Appendices explained

Topic Progress:

Appendix 1: British Standards Referenced by the Regulations

Appendix 1 is described as normative, all other appendices are informative. This appendix lists all the British Standards which a referred to in BS7671.

Normative means that this Appendix provides reference to British Standards.

Informative (for all other appendices) means that these appendices provide additional information and help you to understand the regulations

Appendix 2: Statutory Regulations and Associated Memoranda

Electrical installations are to comply with not only BS 7671 but to specific Statutory Regulations. Refer to Appendix 2 for the up to date list of Statutory Regulations

Appendix 3: Time / Current Characteristics of Overcurrent Protective Devices and Residual Current Devices

Appendix 3 provides the Time/current characteristics of overcurrent protective devices and RCDs.

For the 18th Edition characteristics of earth fault loop impedance have been moved from Appendix 4 (in the 17th Edition), to Appendix 3.

The characteristics are provided in the form of graphs and by plotting time in seconds against prospective current in amperes. By referencing the graphs the disconnection time of the selected device can be ascertained when the current passing through the device is known, conversely the current required to disconnect the device in a given time can also be known.

These graphs use a logarithmic scale and this must be kept in mind or mistakes or miscalculations may be made. For this reason a table has been introduced for quick reference.

The information within these graphs, are interrelated onto various other tables throughout BS7671 and also in the IET Onsite Guide.

Appendix 4: Current-Carrying Capacity and Voltage Drop for Cables and Flexible Cords

This section was significantly changed in the 17th edition from the 16th edition, so if you are a bit behind the curve, care is required in the reading of this appendix.

Appendix 5: Classification of External Influences

The Classification of External Influences is noticeably mentioned throughout BS7671 and are intended to give the classification and codification of the external influences of the installation

The First letter relates to the general category of the external influence.

The Second letter relates to the nature of the external influence.

The number relates to the class within the external influence, i.e. AD4

‘A’  represents the Environment:

  • Ambient temperature
  • Temperature and Humidity
  • Water
  • Foreign bodies
  • Corrosion
  • Impact
  • Vibration
  • Flora fauna
  • Electromagnetic
  • Solar
  • Seismic
  • Lighting
  • Lightning
  • Movement of air
  • Wind

‘B’ represents the Utilization of the building:

  • Capability of the occupants
  • Contact of the occupants with Earth potential
  • Conditions of evacuation in an emergency
  • Nature of stored materials

‘C’ represents the construction of the building:

  • Construction — combustible or non-combustible
  • Building design

It is recommended that installers and designers become familiar with these classifications. There is also a high probability of several exam questions occurring concerning external influence codes.

Appendix 6: Model Forms for Certification and Reporting.

The forms required for the Certification of Electrical Installations are:

  • The Electrical Installation Certificate
  • The Minor Electrical Installation Works Certificate
  • Electrical Installation Condition Report

Further information on inspection and testing and periodic inspection, testing and reporting can be found in IET Guidance Note 3

As mentioned in Part 6 Testing and Reporting of Electrical Installations is covered more thoroughly in the 2391 Inspection and Testing qualification.

Appendix 7: Harmonized Cable Core Colours

This section indicates the colour identification and harmonization of conductors and the interconnection with existing installations.

Appendix 8: Current-Carrying Capacity and Voltage-Drop for Busbar Trunking and Powertrack Systems

The rating factors which apply to current-carrying capacity have been changed. The use of Greek character for the subscript of k (a 17th Edition rating factor could be indicated by kα) is replaced by letter/number combinations, for instance k1A

Appendix 9: Definitions — Multiple Source, DC and Other Systems

Covers specialized supply systems.

Appendix 10: Protection of Conductors in Parallel Against Overcurrent

Provides information and guidance in relation to overcurrent protection (for overload and short-circuit currents) where conductors are connected in parallel (normally larger current designs).

Appendix 11: Effect of Harmonic Currents on Balanced Three-Phase Systems

Not used

Appendix 12: Voltage Drop in Consumers Installations

Not used

Appendix 13: Methods for Measuring the Insulation Resistance/Impedance of Floors & Walls to Earth or to the Protective Conductor System

This is a specialized subject and is not addressed in everyday testing and reference is to be made to the IET Guidance Note 3 concerning this procedure.

Appendix 14: Determination of Prospective Fault Current

Regulation 434.1 requires the prospective fault current to be determined at every relevant point of the installation.

Zs (m) is the measured impedance of the earth fault current loop up to the most distant point of the relevant circuit from the origin of the installation (Ω)

Uo is the nominal AC rms line voltage to Earth (V)

Ia is the current in amperes (A) causing operation of the protective device

Cmin is the minimum voltage factor to take account of voltage variations depending on time and place, changing of transformer taps and other considerations.

In a single phase system the prospective fault current is the greater of either the fault current between the line conductor and neutral or the fault current between line conductor and Earth

Appendix 15: Ring and Radial Final Circuit Arrangements

Sets out options for the design of ring and radial final circuits for household and similar premises

Appendix 16: Devices for protection against overvoltage

Refer to Appendix 16 for information on the installation of surge protective devices (SPD)

Appendix 17: Energy efficiency

New for 18th Edition. Refer to the separate topic.


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