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Q: In gas-insulated systems, for example in 110-kV voltage or above, it is not easy to meet the requirement of protecting voltage transformers with primary fuses. I have not found any manufacturer that gives a solution. The question is: Is this requirement absolutely compulsory in USA? ( see Article 450.3 (C) NEC)

A: Because of the modular design of GIS and the fact that it is an enclosed equipment with SF6 used for insulation, it does not lend itself to the installation of power fuses between the bus and the VT's. Customers worldwide have accepted the fact that VT's become part of the protection zone in which they are connected. For instance, bus VT's become part of the bus differential zone of protection.

Best Regards,
Pete Santoro
Strategic Projects Account Manager

ABB

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