SS 638:2018 (Code of Practice for Electrical Installations) is the primary Singapore standard governing the design, installation, and testing of all LV electrical installations — compliance is mandatory under the Electricity Act (Cap. 89A) and enforced by the Energy Market Authority (EMA). All new electrical installations above 45 V must be tested and certified by a Licensed Electrical Worker (LEW) before they can be energised or connected to the supply. This guide covers every test required under SS 638, the instruments needed, and how calibration fits into the compliance picture.
What SS 638 Covers and Who Must Comply
SS 638:2018 is the Singapore equivalent of the UK's BS 7671 (IET Wiring Regulations) and aligns closely with IEC 60364. It covers electrical installations in buildings and structures that operate at voltages up to 1000 V AC or 1500 V DC — encompassing virtually all domestic, commercial, and light industrial wiring in Singapore. High-voltage installations above 1 kV are governed by the EMA's Electrical Licensing Regulations and SS 555.
Compliance is not optional. Under the Electricity (Electrical Installations) Regulations:
- All electrical installation work must be carried out by or under the supervision of a Licensed Electrical Worker (LEW).
- New installations and major alterations must be inspected and tested before connection to supply.
- The LEW must complete a Schedule of Inspections and Tests (Form EL1 or equivalent) confirming all tests have been performed and results are satisfactory.
- Existing installations must be periodically inspected and tested — typically every 5 years for domestic, 3 years for commercial, and annually for high-risk premises.
Non-compliance — including failure to test or using uncalibrated instruments that produce unreliable results — can expose building owners and LEWs to prosecution under the Electricity Act, and void insurance coverage in the event of a fire or accident.
Visual Inspection: The First Step Before Any Instrument Test
SS 638 Chapter 6 (Inspection and Testing) requires a thorough visual inspection before instrument testing begins. Visual inspection identifies faults that instruments cannot detect — missing earth wires, improperly terminated conductors, unauthorised modifications, and equipment not suitable for the environment. Items to check include:
- Correct cable routing, support, and protection from mechanical damage
- All conductors correctly identified (phase, neutral, earth) using Singapore colour code (brown/blue/green-yellow)
- Correct protective device ratings for the conductor sizes used
- All junction boxes and enclosures closed and properly sealed
- Absence of visible damage, burning, or moisture ingress
- Correct labelling of distribution boards and circuits
- All earthing and bonding connections present and secure
Visual inspection must be completed and any defects corrected before proceeding to instrument testing. Documenting the visual inspection is a requirement under the certification process.
Test 1: Continuity of Protective Conductors
This test verifies that every protective conductor — earth wires in cables, conduit used as a CPC (circuit protective conductor), and structural bonding conductors — is continuous from the point of origin to all points served. A single open circuit or high-resistance joint in an earth wire renders the entire earth protection system ineffective.
The test uses a low-resistance ohmmeter applying at least 200 mA DC test current. The instrument is connected between the main earth terminal and each socket outlet earth pin, appliance frame, or bonded metalwork in turn. Readings are compared against calculated values based on conductor length and cross-section.
Additionally, the R1 + R2 measurement (resistance of phase conductor plus resistance of protective conductor for each circuit) is required — this is used later to calculate the expected earth fault loop impedance. This measurement is taken with a wander lead connected between the phase and earth at the distribution board, then measured at each outlet point.
Test 2: Continuity of Ring Final Circuit Conductors
Ring final circuits (ring mains) — common in older Singapore commercial buildings and some residential properties — require a specific three-measurement test to confirm the ring is electrically continuous and has no "borrowed neutrals" or spurious connections. The test verifies end-to-end continuity of the phase, neutral, and earth conductors and checks that the ring is not open or crossed.
This test is performed using the same low-resistance ohmmeter as the protective conductor test. The result should show that the resistance measured at the midpoint of the ring is approximately one quarter of the end-to-end resistance — confirming the ring geometry is intact.
