This post explains the use of electronic fencing equipment, and how they work.
Fencing is a fast sport, and with the rapid movements and constant whirring of blades, it takes a keen eye to follow the bouts and judge them correctly. Luckily for referees, the invention of electronic scoring equipment came about and is now used in tournaments, as well as national competitions, including the Olympics.
Equipment

Body cords - in a bout, each fencer is connected to a body cord
that runs up through their jacket, out the sleeve of their sword arm, and to the special plug on the inside of the bell guard of the electric weapon. The other end of the cord is attached to the reel that runs to an anchor at either end of the strip, which runs back to the scoring machine located parallel to the center of the strip. [1]
Scoring machine - the machine has four lights - two white lights and one colored light for each fencer. When a touch ensues, a buzzer goes off and a light lights up on the side of the fencer who was touched. If both lights go off at the same time, the referee determines who touched first,
or in the case of foil and sabre, who had right of way. [1]
Electric Scoring
Foil
For foil, the fencer must wear a lamé made of a metallic material that acts as a conductor for an electrical current. The tip of the electric foil has a button on the end so that when a certain amount of force (0.1lb) is applied, the button depresses, activating the current. When a touch lands on an opponent's lamé the scoring machine lights up, indicating a valid hit. If the touch was off-target (not on the lamé), the scoring machine would light up white, indicating an invalid hit. [1]
Épée
For épée there is an open circuit set up by the system, which is closed when a touch is landed. If the tip touches the opponent's bell guard or the piste nothing happens, but otherwise when a touch is landed the buzzer goes off and a colored light lights up, as there are no off-target hits in épée. [1]
Sabre
As all hits above the waist are valid in sabre, the fencer must wear a lamé that covers the torso and arms, as well as a metal mask and a metal glove. The electric system is a closed circuit like foil, but as sabre is a slashing weapon the entire blade is hot and the target lamé closes the circuit connection. The scoring is the same as for foil, except that off-target hits don't register on the machine. [1]
New modern, wireless scoring systems now eliminate the need for the constant tug of the reel connecting to the body cord, as a wireless receiver-pack was created that is worn on the back of the fencer's waist, about the size of a cigarette pack. [1]
Sources
1. How Fencing Equipment Works. howstuffworks.com. Retrieved 2013-12-3.
