Explosion vent is provided on the transformer tank to relieve pressure if the pressure in the transformer exceeds 5 psi. It is swan neck shaped having two Bakelite diaphragms. One at top and another at bottom. These break if the static pressure increases to 5 psi. Wire meshes are provided below the bottom diaphragm and above the top diaphragm. When there is any breakage due to excess pressure the bottom wire mesh prevents broken pieces from entering transformer tank and the wire mesh provided above the top diaphragm protects the diaphragm from any external damage. There is an oil level indicator provided above the bottom diaphragm. It indicates the level of oil in the vent if the bottom diaphragm ruptures.
A ruptured diaphragm must be immediately replaced. Also we should check the top diaphragm for any external damage.