Distribution boards in commercial buildings: protection requirements and the division of responsibility

The distribution board of a commercial building performs two functions absent from residential installations: it protects people who do not know the installation and are under no obligation to know it, and it defines the technical boundary of responsibility between landlord and each tenant. The second function is the one most often treated superficially, and the consequences typically become apparent only after an incident.

1. Arc fault detection in publicly accessible areas

A series arc fault — the intermittent contact produced by a loosened connection or a damaged conductor — generates neither an overload nor an earth fault current. It is therefore detected by neither the circuit breaker nor the residual current device. The energy dissipated locally produces a sustained temperature rise capable of initiating a fire.

In commercial premises, the factors that make this scenario more probable than in residential buildings are structural: cable routes run through suspended ceilings and inaccessible voids, background noise levels are high, and the building is unoccupied outside trading hours. The indicators a householder would notice — smell, localised heating, audible buzzing — reach nobody.

The arc fault detection device (AFDD) is the only protection that responds to this class of fault. Recommended order of priority when specifying:

• Lighting circuits in publicly accessible areas — luminaires installed in suspended ceilings above occupied space, without routine visual inspection.
• Socket circuits accessible to the public — where neither the connected appliances nor the condition of their flexible cords is under the operator's control.
• Loads connected directly at the board — machinery, refrigerated display units, air conditioning, supplied through long fixed routes frequently embedded in the structure.

Combined devices are available providing short-circuit, residual current and arc fault protection in a single module. Unit cost is higher and the space occupied in the board is lower, which is a material consideration when upgrading a board with no spare ways.

2. Metering and an isolating device per lettable unit

Each unit intended for letting requires two distinct provisions, frequently conflated in practice.

Individual metering. Apportioning consumption in proportion to floor area does not reflect actual demand: a unit with a commercial kitchen and an office unit of the same area differ by an order of magnitude. Proportional apportionment transfers cost from one tenant to another and constitutes a predictable source of contractual dispute.

A dedicated, accessible isolator. An isolating device on the supply to each unit, appropriately labelled and positioned so that it can be operated without entering another tenant's demise. Its function is not metering but the ability to isolate for maintenance: an electrician attending at the tenant's request must be able to make the unit dead without interrupting the whole building and without depending on a third party's availability.

3. Earthing and equipotential bonding

All exposed and extraneous conductive parts are bonded: structural steelwork, ventilation ductwork, water and gas services, cable containment and, where applicable, metallic facade elements. In a building where systems have been installed by different contractors in different phases, the potential difference between metallic parts that can be touched simultaneously is the principal mechanism by which electrical accidents occur.

Verification of the earth electrode resistance and of the continuity of the protective conductor precedes any technical proposal. Installing a new board over a non-compliant earthing arrangement does not improve the safety level of the installation.

4. Residual current sensitivity: 300 mA and 30 mA are not interchangeable

On the main supply and principal distribution circuits of a commercial building, the landlord commonly installs residual current devices rated at 100 mA or 300 mA. Their function is protection against fire caused by earth fault currents, by limiting the period during which an insulation fault dissipates energy into the building fabric.

Installing a 30 mA device on the main supply of a building is not technically achievable. Cumulative natural leakage currents — EMC filters in switch-mode supplies, variable speed drives, long cable runs, large numbers of loads — exceed the operating threshold in the absence of any fault. Discrimination would additionally be lost: a fault confined to a single unit would interrupt the supply to every tenant.

The division of responsibility

The customary allocation of technical obligations is as follows:

• The landlord provides a supply to the let unit, sized for the contracted capacity, with individual metering, an accessible isolating device, and fire-type residual current protection (100 or 300 mA) on the general distribution. Responsibility customarily ends at the point of supply.
• The tenant is responsible for the installation within the demise: their own board, the division into circuits, 30 mA residual current protection for the protection of persons, AFDDs where justified, and sizing appropriate to the equipment they introduce.

The common misreading, on the tenant's part, is the assumption that residual current protection existing at building level also covers their internal installation. Between the landlord's 300 mA device and the sockets used daily by staff there is no protection of persons unless the tenant has installed it.

The technical recommendation to both parties is that the boundary be recorded contractually: the location of the point of supply, the protective devices upstream of it, the contracted capacity, and the allocation of responsibility for the installation downstream.

5. Survey and measurement before quotation

For a commercial building, the first stage is not the quotation but the survey: identifying the loads served by each outgoing way, an inventory of existing protective devices and their sensitivities, verification of the earthing arrangement, and measurement of actual demand at peak.

In commercial premises, the difference between declared connected load and measured demand is significant in most cases. The scope of works, its phasing and the duration of interruptions are established on the basis of this data.

Related services

• Electrical panels
• Commercial electrical installations
• Test and measurements