5.1 Coding Scenarios: Earthing, Bonding & Shock Protection Defects
Key Takeaways
- Complete absence of main protective earthing is classified as Code C1 (Danger Present) requiring immediate remedial action to prevent fatal electric shock.
- Inadequate or missing main protective bonding conductors to extraneous-conductive-parts are classified as Code C2 (Potentially Dangerous) under BS 7671:2018+A2:2022.
- Socket-outlets rated up to 32 A without 30 mA RCD protection generally warrant Code C2 in domestic/general locations, but may be Code C3 in non-domestic premises supported by a valid risk assessment.
- Measured earth fault loop impedance (Zs) exceeding maximum permitted values in BS 7671 Table 41.2–41.4 without RCD fault protection is coded C2 due to failed disconnection times.
Classification Principles for Earthing, Bonding & Shock Protection
Periodic inspection and testing under BS 7671:2018+A2:2022 and IET Guidance Note 3 (GN3) requires inspectors to exercise sound technical judgment when assigning classification codes to observed defects. The primary objective of an Electrical Installation Condition Report (EICR) is to report on the safety of the electrical installation for continued service.
Classification codes are defined as follows:
- Code C1 (Danger Present): Risk of injury exists. Immediate remedial action is required.
- Code C2 (Potentially Dangerous): Urgent remedial action is required to remove potential danger.
- Code C3 (Improvement Recommended): Does not comply with current regulations, but does not present an immediate or potential danger.
- Code FI (Further Investigation): Requires further investigation without delay to determine safety.
Summary Classification Table: Earthing & Bonding Defects
| Defect / Observation | Classification Code | Regulatory Standard / Guidance | Technical Justification |
|---|---|---|---|
| Missing Main Earthing Conductor | C1 | BS 7671 Reg 411.3.1.1, GN3 | Exposed-conductive-parts cannot clear faults; live voltage persists during insulation failure. |
| Missing / Undersized Main Protective Bonding | C2 | BS 7671 Reg 411.3.1.2, 544.1.1 | Risk of dangerous potential differences between metalwork under external supply fault conditions. |
| Class I Equipment Without CPC | C2 | BS 7671 Reg 411.3.1.1 | Exposed metal casing accessible to touch could become live under single fault condition. |
| Socket Outlet Outdoors Without 30 mA RCD | C2 | BS 7671 Reg 411.3.3 | High-risk outdoor environment supplying mobile equipment; heightened shock hazard. |
| Zs Exceeds Max Permitted Table Value | C2 | BS 7671 Reg 411.3.2, Tables 41.2–41.4 | Disconnection time (0.4s/5s) exceeded; automatic disconnection of supply fails to protect. |
Earthing & Protective Bonding Walkthrough Scenarios
Scenario 1: Absence of Main Earthing Conductor (Code C1)
During a periodic inspection of a 230 V single-phase TN-S commercial installation, the inspector opens the main intake equipment and observes that the main earthing conductor connecting the main earthing terminal (MET) to the distributor's lead sheath has been severed and disconnected.
- Analysis: Without a main earthing conductor, the installation lacks a protective earthing path. In the event of an earth fault on any Class I circuit, protective devices (MCBs or fuses) will fail to operate, leaving exposed-conductive-parts continuously energised at line voltage (230 V).
- Classification Code: C1 (Danger Present).
- Required Inspector Action: The inspector must immediately notify the duty holder in writing before leaving the site and strongly advise isolation of the installation until earthing is restored.
Scenario 2: Missing or Inadequate Main Protective Bonding (Code C2)
In a domestic PME (TN-C-S) installation, the main incoming water pipe enters the premises in copper metalwork. The inspector finds no main protective bonding conductor connected to the water service pipe within 600 mm of the meter or point of entry. Alternatively, the existing bonding conductor is sized at 4 mm², whereas Table 54.8 requires a minimum of 10 mm² copper for a PME installation with neutral conductors up to 35 mm².
