When contacts are moving towards each other, a point will be reached where the
dielectric strength of the remaining contact gap is unable to withstand the voltage
stress that is being imposed upon it, and electrical current will start to flow. This will
result in a measure of arc erosion of the contacts taking place. At the instant when
the contacts initially touch, there will be no contact load, and further erosion of the
contacts will take place. This contact erosion will continue until the minimum contact
loads to prevent burning are established. The current will have started at zero, at the
instant of initiation of pre-arcing, and will have risen as a sinusoidal wave regardless
of the contact loading and any contact burning. It is, therefore, important that the
speed of contact closure is as fast as possible, and that contact loading is established
as early as possible. However, as with most things in engineering, a compromise
needs to be made between minimising pre-arcing by minimising the time to close and
provide full contact loading, on the one hand, and the life, size and cost of the circuit
breaker on the other. It must be recognised that pre-arcing cannot be eliminated and,
therefore, contact closure under short-circuit fault conditions will always lead to a
measure of contact erosion.