Ss 551 Code Of Practice For Earthing Review

Before diving into clauses, it is vital to understand the three primary functions of an earthing system as defined by SS 551:

To ensure adherence to SS 551:

SS 551 adopts the IEC classification for earthing arrangements. The standard does not mandate a single type but provides conditions for each. ss 551 code of practice for earthing

TT System: The source neutral is earthed, but the installation’s exposed conductive parts are connected to a local earth electrode independent of the source. This is often used for temporary supplies or specific outdoor equipment. SS 551 mandates that all TT systems must be protected by Residual Current Devices (RCDs) of 100 mA or less.

IT System: The source is isolated from earth or connected via a high impedance. Used in medical facilities and data centers to maintain supply continuity. SS 551 requires insulation monitoring devices (IMDs) for this configuration.

SS 551 adopts the IEC classification of earthing systems for LV installations (up to 1,000 V AC): Before diving into clauses, it is vital to

| System | Description | Typical Application in Singapore | |--------|-------------|--------------------------------| | TN-S | Neutral and protective conductors separate throughout. Earth fault returns via dedicated PE. | Preferred for new buildings. | | TN-C-S | Neutral and protective combined (PEN) upstream, separated downstream. | Existing supply conversions. | | TT | Installation’s earth electrode independent of source’s earth. | Outdoor equipment, temporary supplies, high earth leakage risk areas. | | IT | No direct connection of live parts to earth; all exposed parts earthed. | Medical IT rooms, uninterruptible critical processes. | SS 551 adopts the IEC classification for earthing

SS 551 mandates that for TN systems, the maximum disconnection time for final circuits shall not exceed 0.4 s for 230 V (phase-to-earth) and 5 s for distribution circuits.

| Installation type | Maximum interval | |-------------------|------------------| | Residential | 5 years | | Commercial / Offices | 3 years | | Industrial / High-risk | 1 year | | Construction sites | 3 months |