^Paul Cook (2002). Commentary on IEE Wiring Regulations 16th Edition, BS 7671 : 2001: Requirements for Electrical Installations Including Amendment No. 1 : 2002. IET. p. 2. ISBN978-0-85296-237-4.
^Hermann J. Koch (2017). Practical Guide to International Standardization for Electrical Engineers: Impact on Smart Grid and e-Mobility Markets. Wiley. p. 90. ISBN978-1-119-06743-6.