Transformer LV Bushing Air Clearance and Creepage Distance

IEC 60076-3 does not mention minimum clearance between phase to phase and phase to earth for low voltage (<1000V), which standard should be referred to?

IEC 60815 only applicable for high voltage bushing, which standard should be referred to for low voltage bushing (<1000V),I'm looking for minimum creepage distance info.
 
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