Test 3: Insulation Resistance
Insulation resistance testing under SS 638 verifies that the insulation between live conductors and earth is intact and meets the minimum resistance threshold. Standard test conditions:
- Test voltage: 500 V DC for all circuits rated up to 500 V (including 230 V and 400 V TPN systems)
- Minimum acceptable IR: 1 MΩ for each circuit tested
- Circuit configuration: All switches closed, all lamps removed, all equipment disconnected. Test phase-to-earth, neutral-to-earth, and phase-to-neutral for each circuit.
Electronic dimmers, RCDs, electronic ballasts, and surge protective devices (SPDs) must be disconnected or bypassed before the 500 V test to avoid damage. SPDs in particular will conduct (and may be destroyed) if the test voltage exceeds their clamping level. See our detailed insulation resistance testing guide for full procedure including the Polarisation Index test for motors and large installations.
Test 4: Polarity
Polarity testing verifies that single-pole protective and switching devices are connected in the phase conductor (not the neutral), and that socket outlets and luminaire connections are correctly polarised. Incorrect polarity — a live and neutral reversal — leaves a switch that appears to turn equipment off but actually disconnects the neutral, leaving the equipment live and dangerous.
In Singapore's wiring system, correct polarity is: Brown = Line (Phase), Blue = Neutral, Green/Yellow = Earth. This was adopted in 2009, aligning Singapore with IEC harmonised colours. Older installations may use the legacy colours (Red = Phase, Black = Neutral) — these must be identified during inspection.
Polarity is verified using the multifunction installation tester's continuity or voltage functions. The test is performed both at de-energised stage (checking wiring connections) and confirmed at live stage during the socket outlet functional test.
Test 5: Earth Fault Loop Impedance
Earth fault loop impedance (EFLI) testing is performed on the energised installation to verify that the total impedance of the fault current path is low enough for the protective device to operate within the required disconnection time — 0.4 seconds for final circuits (as per ADS requirements in SS 638 Chapter 4).
The tester momentarily applies a known load across the circuit and measures the resulting voltage drop. Modern testers display Zs directly in ohms. The LEW compares the measured Zs against maximum values in SS 638 Appendix 3, which provides tables for every common MCB rating and type (B, C, D), rewireable fuses, and semi-enclosed fuses.
Loop impedance testing is the live test that most directly confirms the installation will protect people from electric shock under fault conditions. It is a mandatory test for every final circuit, every distribution circuit, and at the supply origin. Multifunction installation testers from Unitest Instruments include an NI (No-Trip) mode that measures loop impedance on RCD-protected circuits without causing unwanted tripping.
For further reading on the earth continuity principles underlying this test, see our article on earth continuity testing methods and acceptable values.
Test 6: RCD Testing
Residual current devices (RCDs) are mandatory in Singapore for all socket circuits in domestic properties (EMA requirement), bathroom and outdoor circuits, and circuits serving portable equipment used outdoors or in wet locations. SS 638 requires functional testing of all RCDs at commissioning.
The standard SS 638 RCD tests are:
| Test | Test Current | Required Disconnection Time |
|---|---|---|
| No-trip test (½ × IΔN) | Half rated trip current | Must NOT trip |
| Trip time at rated current (IΔN) | Rated trip current (e.g., 30 mA) | ≤ 300 ms for general; ≤ 40 ms for Type S |
| Trip time at 5× IΔN | 5× rated current (e.g., 150 mA) | ≤ 40 ms (general purpose) |
| Push-button test | Mechanical test only | Functional check |
RCD test results must be documented on the inspection schedule. The test also confirms the RCD is connected correctly (not bypassed or wired around). Our detailed guide to RCD testing covers the full procedure including testing different types (AC, A, B, F) and 30 mA vs 100 mA trip ratings.
