> Why not get a 1 ohm or likewise crazy low value resistor

That would be more applicable for lower currents. You should
probably be thinking about magentic devices at 600A. Allegro
have quite a few Hall Effect ICs for that sort of thing

http://www.allegromicro.com/

Hall Effect application guide (880kB)

http://www.allegromicro.com/techpub2/an/an27701.pdf

An example on Page 29 of a 1000A current sensor

> What kind of wire is rated for 600A, anyway?!  (wow!)

Exactly. And the dissipation through eg 1 ohm ?

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