![]() I connected a lot of smallish 5 KVA transformers for lighting, 480x120/240 it was. Neutral sizing may be different in each case. ![]() If the SBJ is at the transformer, its 4 wires, 2 hot, neutral and EGC. Or to be more prudent as you have been advised for a slightly different reason, use two RCCBs in the same configuration. For single phase, If the SBJ is at the panel, three wires to the xfmr. So, to answer your question, by all means do use protection on the secondary side, but make it two fuses, one on each secondary wire, to protect against that admittedly rare primary to secondary short circuit. the cable connectors and distribution equipment are color coded as follows: Phase Brown. Those two secondary fuses not only protected the secondary of the transformer against overload, but they also provided (admittedly minimal) protection for the user of the transformer against the chance of a short circuit between primary and secondary windings, thus completely negating the primary people-protective reason for the transformer. A transformer has two coils in close proximity to one another. In days of yore it was, I believe, a Regulatory requirement to fuse both sides of the secondary (and, to a person performing checks outside the transformer case, it always appears as though the two fuses were in series, and until I checked by looking inside the case and also checking old Regulations, I wondered at the point of "two fuses the same value" in "series"). An old question, but hopefully I have a still-relevant comment to do with people-protection rather than DUT protection.
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