One danger of a sparking switch is that it will generate heat and start to burn the plastic on the fitting - this will form carbon which is a conductor of electricity and could lead path for a sock.
A spark is normally a small speck of metal which is on fire such as from a grinder working away on iron. Electrical discharges do not jump very far and in the case of 240 volt ac mains this produces a peak voltage of approximately 350 volts which will jump only a mm or so in dry air as the current is switched ... it is also possible that the current is switched as the voltage passes thro' a polarity change (reversing current direction) when no spark will occur as the switch is opened.
To check on sparking hold a portable radio near the switch as you operate it and listen for the buzz.
best wishes Nyge C
March 2011