Insulation Resistance and Loop Impedance Testing for Installation Commissioning
Insulation resistance and loop impedance measurements are the two most critical commissioning tests for any new electrical installation. Together they prove that the cable insulation is sound and that protection devices will operate fast enough to prevent electric shock injury under earth fault conditions.
Purpose and applicable standards
BS 7671 Section 643 and IEC 60364-6 clause 6.3 mandate both insulation resistance and loop impedance testing as part of initial verification for every new electrical installation. These tests must be completed before the installation is energised for normal use and before any Electrical Installation Certificate is issued.
| Test | Standard clause | Purpose | Instrument |
|---|---|---|---|
| Insulation resistance (IR) | BS 7671 S643 / IEC 60364-6 cl.6.3.3 | Verify cable insulation integrity — no breakdown or moisture ingress | Megger MIT400, Fluke 1587FC |
| Earth continuity | BS 7671 S643 / IEC 60364-6 cl.6.3.2 | Verify PE conductor connected throughout — no open circuits | Low-resistance ohmmeter (mΩ) |
| Loop impedance (Zs) | BS 7671 S643 / IEC 60364-6 cl.6.3.4 | Verify fault clearance time within 0.4s — MCB will trip fast enough | Fluke 1662, Megger MFT1741 |
| Prospective fault current (PSCC) | BS 7671 S434 / IEC 60364-4-43 | Verify SCPD breaking capacity ≥ available fault current at incomer | Fluke 1662 PFC function |
Insulation resistance test instrument and minimum values
The insulation resistance (IR) test applies a high DC voltage to the cable and measures the resulting leakage current through the insulation. Good insulation allows only an extremely small current — measured in megaohms of resistance. Damaged, wet, or contaminated insulation shows lower resistance.
IR test voltages and minimum values (BS 7671 Table 64)
Circuit nominal voltage Test voltage (DC) Min IR (MΩ) SELV / PELV (≤ 50V AC) 250V DC ≥ 0.5 MΩ 230/400V (standard) 500V DC ≥ 1 MΩ Above 400V 1000V DC ≥ 1 MΩ Test duration: 1 minute (steady-state reading) The reading should stabilise — a rising reading over 60 seconds indicates good insulation (polarisation effect). A falling or fluctuating reading indicates moisture or fault. Practical interpretation: New installation, good cable: ≥ 100 MΩ (often 1000+ MΩ) Acceptable minimum (BS 7671): ≥ 1 MΩ Investigate if: < 10 MΩ — possible cable damage or damp Fail (mandatory investigation): < 1 MΩ Temperature effect: IR halves for every 10°C rise in cable temperature. Test at ambient temperature and record temperature — adjust if comparison needed with future tests.
IR test procedure for KNX panel circuits
KNX panel circuits include both standard 230V final circuits and low-voltage KNX bus circuits. Each requires different test voltage and different preparation. Applying the wrong test voltage to KNX devices will cause permanent damage.
Step-by-step IR test for 230V circuits in KNX panels
Step 1: Open all MCBs and RCBOs in the panel
Step 2: Remove all plugs from socket outlets
Step 3: Disconnect all sensitive electronic loads:
- KNX PS640 power supply (remove from DIN rail or
disconnect bus cable connections)
- All KNX actuators with 230V outputs
(disconnect load cables, not KNX bus connections)
- LED drivers, DALI power supplies
- Any device with semiconductor input filtering
- Variable speed drives, UPS modules
Step 4: Short L and N together at the origin
(use a shorting test lead with crocodile clips)
Step 5: Connect Megger MIT400 or Fluke 1587FC:
Test lead A → L+N shorted terminal
Test lead B → PE (earth) terminal
Step 6: Set instrument to 500V DC
Step 7: Apply test for 60 seconds — read IR at 60s
Step 8: Record IR (MΩ), date, time, ambient temperature
Step 9: Discharge cable (instrument auto-discharges) before
touching conductors — allow 1 second per 1 MΩ of IR
Fault isolation if IR < 1 MΩ:
Disconnect each cable at panel end, retest each section
Progressively isolate until faulty section identified
Inspect for: damaged cable sheath, wet conduit, crushed cableNever apply 500V DC to KNX bus cables or devices: KNX TP bus cable is rated to 120V DC maximum. KNX devices have input capacitors and transient suppressors that will be destroyed by 500V DC. Always disconnect and isolate all KNX equipment before applying 500V IR test voltage, and use the separate 100V DC procedure for the KNX bus cable itself.
