Overview

It is a fundamental phenomenon of electricity that a force is exerted between conductors carrying electrical current. Under normal load current conditions, these forces are very small, however, many engineers will not be aware of the enormous forces that are generated when the normal current is replaced by a short-circuit current which can be 40-50 times larger in magnitude. This force generated phenomenon forms the basis of many desirable aspects of electrical engineering, such as the operation of measuring instruments and electrical motors, but in switchgear these forces are potentially dangerous in terms of the stresses induced in both the conductors and their supporting insulators.

Two factors influence the magnitude of the electromagnetic force that will be experienced. These are the strength of the magnetic field and the current flowing. The field strength can be derived from Laplace's Law. This states that the field strength created at a point in space due to the passage of electric current through a conductor is inversely proportional to the square of the distance between that point and the conductor, and is directly proportional to all other factors. The force experienced by a conductor in a magnetic field is derived from Biot Savart's law in that the force is proportional to the flux density and the length of the conductor.

Figure 5.1 shows two parallel conductors, length L (m) and spacing D (m), one carrying the return current of the other in a circuit. It will be seen that the conductors experience...