Oil-immersed transformers use oil as the primary insulation method of the transformer and rely on oil as the cooling medium (self-cooling, air-cooling, and water-cooling). The reason for putting pebbles in the oil pit of the oil-immersed transformer is mainly for fire prevention and oil leakage.

This part under the oil-immersed transformer is usually called the oil dump or oil discharge, which leads to the oil pit or oil tank at the terminal. In the event of an accident, such as fuel injection or explosion, the transformer's oil will flow into the dump pit through the pebbles, and then flow to the oil pond at the incident. At this time, the pebbles under the transformer will block the fire and reduce the fire, which is conducive to fire suppression. After the oil is cooled by pebbles, the fire can be reduced.

Some of the pools are made of fences, and some are not. The cobblestone is placed on top of the grate, and the cobblestone is placed in the dump when it is not made. Placing pebbles for easy oil removal is one reason. In addition, insulation is convenient for maintenance, inspection by operating personnel and work is also a reason.

Avoid burning the oil when it leaks to the ground when the transformer is on fire, and it will not block the drain hole at the bottom of the drain pit. The oil flows below the gap between the cobblestones of the transformer to avoid oil pollution on the outside and it also plays a role of cooling the oil temperature when the oil is drained to prevent the oil from burning.