is restricted as a rule to the area of the furnace and there mainly to the immedi- ate area of the furnace coil. The first thing to do is check the function of the earth shorting indicator with a 500 Ohm resistance, which is installed in the measure- ment line. In order to establish whether the earth short lies between the melt and the coil, a significantly higher resistance value must be displayed after discon- necting the bottom electrodes. If the earth short is still present with the same value, the short must lie outside the furnace coil. In a dark furnace area, the short may even be visible to the naked eye. If the short lies in the area of the supply lines or the power supply, this might also be visible in dark working areas. In mains frequency systems, earth shorts often occur in the area of the starter resis- tors. One “rough method” is a check using a bridged insulation detector, the earthed neutral point of the transformer using a 25 or 35 A fuse. This method can even be used over a longer period to arrive at an audible diagnosis, e.g. in the balancing choke or between an iron pile and the coil.
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