Re: 250.21 (1) Industrial Electric Furnaces
In the aluminum and chemical industries, some furnaces for refining operate on high currents (100,000 Amps) but low voltages per furnace (3-5V)- sometimes called a "cell" or "pot". The pots are series connected together into potlines with rectifier banks feeding the series circuit at 500- 1000 VDC, 100 -200 kA.
NEC Article 668 describes this type of furnace and the special precautions needed.
The steel and carbon structure of each furnace is part of the circuit. Potline workers are trained in safe methods of working the furnaces while energized. The low DC voltages to adjacent equipment make it a safer environment than it sounds. Since the system is floating, voltage to ground varies from 700V to 0V. Introducing a ground anywhere in the vicinity of the potline creates a hazard. Great care is taken to not short out any portion through an unintentional ground.