Test 7: Functional Testing and Verification
After all instrument tests pass, functional testing confirms that the installation performs its intended purpose — switches operate the correct loads, lighting circuits illuminate the correct luminaires, interlocks work as designed, and protective devices operate. This step catches wiring errors that slipped past the earlier tests, such as a circuit wired to the wrong distribution board way, or a switch leg connected to the wrong luminaire.
Equipment Required for SS 638 Testing
A complete SS 638 test kit for an LEW includes:
| Instrument | Primary Tests Performed | Key Specification |
|---|---|---|
| Multifunction installation tester | IR, continuity, EFLI, RCD, polarity | 500/1000 V IR; Zs ≤ 0.01 Ω resolution; RCD 10/30/100/300/500 mA |
| Low-resistance ohmmeter | Protective conductor continuity, R1+R2 | ≥ 200 mA test current; 0.01 Ω resolution |
| Approved voltage indicator | Safe isolation — verify dead before work | CAT III or IV rated; two-pole preferred |
| Clamp meter / power analyser | Load current, power factor, harmonics | True-RMS; CAT III rated |
All instruments used for SS 638 compliance testing must be calibrated — and for records submitted to EMA or for insurance purposes, calibration by a SAC-SINGLAS accredited laboratory is strongly recommended. Unitest Instruments' calibration lab (LA-2023-0845-C) calibrates all major multifunction testers, continuity instruments, and voltage indicators, with certificates traceable to Singapore's NMC. Standard turnaround is 3–5 working days.
Record-Keeping and Certification Under EMA Requirements
After completing all SS 638 tests, the LEW must issue a Test Report and Inspection Certificate documenting:
- Description of the installation (address, supply details, earthing system)
- All test results tabulated by circuit
- Instrument details and calibration certificate references
- Declaration that the installation complies with SS 638 and is safe to energise
- LEW licence number and signature
This certificate must be retained by the building owner and a copy submitted to SP Group (the electricity retailer/distributor) before the new connection or altered circuit is energised. EMA may audit these records. Falsifying test results or signing off without performing tests is a criminal offence under the Electricity Act — penalties include licence revocation and imprisonment.
Common Failures Found During SS 638 Testing and How to Address Them
Based on the nature of Singapore's electrical installation stock — spanning post-war public housing, 1980s commercial buildings, and modern intelligent buildings — certain defects appear regularly during SS 638 testing and periodic inspection:
- Insulation resistance below 1 MΩ: Most commonly found in older buildings where cables have been wet, overheated, or mechanically damaged. Isolate the affected circuit, trace the fault to the specific cable section or connection, repair or replace, and retest. Often caused by damaged cable insulation at a conduit entry point or by water ingress at a junction box.
- Earth fault loop impedance above the maximum tabulated value: Usually caused by undersized earth conductors, corroded earth connections, or long circuit runs without adequately sized protective conductors. The fix may involve adding supplementary earthing conductors, replacing corroded bonding clamps, or upgrading the protective device to one with a higher instantaneous trip current that allows a higher Zs.
- RCD fails trip time test: Aged RCDs — particularly original devices installed in older HDB flats — may show trip times above 300 ms. Replace the RCD; there is no adjustment possible. Ensure the replacement is the correct type for the circuit (Type A for modern appliances).
- Polarity reversal: Common in older installations where the brown/blue colour change (from red/black) was not correctly applied during modifications. Trace the reversal to the specific socket or junction box and correct the connection. Do not re-energise until polarity is verified at every outlet.
- Open circuit protective conductor: A break in an earth wire — often at a socket outlet where a screw terminal was not properly tightened — results in the equipment being unprotected during a fault. Trace and reconnect; check all terminals in the affected circuit for tightness.
When defects are found, they must be corrected before the installation is energised or returned to service. The LEW must retest the affected circuits after any corrective work and update the inspection certificate accordingly. If you need replacement test instruments or calibration of your installation test equipment, contact Unitest Instruments at +65 6659 8878 — we can also advise on suitable multifunction installation testers for purchase or rental for short commissioning projects.