Separate 'clean earth', bonding sizes and SPDs on a 55 m tower

Foundation earth resistance

For a TN-C-S supply, the electrode resistance is not the safety criterion - the protective earth is the PEN, and disconnection depends on the loop impedance and the supply authority's network. A 6.8 Ω foundation earth is a good result and comfortably meets typical design guidance (≤10 Ω); it should not be used as an argument either way about protective conductor performance.

Bonding conductors

The main protective bonding conductor to each incoming metallic service must be at least half the cross-section of the largest main protective conductor and need not exceed 25 mm², with a minimum of 6 mm² generally and 10 mm² for a PME supply. The 25 mm² to the gas pipe is correct. The 6 mm² to the structural steel is not defensible: the steel is an extraneous-conductive-part and a natural lightning component, it is part of the equipotential zone, and it should be bonded with a conductor to the same rule, practically 25 mm² or the structural equivalent, with a tested connection.

The 'clean earth'

This is the serious finding. Under TN-C-S the equipment protective earth is the PEN, so a server-room bar connected only to a separate rod is not a protective earth at all. It creates a second, unequally bonded earth within one installation. The server's protective conductor from its sockets, its signal cable screens and its metalwork all reference the main earthing terminal, while the local bar sits at a different potential. The correct arrangement is a single main earthing terminal with a common bonding network, a dedicated bonding bar in the server room connected to it by a properly sized conductor, and attention to cable routing and signal reference within that common network - not isolation.

Record the finding formally; this is a danger, not a preference.

Selection and location

• Origin of installation: Type 1 SPD, because the building has a lightning protection system using natural components. For LPL III the per-pole impulse current to design for is typically 12.5 kA (10/350 µs) - confirm against the risk assessment and the manufacturer's declared Iimp.
• Floor distribution boards: Type 2 SPDs, coordinated with the origin device by energy (the origin Type 1 must be rated to take the partial lightning current and the Type 2 must take the residual), and separated by at least the manufacturer's stated cable length, typically 10 m, to allow the first stage to operate.
• Server room: Type 3 SPD at or near the equipment, on the final circuit, in addition to the upstream Type 2.

Parameters to check

1. Voltage protection level Up versus the equipment's rated impulse withstand Uw, with margin for the cable run to the equipment - a common rule is Up ≤ 0.8 × Uw at the equipment terminals, so if the SPD is more than 10 m away the inductive drop in the connecting conductors must be added, not assumed away.
2. In and Imax of each stage, and the Iimp of the Type 1, coordinated with the upstream device's let-through.
3. Short-circuit withstand of the SPD against the prospective fault current at its location, and the rating of its associated disconnector or fuse, including whether the SPD fails safe.
4. Status indication and remote signalling to the BMS - an SPD that has failed silently is worse than none, because the installation is believed protected.
5. Connection length: the SPD's connecting conductors should be short and straight (target under 0.5 m) to keep the let-through voltage low.

The omission at the server room is the weakest point of the current proposal and the one most likely to be exposed by an actual event.

During a strike

The lightning current flows to earth through the natural down conductors and the foundation earth. The impedance of that path, plus the inductive term L·di/dt, raises the potential of the whole earthing system - the main earthing terminal, the structural steel, the bonding network and every bonded service - by a large transient voltage for the duration of the strike.

The isolated server-room bar, connected only to its own rod, stays locally near earth potential. But the server is not isolated: its protective conductor from the socket, its data cable screens and its metal casework are all referenced to the main earthing terminal through the installation. So the full potential difference of the strike appears across the server's own earth connection - from its signal reference to its protective earth. That is precisely the condition the 'clean earth' was intended to prevent, and it is a flashover or component-destruction mechanism inside the most sensitive room in the building.

A second hazard: the separate rod provides a parallel path to earth at a different potential from the main electrode, so strike current can flow through the building fabric or through cable screens between them, with step and touch voltage risk along the route.

Advice to the client

The isolation must be removed and the bar bonded to the main earthing terminal, with the bonding conductor sized to the same rule as other main bonding and routed as directly as practicable. If the IT equipment has a genuine signal-reference requirement, it is met by a common bonding network with a clean, low-impedance, single-point reference within it - not by separation. Recommend the corrective work be completed and tested before the server room is occupied, and that the lightning protection system's continuity and separation distances be verified at the same time, since the rebar down conductors must be electrically continuous and the separation distance to internal services must be checked for the height involved.