Understanding Earthing Systems & Maximum Ze Limits

The external earth fault loop impedance (Ze) is a critical parameter verified during any Initial Verification or Electrical Installation Condition Report (EICR). It represents the resistance of the earth path outside of the installation, trailing back to the distribution transformer. If this value is too high, the protective devices within the consumer unit will fail to disconnect the supply rapidly during an earth fault.

TN-C-S (Protective Multiple Earthing)

In a TN-C-S system, the supplier provides a combined Protective Earth and Neutral (PEN) conductor up to the service head. At the cut-out, this separates into distinct Earth and Neutral conductors. This is the most common supply type for modern UK properties.

• Maximum Declared Ze: 0.35 Ω
• Observation: If you measure a Ze greater than 0.35 Ω (e.g., 2.35 Ω) on a TN-C-S system, the supplier's earth has severely degraded or failed entirely. This is a critical safety issue. The installation is reliant on the mass of earth or bonded pipework rather than the PME terminal. The Distribution Network Operator (DNO) must be contacted immediately.

TN-S (Separate Earth and Neutral)

In a TN-S system, the earth path returns via the metallic lead sheath of the supplier's underground cable. The earth and neutral remain completely separate from the transformer to the consumer unit.

• Maximum Declared Ze: 0.8 Ω
• Observation: Over time, underground lead sheaths degrade, corrode, or are replaced by plastic jointing by DNO contractors without maintaining earth continuity. If the Ze exceeds 0.8 Ω, the structural integrity of the return path is compromised. The DNO should be contacted to either repair the sheath or upgrade the head to a PME (TN-C-S) arrangement.

TT (Local Earth Electrode)

In a TT system, the supplier does not provide an earth terminal. The property relies entirely on a locally driven earth rod or earth mat. Because the fault path relies on the soil back to the transformer, the impedance (Ze or Ra) is significantly higher, mandating the use of an RCD for all fault protection.

• Mathematical Limit: For a 30 mA RCD to operate at 50V touch voltage, the mathematical maximum earth resistance ($Z_s \le 50V / I_{\Delta n}$) is 1667 Ω.
• Stability Limit (BS 7671): Table 41.5 explicitly states that any earth electrode resistance exceeding 200 Ω may be unstable. Soil dries out or freezes, drastically increasing resistance. A reading above 200 Ω requires additional earth rods to be driven to achieve a stable, compliant reading.