Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning 110V distribution board protection and transformer supply, is answered by SELECT:
Answer: Where the transformer supplies a distribution board, then the answer is yes, you should provide secondary fault protection.
For a centre tap transformer, both live poles of the secondary should be protected either by double pole circuit breakers or fuses in each live pole. Where supply is only single pole, then protection should only be provided in the live pole.
Note: where small 230/110V transformers (generally centre tapped) are used to supply only one or two secondary sockets, then protection to the secondary circuits are usually provided by the thermal trip embedded in the transformer winding, which would operate to disconnect the primary supply, and hence isolate both poles of the secondary.
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