What is the function of a transformer oil sump pit?

A sump pit collects liquids from the transformer containment area, including rainwater and any leaked oil. It supports safe removal and separation by routing fluids to an oil water separator or controlled pumping system, rather than allowing uncontrolled discharge. In a substation, the sump also provides a monitoring point for leaks, because unexpected oil sheen or rising oil layer thickness indicates a problem. Proper design includes impermeable surfaces, access for cleaning, and measures to prevent oil bypass during heavy rainfall events.