250.20 (A)

[mbrooke]

250.20(A)

(A) Alternating-Current Systems of Less Than 50 Volts.
Alternating-current systems of less than 50 volts shall be grounded
under any of the following conditions:

(1) Where supplied by transformers, if the transformer
supply system exceeds 150 volts to ground

(2) Where supplied by transformers, if the transformer
supply system is ungrounded

(3) Where installed outside as overhead conductors

What is the rational behind these 3 rules?


Would these set of rules be better?

(1) Where the transformer secondary is without a grounded shield between primary, and tertiary windings.

(2) Where the primary voltage to grounded exceeds the insulation rating of the secondary system conductors.

Or am I way off?