are always required when several furnaces are to be supplied from one or more energy supply systems. The simplest form of switching is furnace current switching from one switching system to 2 following furnaces. An alternative here would be effective current switching, with a condenser bank for each fur- nace. The furnace switching system with the optimum “reserve” for the furnaces and the energy supply is the so-called busbar distributor. With this system, every furnace can be switched to every power supply. This requires the same number of furnaces as energy supply systems, together with the corresponding number of effective current switches for each furnace. In the case of more than 3 furnaces, these systems require too large and complex a busbar system, so that from 4 fur- naces, a 2 x 2 furnace system is used.
In mains frequency systems, the current to the furnace is approx. 4.5x higher than the effective current. 250 Hz systems have a current to the furnace approx. 7.5x higher than the effective current, while for 500 Hz systems, the current is approx. 11x higher than the effective current.
As an alternative to these switching systems, other systems have been developed with unstepped distribution of the output to 2 or more furnaces. These systems also have the advantage that 2 furnaces can be supplied with energy simultane- ously.
Electronic switching has not come into such common use, since in this case only 1 furnace can be supplied with energy at any one time. The switching off and on time is approx. 1 second.