Equipotential grounding: reducing electrical risks in commercial environments

Mapping the metal parts is the first technical step. Kytom’s field experience identifies four areas where failures concentrate on French commercial sites.

• Technical rooms: main LV distribution boards, landing risers, metal cable trays. Minimum 16 mm2 copper cross-section for the main bonding according to NF C 15-100 §411.3.1.2.
• Wet areas: sanitary facilities, kitchenettes, cleaning rooms. Supplementary bonding mandatory on metal pipework, cross-sections of 2.5 to 6 mm2 according to NF C 15-100 §415.2.
• Roof terraces: chiller units, air handling units, telecom antennas, metal guardrails. Continuity with the building’s earth electrode is essential.
• High electrical density areas: server rooms, UPS rooms, data rooms. Reinforced 25 mm2 bonding and specific meshing recommended by standard NF EN 50310 for IT installations.

The main trade-off pits comprehensive coverage against installation cost. The budget impact of equipotential bonding varies significantly depending on the density of technical equipment on the floor. On standard commercial floors of less than 300 m2 with no dedicated technical room, no exposed metal pipework, and supplied via an existing, already compliant floor LV distribution board, the additional investment in secondary supplementary bonding becomes marginal and can be set aside following a case-by-case analysis. Beyond the scope required by NF C 15-100, extending the meshing to modular partitions and low shelving does not reduce the risk in any measurable way and adds 4 to 6 EUR/m2 to the budget with no documented safety benefit. In these configurations, strict compliance with main bonding and wet areas is sufficient, contrary to the widespread practice of systematically bonding all accessible metal parts.

3 design errors responsible for the majority of rework

Three recurring defects undermine the effectiveness of equipotential installations on the commercial sites audited by Kytom. On the commercial sites audited by Kytom, these three families of defects account for the majority of electrical rework identified before delivery.

1. Omission of secondary metal parts: wall-mounted metal shelving, steel-frame modular partitions, suspended ceilings with exposed framing. This equipment, added during the fit-out phase, frequently falls outside the initial scheme.
2. Undersized conductors: use of 4 mm2 cross-sections where the fault current calculation requires 10 or 16 mm2. Excessive resistance creates dangerous touch voltages in the event of an insulation fault.
3. Discontinuity at crossings: unbonded junctions at changes of support, insulating flanges overlooked on mixed pipework.

Best practice consists of establishing a comprehensive scheme of metal parts from the design phase, sizing according to calculated rather than estimated fault currents, and providing accessible connection boxes every 15 to 20 metres. A loop impedance measurement check before handover validates regulatory compliance and avoids post-delivery rework, the cost of which systematically exceeds that of integration during the works phase.

Methodological limitation: this overrun is all the more pronounced on buildings delivered with sealed raised floors, poured concrete slabs or permanent plasterboard partitions, where the trade-off shifts towards a negotiated acceptance of the residual defect with the inspection body, rather than destructive rework.

For the CFO and Asset Manager: turning equipotential risk into a CAPEX/OPEX trade-off

From a financial management perspective, equipotential bonding is not an electrical topic but an asymmetric risk item to be provisioned for. Three indicators structure the investment decision.

• Controlled initial CAPEX: 8 to 15 EUR/m2 integrated during the design phase on a standard floor, i.e. 4,000 to 7,500 EUR for 500 m2. On buildings with high technical density such as data rooms and server rooms, the budget rises to 11 to 21 EUR/m2, with a significant increase linked to equipment complexity and the density of bonding to be carried out.
• Post-delivery rework cost: 24 to 75 EUR/m2 depending on accessibility, i.e. a factor of 3x for accessible rework up to 8x for destructive rework. For an Asset Manager, this delta represents a risk provision to be recorded in the as-built documentation if the initial design has not been validated by an independent audit.
• Rent foregone during rework: rework on a sealed raised floor immobilises 200 to 400 m2 for 2 to 4 weeks. At 350 EUR/m2/year for average Paris-La Defense commercial rent, the vacancy represents 1,350 to 5,400 EUR of lost rent per 200 m2 band.

The favourable ratio is built during the design phase: an audit of metal parts on a floor of 500 to 1,000 m2 secures a works budget out of all proportion to its cost, by making the entire electrical envelope reliable. For an Asset Manager weighing up several assets, the absence of a comprehensive equipotential scheme in the as-built documentation is a signal of residual risk to factor into the valuation, in the same way as the tertiary decree or the energy performance certificate.

Kytom’s 5-phase method: from audit to compliance report

The method applied by Kytom’s technical teams structures the intervention across five sequential phases, with a very low rework rate observed on recent projects.

• Phase 1, audit of metal parts: comprehensive inventory of accessible metal elements, identification of the mandatory connection points according to NF C 15-100.
• Phase 2, sizing: calculation of cross-sections according to fault currents and bonding lengths, integration of routing constraints such as suspended ceilings, technical risers and raised floors.
• Phase 3, wiring scheme: main hierarchy towards the main earth terminal, as-built drawing integrated into the as-built documentation.
• Phase 4, controlled execution: visual inspection of connections, traceability of installed cross-sections, hold points at critical crossings.
• Phase 5, compliance report: loop impedance measurement, certificate of compliance and transfer of ten-year civil liability to the inspection body, among Apave, Bureau Veritas, Socotec or Qualiconsult depending on the project.