Switching on of crucible induction furnaces is carried out as a rule by hand, or automatically with the use of processor controls. Mains frequency systems are usually connected in 3-phase to the 3-phase power supply system. By means of Steinmetz switching, the 3-phase system is also loaded equally over all 3 phases in the event of 1-phase connection of the induction furnace. Depending on the load on the system, the out-of-balance load of the 3 phases amongst each other may also be up to 10% of the maximum phase current. At phase currents above approx. 500 A, the balanced phase is switched on in advance of the field, and a furnace current is switched via a switch-on resistance with reduced switch-on current. A short time after this switch-on process, the bypass contactor is switched in to short-circuit this switch-on resistance. By this means, the high switch-on current of approx. 1,500 A can be reduced to about one third of this level. The starting of a mains frequency furnace via the individual transformer steps offers no benefits, since every individual step is ultimately switched on at full load. The main disadvantage is the multiplication of the hysteresis at the main and bypass contactors.
In the case of medium-frequency systems, the current is brought up, in accordance with the specifications, from 0 to the maximal value of the required current by means of a so-called start-up ramp. For example, some customers have this process set so that in the 1st switch-on phase, the output is brought up to 1,000 kW and then after approx. 10 seconds to the maximum output of 3,200 kW. As soon as the electrical power is switched on, the energy is available in the form of heat, in the same way as a flow heater. For this reason, and before switching on an induction furnace, the water supply to the furnace coil with sufficient volume of water at the right temperature must be ensured.