SS 551 includes specific clauses for:
The Main Earthing Terminal (or Main Earthing Busbar) must be located as close as possible to the incoming supply. The following must be connected to the MET:
Bonding conductor cross-section: Not less than half the size of the earthing conductor and minimum 6 mm² copper.
| Installation Type | Recommended Maximum Earth Resistance | |------------------|----------------------------------------| | General LV supply | ≤ 1 Ω (for high fault current systems) | | TT system (with RCD) | ≤ 200 Ω (practical limit for RCD operation) | | Lightning protection (combined earth) | ≤ 10 Ω | | Telecommunications earth | ≤ 1 Ω (often ≤ 0.5 Ω) | ss 551 code of practice for earthing
SS 551 requires the following tests upon completion and periodically thereafter:
| Test | Method | Acceptance Criteria | |------|--------|----------------------| | Earth electrode resistance | 3-terminal or 4-terminal fall-of-potential method | ≤ design value (typically ≤ 1 Ω for LV supplies) | | Earth fault loop impedance (Zs) | Loop impedance tester at furthest point | Zs ≤ 0.8 × (Uo / Ia) where Ia = operating current of protective device | | Continuity of protective conductors | Low-resistance ohmmeter (200 mA – 10 A) | R ≤ 0.1 Ω (typical for short bonding runs) | | Touch voltage measurement | Voltmeter between exposed conductive part and reference earth | ≤ 50 V AC | | RCD testing | Dedicated RCD tester | Tripping time ≤ 300 ms (general); ≤ 40 ms for 30 mA RCD) |
All test results must be recorded in an Electrical Installation Test Report (as per SS 638 / EMA requirements). SS 551 includes specific clauses for: The Main
To ensure adherence to SS 551:
While human safety remains the paramount goal, the revision of SS 551 has had to pivot toward a modern obsession: data.
In an era where Singapore is a global hub for data centers, earthing has taken on a new dimension. Modern servers operate at incredibly low voltages and are hypersensitive to "electrical noise"—random fluctuations in voltage that corrupt data. Bonding conductor cross-section: Not less than half the
SS 551 provides the framework for a "Clean Earth" or "Functional Earth." It distinguishes between the messy, high-energy earthing required for lightning protection (covered by its sibling code, SS 555) and the "quiet," stable earthing required for sensitive electronics.
"In a data center, a poor earthing system doesn't cause a fire; it causes downtime," notes a facilities management expert. "It causes ghost glitches. SS 551 gives us the baseline to prevent that. It ensures that the physical infrastructure is invisible to the digital layer."