Requirement For Transformer Soak Pit

Oil-filled transformers are extensively used in oil and gas electrical networks. The oil in power transformers causes a major hazard to the electrical installations, especially in case of fault, which may lead to explosion hazards that would have destructive effects on the people, assets, and environment. Therefore, transformer manufacturers and design engineers shall consider suitable devices and mitigation measures in the design of oil-filled transformers to avoid harsh effects in case of failure. Moreover, they need to prevent the event of fire spreading and damage to other assets that surround the transformer area.

Oil Pits Transformer

Transformer oil soak pits

A Soak Pit is a covered, porous-walled chamber that allows water to slowly soak into the ground. Pre-settled effluent from the septic tank is discharged to the underground chamber from where it infiltrates into the surrounding soil.

Oil pits are required to collect the transformer oil – which generally comes out from the transformer because of leakage overflow or sometimes heavy draining out of oil.
The collection meant that the surrounding area of the transformer could not be oily slippery or flooded with oil in any above cases. It comes down through the surrounding channels of the transformer base and comes into the pit through the connected pipe via gravity.

Let’s look at what different standards have to say about Transformer Soak Pits Requirement

Requirement of Transformer Soak Pit

Standard IS 10028, Code of Practice For Selection, Installation, And Maintenance Of Transformers, section 3.6

Provisions for Draining of Oil –

Transformers installed inside buildings, where the oil capacity exceeds 2000 liters of oil, shall be provided with oil soak pits. Oil soak pits may also be made of RCC tanks.

For indoor transformers, oil soak pits shall be provided either under the transformer or outside the transformer room.

If two or more transformers are installed side by side, they shall be separated by fire-separation walls. Fire separation walls are deemed to be adequate from the fire-safety point of view, even if the oil capacity of an individual transformer does not exceed 2000 liters, and the total capacity of all transformers installed side by side exceeds 2000 liters.

Standard IS 1646, Code Of Practice For Fire Safety Of Buildings (General): Electrical Installations, Section 7.3

Each building or compartment housing oil-filled apparatus containing 2000 liters or more of oil shall be provided with oil drains of at least 150 mm in diameter and soak pits, the latter being not less than 2.5 m away from the substation. Floors shall be sloped not less than 1 in 96 towards oil drains.

Central Electricity Authority (Technical Standards for Construction of Electrical Plants and Electric Lines) Regulations, 2010”

1. The bottom of the soak pit below the transformer/reactor shall be connected to the common burnt oil pit with a drain pipe of a minimum of 150 mm diameter for fast draining of oil or water through gravity from the soak pit to the burnt oil pit.
2. Every soak pit below a transformer/reactor shall be suitably designed to contain oil dropping from any part of the transformer/reactor.
3. The burnt oil pit and soak pit, when burnt oil pit is not provided, shall be provided with a suitable automatic pumping facility, to always keep the pit empty and available for an emergency.
4. The disposal of transformer oil shall be carried out in an environment-friendly manner.

Technical Standards for central Electricity Authority (Measures relating to Safety and Electricity Supply) Regulations, 2010, Clause 44

1. A sub-station or a switching station with apparatus having more than 2000 liters of oil shall not be located in the basement where a proper oil draining arrangement cannot be provided.
2. Where a sub-station or a switching station with apparatus having more than 2000 liters of oil is installed, whether indoors or outdoors provisions shall be made for a suitable oil soak pit and where the use of more than 9000 liters of oil in any one oil tank, receptacle or chambers involved, provisions shall be made for the draining away or removal of any oil which may leak or escape from the tank, receptacle or chamber containing the same.

The capacity of the Transformer Soak Pit

Standard IS 10028, Code of Practice For Selection, Installation, And Maintenance Of Transformers, section 3.6

The capacity of the oil soak pit shall be such that to soak the entire oil content of the transformer, it is intended for individual soak pits for each transformer (wherever necessary) with capacity as above or a common soak pit to contain the entire oil content of the biggest of the transformers shall be adequate.

Soak pits shall be designed in such a way to provide for safe draining of liquids to soak pits.

1. In the typical arrangement for a transformer installed in a locality not subject to seepage of the subsoil water, the soak pit should not be plastered inside or outside, and the bottom should be left open to the earth
2. In locations prone to seepage of subsoil water reinforced concrete construction of soak pit should be adopted, the bottom of the soak pit being also scaled to prevent ingress of water
3. The size of the soak pit will depend on the quantity of oil in the transformer and the type of construction of the soak pit, the main consideration being that the entire quantity of oil should be quickly drained away from the transformer room into and/or away from the soak pit.
4. An exhaust fan may have to be provided in the transformer room if site conditions do not permit adequate natural ventilation

Standard IS 1646, Code Of Practice For Fire Safety Of Buildings (General): Electrical Installations, Section 7.3

The soak pits shall be of sufficient capacity to take the entire oil content of the equipment and designed to provide for drainage of liquids to a safe location.

Draft Central Electricity Authority (Technical Standards for Construction of Electrical Plants and Electric Lines) Regulations, 2010”, Clause 26

1. An oil soak pit of adequate capacity shall be provided below each oil-filled transformer/ reactor to accommodate at least 150% of the full quantity of oil contained in the transformer/reactor and a minimum 300 mm thick layer of gravels or pebbles of approximately 40 mm size.
2. Alternatively, a soak pit of adequate capacity to accommodate 1/3rd of the total quantity of oil contained in the transformer/ reactor and a minimum 300 mm thick layer of gravels or pebbles of approximately 40 mm size shall be provided below each transformer/ reactor provided a common remote burnt oil pit of capacity at least equal to oil quantity in the largest size transformer/ reactor is provided for a group of transformers/ reactors.

While designing transformers, some clients had asked us to provide soak pits because they were asked to comply with the regulations, some had asked us not to provide soak pits even if it was required.

The requirement of the soak pit is decided based on not only the type of transformer but also the type of reactor, automatic voltage regulator, and generator.