by means of a melting bath earthed by bottom electrodes offers great safety for operating personnel, and can sometimes also give an indication of the condition of the crucible. In this system, the insulation value between the voltage-carrying furnace coil and the earth potential is determined by means of direct current measurement. In the case of proper electrical insulation between the coil and the iron piles, and the high-current system, the power supply and the earth potential, a low insulation value can be attributed to the resistance between the coil and the earthed melting bath, and thus also the condition of the crucible (page 98).
In the case of a newly concreted coil installation with coil plastering, the poor resist- ance value is usually due to the high moisture content. At resistance values below 400 Ohms, a crucible induction furnace should therefore not be operated at above 1,000 Volt operating voltage. From 400 – 700 Ohms, a level of 1,500 Volt should not be exceeded, since in addition to the measured “earth short” due to moisture between the coil and earth, the winding voltage can also lead to winding shorts. At approx. 60 Volt winding voltage and mains frequency, there is a winding insulation of 3 mm. In the moist condition, the winding voltage should be no more than 10 V/mm, or in other words 30 Volt. In the case of medium-frequency furnaces, one has winding voltages of up to 270 Volt and 15 mm of intermediate layer insulation, so that consequently a winding voltage of 150 V should not be exceeded.