Please note: Firstly If you are not sure about anything written in my notes, lesson plans or other material then I would suggest that you refer to the appropriate British standard or IEC documentation. These course notes are not copyright, however, there maybe parts that have been cribbed from others, for which I will gladly acknowledge ownership, therefore you can use them for personal use in whichever way you like except for financial gain. However, I trust that they will be used to edify your educational needs!