Earthing a Metal Shed or Container?
Here is a Case Study that may help to consider the PEN fault risk.
Technical Assessment: PME Export to Metal Container
Evaluation of exporting TN-C-S (PME) earthing via 16mm SWA to a fixed metal shipping container.
⚠️ Assessment: High Risk / Not Recommended
Exporting PME earth to a large conductive metal structure in direct contact with the ground (like a shipping container) creates a significant risk of electric shock in the event of a supply PEN conductor failure.
While not explicitly "banned" in the same strict language used for Caravans (ESQCR Regulations), a shipping container presents almost identical electrical characteristics. It is a large extraneous-conductive-part.
If the DNO's neutral (PEN) breaks, the container chassis will become live at mains potential relative to the earth passing through it. You cannot effectively bond the "True Earth" (the grass/soil outside).
Liability Implications
As the designer/installer, you have a duty of care. If a PEN fault occurs and causes injury, relying on "it wasn't explicitly banned" is a weak defense against the known physics of the hazard. Industry best practice dictates separating the earthing system (TT).
Safety Comparison
The "Broken PEN" Hazard Simulator
Why is bonding not enough? In a PME system, the Earth and Neutral are combined (PEN). If this cable snaps in the street, the return current looks for a path. If your container is bonded to PME, IT becomes the return path through the ground.
Industry Scenarios
Select a scenario to compare its PME status with your Shipping Container.
Shipping Container
PME High RiskNot explicitly banned by name, but violates core safety principles of Reg 411.3.1.2 due to difficulty of equipotential bonding.
Large surface area contact with user and ground simultaneously. Loss of PEN energizes entire structure.
Treat as TT Island. Gland armour at source, isolate at load.
The Fix: TT Conversion (Island Mode)
Since you have a 16mm 3-core SWA, you are in a perfect position to implement a safe TT system without laying new cables. You will simply stop exporting the earth and create a local one.
Source End (House/Distribution)
Gland the SWA normally. Connect the Armour (and 3rd core if used as earth) to the PME Earth block. The cable armour protects the cable itself.
Load End (Container) - CRITICAL
Gland the SWA into a plastic enclosure (insulating gland plate) OR use a metal box but DO NOT CONNECT the banjo/armour to the container's earth bar. Insulate the armour earth ("Fly lead") and tuck it away or terminate into an insulated block marked "PME Earth - Do Not Connect".
Install Earth Electrode
Drive an earth rod (or rods) near the container. Connect this via 16mm earthing conductor to the container's Main Earthing Terminal (MET).
RCD Protection
Ensure the supply to the container or the distribution board inside the container has RCD protection (e.g., 30mA or 100mA Time Delayed at source + 30mA at load). TT systems rely on RCDs to clear faults; fuse/MCB will likely not trip on an earth fault.
Visualizing the Separation
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