IR testing of KNX TP bus cable
The KNX TP bus cable (YCYM 2×2×0.8mm²) is SELV-rated and requires a lower test voltage than mains wiring. The bus cable insulation must still be verified — particularly on long cable runs where mechanical damage from other trades is possible.
KNX TP bus cable IR test procedure
KNX cable specification: YCYM 2×2×0.8mm²
Insulation rated to: 50V AC, 120V DC
Test voltage: 100V DC (do NOT use 250V or 500V)
Disconnect ALL KNX devices before testing:
Unplug every KNX device from bus terminals
Remove KNX PS640 or disconnect bus terminals
Bus cable under test should be fully isolated at both ends
Test points (three measurements per cable run):
1. KNX+ to KNX− (pair-to-pair)
2. KNX+ to screen/drain wire
3. KNX− to screen/drain wire
Minimum IR: ≥ 1 MΩ for cable run up to 100m
(shorter runs should show proportionally higher IR)
Fail indicators — possible causes:
< 1 MΩ between conductors:
Moisture in a termination sleeve or junction box
Pinched or crushed cable jacket in cable tray
Incorrect cable (non-screened) used on part of run
< 0.5 MΩ to screen:
Screen grounded at multiple points (ground loop)
Screen insulation damaged, cable sheath cut
KNX screen grounding rule (for EMC):
Ground screen at ONE point only (typically at panel)
Other end: screen floating or connected via 100nF capacitor
Verify screen grounding before IR testing — a shorted
screen-to-ground at both ends appears as a faultEarth continuity testing
Earth continuity testing verifies that the protective earth (PE) conductor provides a continuous low-resistance path from every exposed conductive part back to the main earth bar. A broken or high-resistance PE conductor leaves exposed metalwork energised during a fault until the overcurrent device eventually trips.
Earth continuity test procedure
Instrument: low-resistance ohmmeter Examples: Megger DLRO10, Ductor tester, or the continuity function of Fluke 1662 / Megger MFT1741 (limited to mΩ) Test current: ≥ 200mA (to overcome surface oxide films) Test points: Each metal enclosure → main earth bar (MEB) Each cable gland (metal) → MEB Each exposed conductive part → MEB Each DIN rail → MEB (via earthing clip) Maximum resistance (BS 7671 and IEC 60364): Main protective bonding conductors: ≤ 1 Ω Final circuit PE conductors (to furthest point): ≤ 1 Ω KNX DIN rail enclosures — specific check: DIN rail must be PE bonded (earthing clip or copper braid) Test: DIN rail surface → PE terminal block Target: < 0.5 Ω Many DIN rail mounting problems are found here — springs and paint prevent good contact without dedicated earthing clips Test result format: Location → MEB resistance (Ω) → PASS/FAIL
Loop impedance test (Zs)
Earth fault loop impedance (Zs) is the total impedance of the fault current path: source → line conductor → fault point → PE conductor → source. Low Zs enables the overcurrent device to trip within the 0.4 second limit required for final circuits serving socket outlets (TN systems).
Maximum Zs values for MCB disconnection (BS 7671 Table 41.1)
MCB type / rating Max Zs (Ω) for 0.4s disconnection Type B, 6A 9.58 Ω Type B, 10A 5.74 Ω Type B, 16A 2.87 Ω Type B, 20A 2.30 Ω Type B, 32A 1.44 Ω Type C, 6A 4.79 Ω Type C, 10A 2.87 Ω Type C, 16A 1.44 Ω Type C, 20A 1.15 Ω Type C, 32A 0.72 Ω Type D, 16A 0.72 Ω Type D, 32A 0.36 Ω Test procedure: Use no-trip (LoΩ) mode — 15ms current pulse, avoids tripping RCDs during measurement Instruments: Fluke 1662, Megger MFT1741 in LoΩ mode Measure at furthest point of each circuit (socket, actuator) Record measured Zs × 1.20 temperature correction factor Compare corrected Zs against table above If Zs > maximum permitted: Increase PE conductor cross-section (reduce resistance) Add supplementary bonding at the load end Reduce circuit length (add sub-distribution board closer)
Loop impedance for KNX actuator circuits
KNX switch actuators (such as MDT AKD-0824V 8×16A or Schneider MTN6730-0001) act as intermediate connection points between the panel MCB and the final loads. Loop impedance must be measured at the actuator output terminals — not just at the panel — since this is where the 230V load connects and where a fault is most likely.
