Electronic – How to low voltage, high current (kA) power be dangerous

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Aluminum refineries use electricity to separate aluminum from minerals it naturally occurs in. This electricity typically takes the form of low voltage DC ("low" meaning 4 to 6 volts), at very high current (on the order of tens of kiloamps). This much power poses an electrocution danger, but I don't understand how. If the entire electrical system runs at, say, 5 volts, and the human body acts like a resistor, then how can enough current actually make it through a human body to be dangerous? Similarly, how can an electrical arc through air happen, if it takes hundreds of volts to arc over a very short distance?

Best Answer

The voltage for the Hall–Héroult process is inconveniently low (and the current too high) for efficient parallel operation so they use a whole bunch of cells in series.

From this source ("Studies on the Hall-Heroult Aluminum Electrowinning Process"):

The optimum current density is around 1 A cm-2 with a total cell current of 150-300 kA and a cell voltage -4.0 to -4.5 V. A typical cell house will contain about 200 cells arranged in series on two lines.

So the voltage at any given cell with respect to earth can be quite high, and the voltage across a cell if it opens up will be almost 1kV. Currents like that will easily vaporize metal so they can sustain a very long arc if it opens up relatively slowly and does not have a blow-out mechanism (DC is worse than AC).

To understand the efficiency issue- consider a simple full wave rectifier made with 6 silicon rectifiers. It will have a drop of (say) 2V at full current so the loss will be the output current x 2V. At 150kA that's 300kW lost. If you run 200 cells in parallel you would be wasting 60MW. Even at the cheap electricity prices that smelters pay that will add up- of the order perhaps 25-50 million dollars a year. In series, the loss is 'only' 300kW. The capital cost is also much less to make 150kA at 800V vs. 30MA at 4.5V because far more rectifiers and heat sinking would be required.