[2]Electrical Inspection Manual, 2011 Edition], Noel Williams & Jeffrey S Sargent, Jones & Bartlett Publishers, 2012, p. 249 (retrieved 3 March 2013 from Google Books)
[3]17th Edition IEE Wiring Regulations: Explained and Illustrated], Brian Scaddan, Routledge, 2011, p. 18 (retrieved 6 March 2013 from Google Books)