Insulation resistance testing measures the DC resistance of electrical insulation by applying a high voltage (typically 500 V to 5000 V DC) and measuring the resulting leakage current. Healthy insulation shows resistance in the hundreds of megaohms to gigaohms range, while degraded insulation shows progressively lower values. The test is commonly called a "Megger test" after the Megger brand that popularised the insulation resistance tester, though today several manufacturers produce equivalent instruments. In Singapore, insulation resistance testing is required under SS 638:2018 for new electrical installation commissioning, and under MOM Workplace Safety and Health (Electricity) Regulations for maintaining electrical plant safety.
Why Insulation Resistance Testing Matters
Electrical insulation degrades over time from heat, moisture, vibration, contamination and ageing, increasing leakage current, wasting energy, causing nuisance RCD tripping, and eventually arcing or fires. In Singapore's tropical climate, moisture ingress is the primary accelerant — condensation inside motors, switchgear and cable terminations, especially in facilities cycling between air-conditioned and ambient temperatures, dramatically reduces insulation resistance, and regular testing provides early warning before breakdown. Testing is required or expected under SS 638:2018 (initial verification for new or altered installations), MOM WSH (Electricity) Regulations (periodic testing of HV installations and electrical plant), EMA licensing (installations above 1 MVA), and commonly by fire and engineering insurers as evidence of periodic testing on industrial plant.
Selecting the Correct Test Voltage
Applying the wrong test voltage is both dangerous (too high can damage good insulation) and misleading (too low may not reveal marginal insulation). Test voltage is selected based on the rated voltage of the equipment under test:
| Equipment Rated Voltage | Recommended Test Voltage (DC) | Typical Minimum IR (New) |
|---|---|---|
| Up to 50 V (SELV circuits) | 250 V DC | ≥ 0.25 MΩ |
| 50 V – 1000 V (LV, e.g. 230/400 V) | 500 V DC | ≥ 1 MΩ (SS 638 minimum) |
| 1 kV – 11 kV (MV cables and motors) | 1000 V DC | ≥ 100 MΩ (IEEE 43 guideline) |
| 11 kV – 33 kV (HV switchgear) | 2500 V DC | ≥ 1000 MΩ (application-dependent) |
| Above 33 kV (EHV cables, transformers) | 5000 V DC | Application-dependent |
For motor winding testing, IEEE 43 (Recommended Practice for Testing Insulation Resistance of Electric Machinery) provides detailed guidance on test voltage selection and minimum acceptable values based on the motor's rated voltage and winding type. For general LV cable and installation testing to SS 638, 500 V DC is standard.
Instruments like the Fluke 1587 FC insulation multimeter and dedicated megohmmeter testers available through Unitest Instruments offer selectable test voltages to cover the full range of applications.
Test Procedure: Step-by-Step
Insulation resistance testing applies high voltage, so strict safety procedures are mandatory: isolate and lock out equipment following LOTO with warning tags at every isolation point; discharge residual charge in capacitive equipment (cables, motors, large transformers) before connecting leads; disconnect surge arresters, capacitors and sensitive electronics that must not see the test voltage; connect "Line" to the conductor under test and "Earth" to the frame or earth bar; apply test voltage for 1 minute for a basic reading, or 10 minutes for the Polarisation Index; record the reading (IR1min) and note temperature and humidity, both of which affect results; discharge the test voltage and confirm zero before disconnecting; then reconnect equipment and restore isolation devices before re-energising.
Interpreting Results: Minimum Values and Pass/Fail
SS 638:2018 requires a minimum insulation resistance of 1 MΩ for LV installations tested at 500 V DC — an absolute minimum; new wiring typically shows hundreds of megaohms to gigaohms, and any reading below 100 MΩ warrants investigation even if it technically passes. For motors and rotating machines, IEEE 43 gives more nuanced guidance: for machines rated above 1 kV, minimum acceptable IR at 40°C (corrected) is (kV + 1) MΩ — a 6.6 kV motor needs IR ≥ 7.6 MΩ; a "caution zone" sits between the minimum and 5× the minimum warranting investigation; well above that is the "good zone." Always compare against the equipment's historical baseline — a motor that tested at 500 MΩ last year and now tests at 10 MΩ is in a downward trend worth investigating even though it superficially passes. Predictive maintenance programmes treat IR trend data as a key health indicator.
The Polarisation Index (PI) and Dielectric Absorption Ratio (DAR)
A single 1-minute IR reading can mislead — wet insulation may start high then rapidly decrease, while good insulation shows a rising reading as absorption current decays. PI and DAR capture this:
| Index | Formula | Test Duration | Good | Questionable | Poor |
|---|---|---|---|---|---|
| DAR | IR60s / IR30s | 1 min | ≥ 1.6 | 1.0–1.6 | < 1.0 |
| PI | IR10min / IR1min | 10 min | ≥ 2.0 | 1.0–2.0 | < 1.0 |
A PI of 2.0 or above indicates the insulation is absorbing the test voltage in a healthy manner. Absorption currents are decreasing over time, which is characteristic of good, dry insulation. A PI below 1.0 indicates that leakage current is increasing, suggesting moisture, carbonisation, or severe contamination.
The PI test is particularly valuable for large motors, generators, and transformers in Singapore's industrial facilities. Paper, rubber, and varnished insulation all show characteristic PI responses that an experienced tester can interpret. The Fluke 1587 FC and similar instruments calculate PI and DAR automatically.
Temperature Correction for Insulation Resistance Readings
Insulation resistance is highly temperature-dependent — IR roughly halves for every 10°C rise (and doubles for every 10°C fall) — so readings at different temperatures can't be compared without correcting to a reference temperature, typically 40°C per IEEE 43, via IRcorrected = IRmeasured × KT; many modern testers apply this automatically via a thermometer input. In Singapore's climate (ambient typically 28–35°C), equipment measured in the early morning before thermal soak shows higher IR than later in the day — always record temperature and correct before comparing to historical records.
Common Applications and Getting Testers Calibrated
Insulation resistance testing applies across LV cables and wiring (SS 638 commissioning before connection to supply), LV motors (periodic maintenance and pre-start checks), MV cables (annual testing of underground feeder cables in industrial parks, hospitals and data centres), transformers, switchgear (bus insulation during annual outages), and marine/offshore plant (MPA and classification society requirements). A tester that reads incorrectly may pass defective insulation or fail good insulation, so calibration verifies the test voltage output and resistance function are within specification. Unitest Instruments' SAC-SINGLAS accredited laboratory (LA-2023-0845-C) calibrates testers from 50 V to 5000 V DC and kilohms to teraohms, traceable to NMC standards and accepted by MOM inspectors and ISO 9001 auditors, with 3–5 working day turnaround.
Selecting an Insulation Resistance Tester for Singapore Applications
For general LV wiring under SS 638, a 500 V tester up to 2 GΩ is sufficient; motor and MV cable work needs a multi-voltage unit covering 250 V through 5000 V. Useful features: auto-PI and auto-DAR, recording IR at 30 seconds, 1 minute and 10 minutes and computing both indices automatically; temperature input and auto-correction to the 40°C IEEE 43 reference; a guard terminal bypassing surface leakage to reveal true bulk resistance on contaminated surfaces; live circuit detection and lock-out; and Bluetooth data logging exporting results to maintenance management systems. For fleets of test instruments, Unitest Instruments offers a managed calibration programme tracking due dates and maintaining a centralised certificate record — valuable for contractors running SS 638 work across multiple sites. Arrange calibration or call +65 6659 8878.