When a fault occurs in a three phase system,the forces acting upon all three conductors balance each other. In other words,the system is a closed one and no external force will be produced.
This can best be demonstrated by examining the forces on each conductor of a three-phase busbar system which is arranged to have all three busbars in horizontal alignment, as shown in Figure.
- The direction of the forces in the outer phases is largely outwards.
- The direction of the forces in the centre phase is, more or less, equal in both directions.
- The frequency of the forces is twice that of the power frequency.
- At any one instance in time the forces in each direction are equal and opposite
It follows then that the switchgear designer should arrange for any insulation bracing the busbars to be in line with the direction of the expected electromagnetic forces
whenever possible. It is also good to examine all insulation supports carefully after carrying out short-circuit type tests, and not just rely upon a power frequency
withstand test on the insulation.The rate of strain imposed upon the insulation is very high, typically reaching a peak in 5 ms.