Coil short-circuit is a short-circuit between 2 parallel windings, which can occur due to an insulation fault. If for example there is too much damp or moisture in the area of the intermediate layers due to inadequate drying, the voltage must be reduced to a low level, so that no leak currents can flow. As a rule, such leak currents are only very low, and this will not lead immediately to short-circuits. In the long term however, a dangerous point will be created, which can be further damaged with every new lining.
If now in the course of the coil’s life, e.g. after 18 months operating time, and under moist weather conditions and formation of condensation on the coil copper, the coil is activated at too high a voltage (> 1,000 V), this can lead to a short-circuit/winding short. The copper of the coil is eroded as in the case of electrode welding, and water can leak out. If the leaking water cannot escape through the intermediate spaces of the coil, the water will penetrate further and further toward the melt, and this will inevitably lead to a water vapour explosion. In order to prevent this, most furnace manufacturers have chosen an open coil construction with water-permeable construction elements. The “packing of the furnace coil” with glass fibre materials and waterproof coatings has in the past often been proven to be an error.
Winding shorts can also occur due to localised over-heating at the inner edges of the intermediate insulation layers. The lightly carburised intermediate layers can retain the moisture very well, thus providing the ideal conditions for a winding short. Accumulations of scrap always occur in the rear area of the furnace coil. This material cannot fall down when the furnace is tipped. Under the effects of the magnetic field, scrap can also collect in the area of the induction coil in the coil jacket, subsequently leading to a winding or earth short. To avoid this, the lower area of the furnace should be cleaned every 4 weeks or vacuumed out with an industrial vacuum cleaner. Under no circumstances should compressed air be used, since this could blow metal parts behind the “core insulation”. This will result in an earth short, which can only be located laboriously by checking every iron pile.
If dark areas can be seen on the inner surface of the furnace coil after removing the crucible, this area should be carefully cleaned of plaster and the condition of the intermediate layers checked. This check and any subsequent repair may well avoid unexpected failure with devastating consequences.