Ss 551 Code Of Practice For Earthing

| Feature | SS 551:2018 | BS 7430 (UK) | IEEE 80 (US) | IEC 62305-3 (Lightning) | |----------------|-------------|--------------|--------------|--------------------------| | Step/touch calcs | Mandatory >1 kA | Recommended | Mandatory for substations | Not covered | | Soil testing depth | Up to electrode length | Variable | Up to grid diagonal | Not specified | | Corrosion protection | Moderate (material choice) | Basic | Detailed (incl. coating) | Minimal | | Software requirement | No | No | Yes (for large grids) | No | | Tropical adaptation | High | Low | Low | Moderate |

Verdict: SS 551 is more rigorous than BS 7430 but less detailed than IEEE 80 for high-voltage substations. For lightning, it defers appropriately to SS 555.

---

SS 551 specifies that the earth resistance of any electrode shall not exceed 1 ohm for LV installations unless otherwise justified by calculation. However, for TT systems, a higher resistance (up to 200 ohms) is acceptable if RCDs with appropriate sensitivity (e.g., 30 mA, 100 mA, 300 mA) are used. ss 551 code of practice for earthing

The formula used is:

[ R_A \times I_\Delta n \leq 50 , \textV ] | Feature | SS 551:2018 | BS 7430

Where:

Example: For a 30 mA RCD, ( R_A \leq 50 / 0.03 = 1666 , \Omega ). But SS 551 recommends keeping below 200 Ω in practice. SS 551 specifies that the earth resistance of

SS 551 requires that all earth electrodes be inaccessible to unauthorized persons and protected against mechanical damage and corrosion. In corrosive soil (common in Singapore’s coastal reclaimed land), stainless steel or copper-clad steel is mandatory.

---

SS 551 defines earthing as the connection of exposed conductive parts (e.g., metal enclosures of equipment) and extraneous conductive parts (e.g., metal water pipes) to the main earth terminal. The primary objectives are:

| 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) |