Ss 551 Code Of Practice For Earthing ((hot)) Link

SS 551 — Code of Practice for Earthing (Draft Outline)

How to Achieve SS 551 Compliance: A 7-Step Checklist

1. Soil Resistivity Survey: Perform a Wenner 4-pin test at the site before designing the electrode layout.
2. Design Calculations: Use the adiabatic equation (Clause 7) to size all earthing conductors. Do not guess.
3. Select Correct Electrodes: Use copper-bonded (250 microns) or solid copper rods, driven below the water table.
4. Install Main Earthing Terminal (MET): Accessible, labeled, and bonded to the building steel within 3m of the incoming supply.
5. Install All Equipotential Bonding: Connect water, gas, HVAC, and structural steel with unbroken 10mm² copper.
6. Test Thoroughly: Measure $R_E$ (earth resistance) and $Z_s$ (fault loop impedance). Record values on an Electrical Installation Certificate (EIC).
7. Periodic Retesting: Set a calendar reminder for 5-year re-verification. Use thermal imaging to check for loose connections at earthing joints annually.

5. Practical Implementation Challenges (Singapore Context)

• Space constraints: In land-scarce Singapore, achieving low earth resistance (e.g., <1 Ω) often requires deep-driven rods (30+ m) or chemical electrodes, which SS 551 allows but doesn’t provide detailed installation quality checks.
• Contractor competence: Many LEWs (Licensed Electrical Workers) are trained on BS 7671’s simple loop impedance testing, not SS 551’s fall-of-potential method. Enforcement by EMA (Energy Market Authority) is periodic, leading to non-compliance.
• Building completion pressure: Earthing is often last on site. Concrete-encased electrodes (UFER grounds) specified in SS 551 are frequently omitted or incorrectly tied to rebar, reducing fault current capacity.

2. Normative References

• List mandatory standards and documents to consult (example entries):
  • IEC 60364 series — Low-voltage electrical installations
  • IEEE 80 — Grounding of Industrial and Commercial Power Systems
  • Local wiring regulations and safety acts
  • Standards for materials and testing (conductors, clamps, test instruments)

5.1 Continuity of Protective Conductors

• Verify electrical continuity from each exposed metal part back to the MET.
• Use a low-resistance ohmmeter (4-wire method).
• Maximum resistance typically < 0.5 Ω for branch circuits.

1. Key Objectives of Earthing

According to SS 551, a properly designed earthing system must achieve three main goals:

1. Safety (Protection against Electric Shock): Preventing the appearance of dangerous touch voltages on exposed conductive parts (like metal enclosures) during an insulation failure.
2. Protection against Overcurrent: Ensuring fault currents are of sufficient magnitude to operate protective devices (circuit breakers, fuses) quickly to clear the fault.
3. Lightning Protection: Providing a path for lightning discharge currents to earth.

2.1 Key Definitions

• Earth electrode – A conductor or group of conductors in direct contact with the general mass of earth (e.g., copper rod, plate, or buried tape).
• Main earthing terminal (MET) – The central point where the earthing conductor, protective bonding conductors, and functional earth conductors are connected.
• Protective conductor (PE) – A conductor used for safety earthing (green/yellow insulation).
• Neutral conductor (N) – The conductor connected to the star point of a transformer.
• PEN conductor – A combined protective and neutral conductor (allowed only in certain systems like TN-C).