- Analysis: Protective equipotential bonding minimizes dangerous touch voltages introduced into the premises via extraneous-conductive-parts during an open-PEN fault on the supply network or an internal insulation breakdown. While automatic disconnection of supply (ADS) may still function via the CPC during an internal circuit fault, the absence of adequate bonding creates a potential touch-voltage hazard under network fault conditions.
- Classification Code: C2 (Potentially Dangerous).
- Regulatory Reference: BS 7671 Regulations 411.3.1.2 and 544.1.1.
Scenario 3: Lighting Circuits Lacking Circuit Protective Conductors (CPCs)
In an older installation wired in metal-sheathed or early PVC cables without an integral CPC supplying lighting points fitted with Class I (metal) switches and luminaires.
- Analysis: If Class I equipment is accessible, an insulation breakdown will energise the metal switch plate or fitting without triggering ADS, posing a direct shock risk to users.
- Classification Code: C2 (Potentially Dangerous) if Class I fittings or switches are installed. (Note: If all accessories are strictly Class II plastic and a warning label is fitted at the distribution board per Regulation 411.3.1.2, it may be coded C3).
Shock Protection & RCD Coding Matrix
Under BS 7671:2018+A2:2022, additional protection by means of a 30 mA Residual Current Device (RCD) is mandatory for socket-outlets rated up to 32 A (Regulation 411.3.3) and mobile equipment rated up to 32 A for outdoor use.
RCD Coding Guidance Matrix
| Installation Context | Observation | Code | Justification |
|---|---|---|---|
| Domestic Socket Outlets | No 30 mA RCD protection provided | C2 | Users plug in portable appliances with high risk of flex damage or outdoor extension leads. |
| Commercial / Industrial Sockets | No 30 mA RCD protection provided (No Risk Assessment) | C2 | Without a documented risk assessment by the employer/duty holder, 30 mA RCD is compulsory. |
| Commercial / Industrial Sockets | No 30 mA RCD protection provided (Documented Risk Assessment Present) | C3 | Permitted exemption under Regulation 411.3.3 where specific documented control measures exist. |
| Bathroom Final Circuits | Lighting/Shower in Zone 1/2 without 30 mA RCD | C2 | Heightened risk due to reduced human skin resistance in wet areas (Reg 701.411.3.3). |
| Cables Buried in Walls (<50 mm) | Concealed cables without mechanical protection or 30 mA RCD | C3 | Installed under older regulations; poses risk only during DIY drilling/penetration. C2 if high risk. |
Earth Fault Loop Impedance (Zs) Disconnection Failures
When carrying out loop impedance testing on a 32 A Type B MCB protecting a final ring circuit, the measured Zs value is 1.45 Ω. Comparing this to BS 7671 Table 41.3 (maximum permitted Zs = 1.1 Ω for a 32 A Type B MCB at 230 V with ambient adjustment):
- Analysis: Because the measured Zs exceeds the maximum allowable limit, the circuit protective device cannot guarantee automatic disconnection within the mandatory 0.4 s time limit under fault conditions. If no RCD is installed to provide fault protection, the failure of ADS constitutes a C2 (Potentially Dangerous) condition.
During a periodic inspection of a commercial premises supplied by a TN-S earthing arrangement, an inspector discovers that the main protective earthing conductor has been completely disconnected from the main earthing terminal. What classification code must be assigned on the EICR?
On a domestic PME (TN-C-S) installation with main copper supply conductors of 25 mm², what is the minimum required cross-sectional area for main protective bonding conductors to gas and water service pipes, and what code is assigned if found to be undersized at 6 mm²?
An inspector evaluates a domestic ring final circuit supplying 13 A socket-outlets installed in 2005. The circuit lacks 30 mA RCD protection. How should this observation be classified under BS 7671:2018+A2:2022 periodic inspection guidance?
A 230 V radial circuit protected by a 16 A Type B MCB (maximum permitted Zs = 2.19 Ω) measures a Zs value of 3.40 Ω. No RCD is present on the circuit. What is the correct classification code and technical reason?