation resistance (IR), and RCD disconnection time. Each has defined limits in BS 7671, and the checker validates your readings against the appropriate table for the specific protective device and circuit type.

Zs_measured ≤ 0.8 × Zs_max
Zs_measured
= Earth fault loop impedance measured at ambient temperature
Zs_max
= Maximum tabulated Zs from BS 7671 Tables 41.2-41.4
0.8
= Correction factor (80% rule) for conductor temperature rise

IET Guidance Note 3, Table B1

For insulation resistance, the test is performed at the appropriate DC voltage for the circuit (250V, 500V, or 1000V) and the reading must meet or exceed the minimum value from Table 61. RCD tests verify both the trip current threshold and the disconnection time.

Quick Reference — Maximum Zs Values (Type B MCBs)

Maximum Zs and 80% threshold for common Type B MCB ratings

BS 7671 Table 41.3 (0.95 Cmin applied)
MCB Rating (A)Max Zs (Ω)80% Threshold (Ω)
67.676.14
104.603.68
162.872.30
202.301.84
321.441.15
401.150.92
500.920.74

Measured Zs at ambient temperature must not exceed the 80% threshold value.

Quick Reference — Insulation Resistance Minimum Values

Minimum insulation resistance by circuit voltage

BS 7671 Table 61
Circuit Voltage (V)Test Voltage (V DC)Minimum IR (MΩ)
SELV / PELV2500.5
Up to 500V (inc. 230V)5001.0
Above 500V up to 1000V10001.0

These are absolute minimums. Readings below 2MΩ on 230V circuits should be investigated.

Quick Reference — RCD Disconnection Times

Maximum disconnection times for general-type RCDs (BS EN 61008)

BS EN 61008-1, BS 7671 Table 41.5
Test CurrentMax Trip TimeNotes
1× IΔn (e.g. 30mA)300msMaximum at rated residual current
5× IΔn (e.g. 150mA)40msFast disconnection at high fault current
½× IΔn (e.g. 15mA)Must NOT tripVerifies no nuisance tripping

Times shown are for non-delayed (general) type RCDs. Time-delayed (Type S) RCDs have longer permitted disconnection times.

Practical Notes

Never Ignore a Low Insulation Resistance

An insulation resistance below 1.0 MΩ on a 230V circuit indicates degraded insulation or moisture ingress. Common causes include water damage, rodent-chewed cables, heat-damaged insulation near downlights, and aged rubber-insulated wiring. A reading below the Table 61 minimum is a mandatory fail — the circuit must not be energised until the fault is located and rectified.

Disconnect Loads Before IR Testing

Electronic devices, LED drivers, and surge protection devices (SPDs) can give false low insulation resistance readings. Always disconnect or isolate all loads and accessories before testing. If readings improve dramatically with loads disconnected, the wiring is likely healthy — but record the test as performed with loads disconnected and note which items were removed.

Zs Changes with Supply Conditions

External earth fault loop impedance (Ze) varies with the DNO network loading and configuration. A reading taken at 10am on a weekday may differ from one taken at 6pm during peak demand. The 80% rule partially accounts for this variability, but if your reading is borderline, consider retesting at a different time. The DNO's declared maximum Ze should be used for design calculations rather than a single measured value.

Frequently Asked Questions

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