This is one of the most important MGN notices for electrics at cadet/office of the watch level in my opinion; very safety orientated, and due to actual incidents caused by ignoring these simple but key guidances. Have a read, and feel free to ask any questions about it. 
http://www.dft.gov.uk/mca/mgn177.pdf

http://www.dft.gov.uk/mca/mgn177.pdf