Measuring Zs at actuator outputs
For multi-channel switch actuators in sub-distribution boxes remote from the main panel: test Zs at each 230V output terminal of the actuator. This includes the cable resistance from panel to actuator plus actuator terminal resistance.
For long cable runs (over 50m from panel to actuator): calculate expected Zs before site test. If calculated Zs approaches the maximum limit, upsize PE conductor to 2.5mm² or 4mm² regardless of MCB size.
Actuator PE connection verification
Some KNX actuators have a metal housing that must be PE bonded via the DIN rail earthing clip. Verify: actuator housing → DIN rail → panel PE bar, resistance less than 1Ω.
Also verify: PE terminal block at actuator output (load wiring) is connected to the panel PE bar, not floating. A common wiring error is connecting PE only at the panel and not running a green-yellow PE conductor to the actuator output terminal block.
Prospective fault current at incomer
The prospective short-circuit current (PSCC) at the panel incomer determines what breaking capacity the incomer SCPD must have. This must be measured on site — network PSCC varies by location and network topology, and cannot be assumed from nominal supply data alone.
PSCC measurement at panel incomer
Instrument: Fluke 1662 (PFC function), Megger MFT1741 (PFC)
Measures prospective fault current using loop impedance
method — safe, non-destructive
Connection: at panel incomer terminals (before MCB)
Requires supply to be live — coordinate with client
Use appropriate PPE: insulated gloves, face shield
Measurements required:
Line-to-neutral PSCC (L-N): e.g. 3.2 kA
Line-to-line PSCC (L-L): e.g. 5.5 kA
Use the HIGHER value (L-L is typically higher) as worst case
Actions based on result:
PSCC ≤ MCB breaking capacity → no action required
PSCC > MCB breaking capacity → options:
Upgrade MCB to higher breaking capacity (B-class → H-class)
Install HRC fuse upstream to limit fault energy
Install MCCB with adequate breaking capacity (10–50 kA)
Record on Schedule of Test Results:
Measured PSCC (kA) at incomer
SCPD type and rated breaking capacity
Result: SCPD adequate PASS / FAILSchedule of Test Results documentation
The Schedule of Test Results is the legal document that records all commissioning test results. It forms part of the Electrical Installation Certificate (EIC) for new installations or the Electrical Installation Condition Report (EICR) for periodic inspection. Both BS 7671 and IEC 60364-6 Annex C define the required format.
Schedule of Test Results — required fields
Distribution board section: Board reference, location, supply voltage/frequency Earthing arrangement: TN-C-S / TN-S / TT / IT PSCC at origin (kA) Earthing conductor size (mm²) Main bonding conductor sizes (mm²) Per circuit (one row per circuit): Circuit reference and description (e.g. "L1 — KNX Lighting West") Number of points (sockets, luminaires, actuator outputs) Circuit type: radial / ring / spur Conductor cross-section: line / neutral / PE (mm²) Max Zs permitted (Ω) — from Table 41.1 Earth continuity resistance (Ω) Insulation resistance (MΩ) between: L+N → PE Polarity: correct (C) / incorrect (I) Measured Zs (Ω) at furthest point RCD rated residual current I△n (mA) RCD measured trip time at I△n (ms) MCB / RCBO rating (A) and type (B/C/D) Overall result: PASS / FAIL Signatures required: Inspection and testing engineer (name, signature, date) Responsible person (company, registration number) Retention: minimum 10 years commercial, life-of-installation residential Copies to: building owner, local authority (if required), insurer
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