Sprinkler System Pipework Main Protective Bonding

For me, this question highlights an area where the interpretation and application of wiring regulations can sometimes lead to different outcomes over time, or where a design decision might be based on specific context.

The Core Requirement for Main Protective Bonding

The requirement for main protective bonding (also known as equipotential bonding) is laid out in Regulation 411.3.1.2 of BS 7671. This regulation mandates that the following extraneous-conductive-parts must be connected to the main earthing terminal (MET) of the installation:

• Water installation pipework.
• Gas installation pipework.
• Other service pipework and ducting.
• Central heating and air conditioning systems.
• Exposed metallic structural parts of the building.

The key term here is "extraneous-conductive-part".

Why Sprinkler Pipework Might Not Be Bonded

The crucial distinction lies in whether the sprinkler pipework is truly an extraneous-conductive-part in the electrical sense, meaning it is liable to introduce an earth potential (a potential other than that of the installation's earth) into the building.

Here are the most common and reasonable explanations for why a qualified designer might deem bonding unnecessary for sprinkler pipework:

1. Isolation from True Earth Potential

A sprinkler system often forms a closed circuit (or a semi-closed circuit) that may be entirely non-conductive to true earth within the building.

• Pump House Isolation: In the pump house, the connection to the main water supply (if any) might pass through sections of plastic pipework or isolating rubber couplings designed to dampen vibration. If the metallic pipework within the building is physically isolated from the general mass of earth outside the building by these non-conductive breaks, it does not strictly meet the definition of an extraneous-conductive-part and would not require main bonding.
• Contained System: The metallic pipework, especially within a modern school building with non-metallic structure or foundations, might only be connected to the installation's earth via local supplementary bonding, or not at all, provided it is electrically separated from the external earth.

2. Design Intent (Not an Extraneous Part)

If the designer confirmed, perhaps through testing or detailed design review, that the resistance between the sprinkler pipework and the main earth terminal (MET) exceeds the required threshold (often considered to be 22 kΩ for being "non-extraneous" in the context of touch voltages), then bonding is not required. Essentially, they deemed it an "unlikely conductor of fault current" or incapable of importing earth potential.

3. Change in Interpretation and Standards

You mentioned this is now being flagged after many years. There are two possibilities here:

• Increased Scrutiny: Over the last 10-15 years, the level of detail and scrutiny required by the electrical safety industry has increased significantly. Newer inspectors are often applying the regulations with a stricter interpretation, or their risk assessment methodology may lead them to err on the side of caution and bond any significant metallic service.
• Physical Changes: Has any work been done on the system? Perhaps an original non-metallic section was replaced with metal, or a new metallic connection has been added that inadvertently provides a path to earth, thus turning it into an extraneous-conductive-part.

Conclusion and Recommendation

While it is possible that the original design intention was sound based on the pipework being electrically isolated (Scenario 1 or 2), the fact that it is now being flagged as a C2 (Potentially Dangerous) or C3 (Improvement Recommended) on an EICR requires action or justification.

My recommendation would be the following:

1. Review the Original Design/Installation: Check the pump house for any deliberate non-conductive sections (e.g., plastic or rubber couplings) between the public water supply and the internal sprinkler pipework.
2. Conduct Resistance Testing: A competent electrical person should test the resistance between the metallic sprinkler pipework and the installation's MET. If the resistance is low (e.g., less than 22 kΩ, but a definitive low resistance path to earth), it must be bonded to comply with the protective measures against electric shock.
3. Default to Bonding : Given the safety critical nature of a school environment, and the cost-effectiveness of fitting a protective bond compared to the liability of leaving a C2 non-compliance, the most straightforward and pragmatic approach is often to install the main protective bonding conductor (typically 10 mm² or 16 mm² depending on the installation size) to the pipework where it enters the building, or at the MET/pump house. This eliminates the uncertainty and clears the EICR defect.

In short, the original designer could have been correct, but a modern inspector is now challenging the assumption of isolation. Protective bonding is a fundamental safety measure, and if there is any doubt about the pipework's extraneous status, bonding it is the safest and most compliant route forward. safest and most compliant route forward.

Thanks for the very informative reply David, certainly going to have to look into this a bit further before we action works to install main protective bonding conductors, as this affects quite a few of our sites.

Thanks for the very informative reply David, certainly going to have to look into this a bit further before we action works to install main protective bonding conductors, as this affects quite a few of our sites.