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A high tension (HT) supply of, say, 6 kV...

A high tension (HT) supply of, say, 6 kV must have a very large internal resistance.Why?

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Because I = `(epsilon)/(R + r)` (Where emf `epsilon` is very large for high tension supply)
In case of short circuit, R = 0 and so value of I will be more than current capacity of given circuit which may cause damage. Hence, in this case value of I can be kept smaller than current capacity by keeping large internal resistance. .
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