Ss 551 Code Of Practice For Earthing //top\\ File
Technical Write-Up: SS 551:2009 – Code of Practice for Earthing
7. Final Verdict
SS 551 is a strong, safety-first code that rightly prioritizes touch/step potential over a simplistic “low ohms” target. It is well-adapted to Singapore’s lightning-prone, corrosive, and space-limited environment. However, it suffers from:
- Under-specification of verification methods,
- No software requirement for complex grids,
- Lag in covering DC and renewable energy systems.
Who should use it: Mandatory for all licensed electrical installations in Singapore. Recommended as a reference for tropical countries (Malaysia, Indonesia, Thailand) lacking their own detailed earthing code.
Best paired with: IEEE 80 (for HV substations), BS 7671 (for final circuits), and SS 555 (for lightning protection).
If you need a specific section analyzed (e.g., Clause 9.2 formulas, Clause 10 bonding for data centers, or Clause 15 lightning integration), let me know.
SS 551 Code of Practice for Earthing is a Singapore Standard that provides comprehensive guidance on the design, installation, and maintenance of earthing systems for land-based electrical installations.
The primary objectives of SS 551 are to ensure the safety of human life and property by limiting potential differences that could cause electric shocks or fires, and to facilitate the proper operation of electrical systems. This essay explores the scope, core principles, and technical requirements of the standard. 1. Scope and Application SS 551 applies specifically to land-based installations in and around buildings. It covers: Low voltage (LV) installations : Providing protective earthing requirements aligned with (Code of Practice for Electrical Installations). HV/LV Interfaces ss 551 code of practice for earthing
: Managing the earthing connection between high voltage and low voltage substations within buildings. Generator Sets
: Detailing earthing arrangements for standby or local power generation units.
The code explicitly excludes specialized systems such as ships, aircraft, offshore platforms, medical equipment, and internal earthing of hardware. 2. Core Principles of Earthing
The standard distinguishes between two fundamental aspects of earthing: System Earthing
: Concerned with the intentional connection of current-carrying conductors (like the neutral point) to earth to ensure the proper operation of the power system and limit potential. Equipment Earthing Technical Write-Up: SS 551:2009 – Code of Practice
: Focused on connecting non-current-carrying metal parts of equipment (casings and frames) to earth to protect humans and animals from electric shock in the event of a fault. 3. Key Technical Requirements
SS 551 provides detailed specifications for components and design to achieve a low-impedance path to the ground: 07) - SS 551 - 2009 (Earthing) | PDF - Scribd
7. Inspection and Testing
Verification of the earthing system is mandatory before energizing an installation and periodically thereafter.
Key Tests:
- Earth Electrode Resistance Test: Typically performed using a fall-of-potential earth tester.
- Continuity Test: Verifying the continuity of the protective conductors from the MET to the furthest point in the circuit (socket outlets, exposed metal).
- Loop Impedance Test: Measuring the earth fault loop impedance ($Z_s$) to ensure the protective device will trip within the required time.
A. TN System (Terre Neutre)
The neutral point of the supply source is earthed, and the exposed conductive parts of the installation are connected to the neutral via a protective conductor. Who should use it: Mandatory for all licensed
- TN-S System (Separate): The Neutral (N) and Protective Earth (PE) conductors are separate throughout the entire system. This is the standard for modern industrial and commercial buildings in Singapore due to lower electromagnetic interference and better safety.
- TN-C-S System (Combined-Separate): The N and PE are combined (PEN) in part of the system and separate in others.
10. Comparison with International Standards
| Feature | SS 551 | BS 7430 | IEC 60364-4-44 | |---------|--------|---------|----------------| | Max touch voltage (general) | 50 V | 50 V | 50 V | | Preferred electrode material | Copper-bonded steel | Copper, steel, cast iron | Any suitable material | | Earthing conductor min. size (Cu) | 16 mm² | 10 mm² | 6 mm² (if mechanically protected) | | Lightning earth bonding | Mandatory | Mandatory | Not covered | | Soil resistivity testing | Required for > 1000 m² | Recommended | Not specified |
SS 551 is generally more conservative than BS 7430 in sizing and more rigorous regarding bonding requirements due to Singapore’s lightning density (≈ 15–25 strikes/km²/year).
5. Testing and Verification per SS 551
Every earthing installation must be tested before energization and periodically thereafter. SS 551 outlines the following mandatory tests:
4.4 Earthing and Bonding
SS 551 distinguishes clearly between:
- Earthing – connecting exposed conductive parts to the MET.
- Protective bonding – connecting extraneous conductive parts (water pipes, gas pipes, structural steel) to the MET to prevent potential differences.
Mandatory bonding applies to:
- Metallic water supply pipes entering the building (even if plastic inside).
- Metallic gas pipes (upstream of an insulating section).
- Central heating and air-conditioning pipes.
- Structural steel exposed to touch.
Prohibited – Using gas or water pipes as earth electrodes.
