I am replacing my 500 watt external security fitting as had problems with the previous one. My supply cable has 4 wires, the normal 3 plus a black wire. The black wire went into the connector block and matched with a double white wire. The new fitting doesn't have white wires and the instructions only tell you do connect the normal 3 to the unit. What is the black wire for and what should I do with it? Needless to say I didn't put the original unit in otherwise I wouldn't be asking!!