Epee overview
Epee developed from the dueling swords that
developed in the
court of Louis XIV of
France. In Epee the weapon used is a maximum
of 770grams, with a maximum length of 110cm. The
valid target area is the whole body, head to toe, including
arms and hands. Any hit upon the earthed piste or on the
opponent’s grounded weapon will not cause the apparatus to
register. Epee evolved from the sword, favored by duelists of the
time. It follows that, in modern fencing, the whole body is valid
target area for epee, and that the basic rule of engagement is
that the first to hit his or her opponent, has scored. Training for
sword fighting (for dueling or combat purposes) concentrated on
thrusts to the torso, where hits would have the most lethal effect.
This training led to the development of blunt lightweight weapons, protective
clothing, and limited "valid" target areas, which are now applicable to foil
fencing.

How a hit is made.
The Epee circuit is normally "open" and is "closed" when the point is depressed, registering the hit. The epee point travels in and out against the pressure of the larger weightspring and carries with it a smaller contact spring. See the parts section here for details. When the contact spring touches the ends of the two blade wires it closes the circuit. The Regulations for Competitions require that the weightspring must be able to support a 750 g weight before a hit is registered and that the contact spring must not close the circuit until the last 0.5 mm of its travel. In addition, the total travel of the point must be at least 1.5 mm

1 In the normal state there is no connection through the apparatus. This state produces no light and would be the state of the circuit as the fencers come on guard.
2 When the point is depressed by any surface except the opponents guard or a connected metallic floor (piste) the circuit is made and a red or green "hit" light comes on.
3 When the point is depressed at the same time as it is in contact with the opponents guard or piste a new circuit is made. This new circuit causes no light to come on.
As well as being electrical, the system also has a mechanical component. That is, moving parts and electronics working in conjunction cause the lights to come on.

The nature of the system means that both mecanical and electrical faults can cause the system to function incorrectly.