Various legislation and regulations govern practice concerning electricity in buildings. As with most regulation and standards covering buildings, the prime purposes are for health and safety, both during construction and when the building is occupied.

Electricity has the potential to kill if systems or equipment is incorrectly installed or maintained. The number of regulations has grown in recent years to include other aspects such as electromagnetic compatibility and power quality (e.g. voltage and frequency limits).

The guide provides a list and brief description of the most important legislation relating to electricity systems currently in force and includes a description of BS 7671, usually better known as the IEE Wiring Regulations.

Load assessment

Modern buildings contain an increasing number of electrical systems and equipment which has in turn increased the demand for electrical power. Operating load requirements must be estimated realistically if the electrical distribution system is sufficient to handle current and forecast loads.

With new building projects, detailed information about the equipment to be installed is unlikely to be available. Nevertheless, the building services engineer must produce load estimates and liaise with the electricity supply company to ensure that the power supply is sufficient for the major elements of plant and service equipment specified.

The guide provides advice and guidance on assessing the load within the building. It describes the load factor, load assessment, load calculations and diversity and demand. There is also a subsection on harmonics generated by equipment such as inductive luminaires, computers and uninterruptible power supplies.

Off-site power supply and on-site power generation

Off-site supplies are still the main source of electrical power in buildings. However, there is increasing interest in on-site generation which has the potential benefit of providing surplus electricity for sale to the National Grid.

The guide describes the regulatory environment of electricity supply following the Electricity Act 1989 and briefly discusses the wholesale electricity market and tariffs. It also covers sub-station planning and construction and the options for on-site generation including testing and commissioning.

On-site generation provides several advantages. It can give security of supply in the event of disruption to or failure of the public supply; the use of renewable energy such as photo-voltaics (PV) and wind generators enables at least a proportion of the electricity to be generated through sustainable means; and combined heat and power (CHP) installations provide a highly energy-efficient solution for buildings.

The guide describes the different types of generator available and gives extensive information of system selection and all the other considerations necessary, such as space, environmental requirements and choice of primary fuel.

Uninterruptible Power Supplies

The need for clean, reliable power is increasing and is being met today by the provision of uninterruptible power supply (UPS) systems. Traditionally, they were used to protect vital services such as airport lighting, communications systems and mainframe computers. Their purpose is to cover the time period between the cut-off of external power supply and the start-up of standby generators. Today they are used for much wider protection, including critical building services such as lights in elevators and smoke ventilation controls. The guide describes the basic types of UPS system, system selection and battery/stored energy selection.

Earthing and bonding

These are two of the most important aspects of electricity for plumbing and heating contractors. Whilst this work is normally left to qualified electricians, it is useful for HVAC technicians and engineers to understand the issues involved. Earthing is the process of providing an electrical path to the general mass of earth for those conductive parts of an installation (e.g. metal casing) which are not normally subject to a voltage or electrical charge.

In some instances earthing may be applied also to certain conductive parts which will normally carry a small and controlled voltage or charge. Bonding is the related process of interconnecting such conductive parts to ensure that even under fault conditions the voltage differences between adjacent conductive parts are restricted to safe levels. It should be noted that bonding does not ensure that adjacent surfaces remain at the same potential under all conditions. Earthing and bonding have three main objectives:

• Safety: which is achieved primarily by limitation of the voltage which may appear on conductive surfaces under fault conditions and by limiting the period of time for the condition to persist.

• Protection of buildings, plant and equipment: which is achieved partly by voltage limitation but mainly by ensuring that fault currents are sufficiently large to produce effective operation of fuses or circuit breakers.

• Correct and precise operation of equipment: which is achieved by providing a constant and noise-free electrical reference plane.

There are various regulations, standards and definitions covering earthing and bonding, including: the Electricity at Work Regulations 1989; BS 7430:1998 Code of practice for earthing; BS 6651:1999 Code of practice for protection of structures against lightning; BS 7671:2001 Requirements for electrical installations - IEE Wiring Regulations. Sixteenth Edition; BS 7361:Part 1 1991: Cathodic protection - Code of practice for land and marine applications.

By convention the general mass of earth is taken to be at zero voltage. Connecting the neutral point of an electricity supply system to earth therefore creates a reference plane against which all system voltages are measured and limits the possible rise in potential of any conductive surface.

The mass of earth, however, is not a sink. Current flow into it from an installation will flow out at the supply point neutral earth connection(s), to complete the current circuit. Because it is a reasonably good conductor the earth performs this function quite well. Enhanced performance is obtained by the provision of suitable protective conductors between conductive surfaces and the source neutral, as much lower and more controllable circuit impedances can be achieved.

Electromagnetic compatibility

The world is bathed in electromagnetic waves. Much electrical equipment relies on receiving and transmitting electromagnetic radiation and the sensitivity of such equipment to extraneous radiation and the amount of ambient radiation must be compatible. Furthermore, there are statutory requirements to avoid causing interference to radio and television systems. It is important to minimise the response to extraneous signals and to control or eliminate the coupling of electromagnetic interference to electronic circuits and components. This is achieved through ensuring electromagnetic compatibility (EMC), the ability of an electronic component, unit or system to co-exist with its electromagnetic environment.

As electronics finds wider usage in a diversity of applications, the importance of ensuring that EMC is achieved will continue to increase. The UK’s Electromagnetic Compatibility Regulations 1992 came into force to implement the European Directive on EMC. The Guide will assist electrical engineers to provide electrical building services installations that comply with the requirements of these regulations.

Guide K is a 146 A4-page document that will be a useful resource in the offices of a wide range of users. It is not a text book but will be a useful reference source for a variety of professionals.

The new publication, ‘Guide K Electricity in buildings’ is available from CIBSE Publication Sales on 020 8772 3618 or online at www.cibse.org/publications The cost is £22 (plus P&P) for CIBSE members and £86 (plus P&P) for non-members.

Richard Cleaver is a Partner at Ridge and chaired the CIBSE Guide K task group.

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