normally that is used to indicate that it is areturn live wire from the switch.
usually a 2 core cable is used consisting of a black and a red wire.
so to mark that it is in fact a live wire and not a common one it is marked in some way ie red sleeving,red tape etc
normally that is used to indicate that it is a return live wire from the switch.
usually a 2 core cable is used consisting of a black and a red wire from the switch to the ceiling fitting
so to mark that the black wire is in fact a live wire and not a common one it is marked in some way ie red sleeving,red tape etc
Look at the upper picture under the section titled 'Two Wire Method'. http://diydata.com/planning/electric_lights/el ectric_lights.php
(scroll down the page)
Your 'black one with the red sleeving on' is the one on the left of the diagram, the cable coming from the switch (it is not shown with sleeving on in this diagram).