1st isolate circuit and prove isolation
2nd disconnect the cables from the pull switch, becareful not to touch the neutral(black) and the copper(green and yellow sleeved) conductors together.
strip the cable off which is burnt back to a point where it can be made off again and cut off the damaged bit of cable.
reconnect the new pull switch as per the connections you disconnected it from
secure the pull switch back up.
turn the power back on and test.
chris gray, February 2010