BS EN 50588-2:2018 Medium power transformers 50 Hz, with highest voltage for equipment not exceeding 36 kV Transformers with cable boxes on the high-voltage and/or low-voltage side. General requirements for transformers with rated power less than or equal to 3 150 kVA

Three-phase transformers
Boxes (electric)
Electrical connections
Rated frequencies
Flanges
Cable junctions
Rated voltage
Power transformers
Alternating-current transformers
High-voltage equipment
Transformers
Oil-immersed transformers
Distribution transformers

Link: BS EN 50588-2:2018 Medium power transformers 50 Hz, with highest voltage for equipment not exceeding 36 kV Transformers with cable boxes on the high-voltage and/or low-voltage side. General requirements for transformers with rated power less than or equal to 3 150 kVA
Source: BSI Standards