Wa-a-ay back in the day, the first significant taps were in electro-chemical smelters. (Copper & Aluminum)
The main conductors were bus-duct.
Hence, term of a bus-tap got started. It was soon carried to heavy industry -- particularly Big Auto.
There are still special Code exceptions for these situations.
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Copper was the first industry to use electro-separation. It became evident very quickly that only really pure copper had good electrical properties. Old methods were too costly.
In practice, miners send their ore to crushers to produce a mineralized dust. Next the dust is sent through floatation tanks where a xanthate soap and forced air bubble-lifts the ultra-fine ore to the surface as a film on the bubbles. They slop over and, after drying, are roasted/reduced to crude metals -- mostly copper -- called dore metal.
This dore is then shipped away to where electric power is cheap. ( Mongolian gold an copper is shipped to Canada in one case. ) The dore is rolled/recast into plating sheets surrounded by acids. Pure target copper sheets are placed near. Between them a porous bag/barrier operates as a diaper for the precious metals.
At this point mega-amps of DC current move the copper to the pure target. Voltages being unique to each element -- the gold, silver and what not drop off and sink. Iron and such stays in solution -- which gets refreshed.
The target plates gain weight and are removed, washed, and remelted under a vacuum to out gas any oxygen. Next the copper is shipped out as 'wire-bar' to the wire spinning companies so familiar to us all.
With all of that high power DC to shunt around bright minds came up with massive solid copper bars as in plant conductors. At the low voltage they were using, the original bus-conductors were NAKED.
What had been established for copper purification proved essential for the reduction of aluminum.
Like copper, the mines ship out a purified product that is converted into metal where electric power is cheap. Unlike copper, alumina is so pure that the only metal coming out is aluminum. But the process uses juice like no other!
