The reason for placing pebbles under the transformer

1、 Core reason: Safety and protection

The pebble layer is laid under the transformer, mainly serving the following key safety functions:

Oil leakage and prevention of oil accumulation (core function)

The function and risks of transformer oil: Large Oil Immersed Transformers contain a large amount of insulating oil inside for insulation and cooling. If the transformer malfunctions (such as internal short circuit, sudden increase in pressure), it may cause the oil tank to rupture or the explosion-proof membrane (pressure relief valve) to actuate, splashing out the hot transformer oil.

Advantages of pebbles:

Excellent permeability: There are numerous gaps between the pebbles. The sprayed transformer oil can quickly flow through these gaps into the accident oil pit (or drain pool) below, without accumulating on the surface.

Preventing the spread of oil fires: If the leaked oil is ignited, the gaps between the pebbles can hinder the spread of flames and disperse them, avoiding the formation of large areas of fire pools.

Easy to clean: After an accident, oil contaminated pebbles can be easily removed and replaced with new ones. If it is a flat concrete surface, oil stains will flow everywhere, making it difficult to collect and treat, causing serious environmental pollution.

2. Fire and explosion prevention

Isolation of combustibles: Pebbles are non combustible inorganic materials that are inherently non combustible and provide natural fire isolation.

Cooling and extinguishing sparks: High temperature transformer oil or burning oil droplets released from transformers, when flowing through the pebble layer, will exchange heat with the pebbles, playing a certain cooling role. Meanwhile, pebbles can cover and extinguish potential sparks.

Preventing the formation of explosive mixtures: If oil leaks in a sealed or poorly drained space, its volatile oil and gas may mix with air and form explosive mixtures. The permeability of the pebble layer helps with the diffusion of oil and gas, reducing this risk.

3. Drainage and moisture prevention

The transformer area needs to maintain good drainage to prevent water accumulation. The pebble layer is an excellent drainage layer, allowing rainwater or other moisture to quickly drain through the stone joints, avoiding long-term immersion of equipment foundations in water and protecting concrete foundations and internal metal components from corrosion.

At the same time, it can also block surface water vapor from rising directly to the transformer body.

4. Protection and stability

Inhibiting weed growth: The pebble layer can effectively prevent weeds from growing under the transformer. Weeds not only hinder inspections, but also pose a potential fire hazard during dry seasons, and may attract small animals to enter, causing a risk of short circuits.

Providing a stable foundation: The pebble layer provides a stable and non water accumulation working surface for maintenance personnel and equipment.

2、 Why is it just 'pebbles'?

Why not use sand, gravel or other materials? This is because pebbles have unique physical properties that make them the best choice:

Uniform gaps and good permeability: The natural shape of pebbles allows them to form uniformly sized and stable pores when stacked, ensuring smooth drainage of oil and water.

Smooth surface: The smooth surface makes it difficult for oil stains to adhere, making it easy to clean and replace. Sharp gravel is prone to getting stuck, and oil stains adhere more severely during cleaning.

Not easy to pulverize: Pebbles have a hard texture and will not weather or pulverize like some crushed stones after long-term use, thus blocking drainage and oil discharge channels.

Strong bearing capacity: able to withstand the weight of maintenance personnel and equipment.

3、 Related standards and specifications

This practice is not arbitrary, but is clearly defined by national standards and electricity industry norms.

In China's "Code for Fire Protection Design of Thermal Power Plants and Substations" (GB 50229), it is explicitly stipulated that:

Electrical equipment with a single oil volume of over 1000kg outside the house should be equipped with oil storage or oil blocking facilities. The volume of oil blocking facilities should be designed at 20% of the oil volume, and facilities should be installed to discharge accident oil to a safe location. When the above requirements cannot be met, oil storage facilities that can accommodate the entire amount of oil should be installed. Oil storage or oil blocking facilities should be larger than 1 meter on each side of the equipment outline. A pebble layer should be laid inside the oil storage facility, with a thickness of not less than 250mm and a pebble diameter of 50-80mm

4、 Summary
Laying pebbles under the transformer is a multiple safety measure that integrates oil leakage, fire prevention, drainage, and protection. It is like setting up an intelligent safety pad for transformers, which is conducive to drainage maintenance in normal times, can quickly guide away high-temperature fuel in case of accidents, prevent the expansion of fires, and provide convenience for post cleaning.

It is a classic and effective engineering detail to ensure the safe operation of substations.