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Low-impedance faults are difficult to contain from the point of view of cover retention and tank rupture in pole-type distribution transformers. When evaluating a transformer's containment capabilities, one needs to consider two aspects: the overpressure caused by an arc of given parameters and the pressure withstand of the tank. This paper presents two methods to calculate overpressures as a function of the arc and all of the relevant geometrical parameters: tank diameter, air-space height, and the depth of the arc below the oil. Both methods have been validated with experiments. Results of tests to determine the pressure withstand of typical tanks are presented. It was found that an average value of the overpressure withstand capability is approximately 100 kPa. For a given tank geometry and fault parameters, a current-limiting fuse would be required if the methods proposed here predict an overpressure of more than 100 kPa. Index Terms--Arc discharges, explosions, finite-difference methods, prediction methods, pressure effects, transformers.

When an electric arc is generated in transformer oil, a strong overpressure may rapidly develop in the tank, especially for low-impedance faults. This paper presents the test method and results obtained in a project aimed at assessing the human and environmental risks associated with this type of fault. Limit energy and pressure values were defined in terms of different safety levels. The best method of limiting the energy and pressure at a safety level in the transformer tank during internal faults of greater than 1.7 kA is to protect the transformer with a current limiting fuse. Other ways to increase the mechanical withstand of transformer tanks under such faults are also discussed. Index Terms--Arc discharge, fault current, fuse, protection, safety, test pressure, transformers.