Heatshrink Tubing 3/32" Diameter is the safest way to seal two wires that have been soldered together.
Significantly better and more secure than using black electrical tape. Simply slide a small section of heatshrink tubing over your soldered joint, apply heat from a small flame, soldering iron, hair dryer or virtually any type of heat approximately 100°C and this tubing will shrink around your solder joint, effectively insulating it!
Sold by the foot. Most solder joints utilize about 1.5" of this tubing.