Electrical Testing Sequence
Dead and live test order, instruments, pass/fail limits, and initial verification vs EICR
BS 7671 Chapters 64 & 65, IET Guidance Note 3
Why the Sequence Matters
The BS 7671 testing sequence is not arbitrary — each test validates the prerequisites for the next. Continuity must be proven before insulation resistance, because the IR test voltage needs a confirmed path to reach all parts of the circuit. All dead tests must pass before the installation is energised for live testing.
Dead first, live second. Regulation 641.1 requires this to minimize risk to the tester.
Initial Verification — Full Testing Sequence
| # | Test | Phase | Instrument | Pass Limit |
|---|---|---|---|---|
| 1 | Continuity of Protective Conductors Method 1 (bridge) or Method 2 (wander lead). Null test leads first. Confirms ADS path. | DEAD | Low-resistance ohmmeter (4–24 V, ≥200 mA) | Record R1+R2 or R2 value in Ω |
| 2 | Continuity of Ring Final Circuits Three-step cross-connection test. Highest Step 3 reading = R1+R2 for the ring. | DEAD | Low-resistance ohmmeter | Consistent readings across ring; r1 ≈ rn, rcpc ≈ 1.67 × r1 (2.5/1.5 T&E) |
| 3 | Insulation Resistance Test L–N, L–E, N–E. Disconnect sensitive electronics. Use 250 V DC for IT/SPDs. | DEAD | Insulation resistance tester (500 V DC for ≤500 V circuits) | ≥ 1.0 MΩ (new installs typically >200 MΩ) |
| 4 | Polarity (dead) Confirmed during R1+R2 test if Method 1 used. Check single-pole switches are in line conductor. | DEAD | Low-resistance ohmmeter / continuity tester | Correct polarity at all points |
| 5 | Earth Electrode Resistance (TT only) 61.8% rule for spike placement. Test in dry conditions for worst case. | DEAD | Earth electrode tester (Fall-of-Potential method) | Ra × IΔn ≤ 50 V (typically Ra ≤ 200 Ω for 30 mA RCD) |
| 6 | Supply Polarity & Phase Sequence First live test. Verify supply before energising installation. | LIVE | Approved voltage indicator (GS38) / phase rotation meter | L–N ≈ 230 V, L–E ≈ 230 V, N–E ≈ 0 V; correct L1-L2-L3 rotation |
| 7 | Earth Fault Loop Impedance (Ze) Disconnect main earthing conductor from MET. Measure at origin only. | LIVE | Loop impedance tester (high current) | TN-C-S ≤ 0.35 Ω, TN-S ≤ 0.8 Ω |
| 8 | Earth Fault Loop Impedance (Zs) Or calculate Zs = Ze + (R1+R2). Compare against Tables 41.2–41.4. | LIVE | Loop impedance tester (no-trip for RCD circuits) | Measured Zs ≤ 80% of BS 7671 table value |
| 9 | Prospective Fault Current (PFC) Measured at origin. Highest of PSCC (L–N) and PEFC (L–E) is recorded. | LIVE | Loop impedance / PFC tester | PSCC and PEFC must not exceed device breaking capacity |
| 10 | RCD Testing Test at rated current (IΔn). Also test trip button function. | LIVE | RCD tester | Trip at IΔn within 300 ms (non-delay); 5×IΔn within 40 ms (additional protection) |
| 11 | Functional Testing Includes SPDs, lighting controls, interlocking devices. | LIVE | Visual / manual operation | All switches, isolators, interlocks operate correctly |
Initial Verification vs Periodic Inspection (EICR)
| Feature | Initial Verification | Periodic Inspection (EICR) |
|---|---|---|
| Regulatory basis | Chapter 64, BS 7671 | Chapter 65, BS 7671 |
| Objective | Verify design and installation | Assess condition for continued service |
| Testing sequence | Mandatory, strictly sequential | Flexible, guided by professional judgment |
| Supply during dead tests | Installation de-energised | Often energised; begins with supply checks |
| Documentation | EIC (Electrical Installation Certificate) | EICR (Condition Report) |
| Visual scope | Compliance with current BS 7671 | Deterioration, damage, and changes of use |
Insulation Resistance Limits (Table 6.3)
| Circuit Voltage | Test Voltage | Minimum Resistance |
|---|---|---|
| SELV / PELV | 250 V DC | ≥ 0.5 MΩ |
| Up to 500 V (standard) | 500 V DC | ≥ 1.0 MΩ |
| Over 500 V | 1000 V DC | ≥ 1.0 MΩ |
EICR Classification Codes
Immediate risk of injury. Make safe before leaving site.
Would become dangerous if a subsequent fault occurred. Urgent remedial action required.
Compliant with previous standards but not current BS 7671.
Issue discovered that needs more in-depth exploration beyond the standard inspection scope.
Common Testing Mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| Testing before isolation is proven | Electric shock risk to tester | Always use AVI + proving unit (GS38) |
| Not nulling test leads | Lead resistance added to reading (>0.2 Ω error) | Zero leads before every session |
| Skipping continuity before IR test | IR voltage may not reach all conductors | Follow the sequence — each test validates the next |
| Using 500 V IR on sensitive equipment | Damage to MOVs, SPDs, electronic controls | Reduce to 250 V DC or disconnect equipment |
| Comparing ambient Zs to table max (100%) | False pass — fails at operating temperature | Apply the 80% rule (or calculate temperature-corrected Zs) |
| Incomplete isolation during IR test | Parallel paths give false high readings | Ensure circuit under test is fully isolated |
Safety Reminders
- - Safe isolation before any dead testing — AVI + proving unit, lock off, prove dead
- - Do not proceed to the next test if the current test fails — rectify the fault first
- - Test leads must comply with GS38 — finger guards, fused, 4 mm max exposed tip
- - Instruments must be calibrated to BS EN 61557 and within calibration date
- - In occupied premises, communicate with occupants before isolating circuits
Key Points
- - Dead tests first (1–5), live tests after (6–11) — minimises risk to tester
- - Each test validates prerequisites for the next — do not skip or reorder
- - Initial verification has a mandatory sequence; EICR sequence is flexible
- - 80% of defects can be found during visual inspection alone
- - Compare measured Zs against 80% of BS 7671 table values (temperature correction)
- - Record every circuit — even untested ones (note the limitation and reason)