What goes into a distribution substation room?
A typical building substation in the GCC has an 11 kV RMU, one or two 11/0.4 kV transformers, an LV main switchboard (sometimes in a separate LV room), a cable trench or basement for MV and LV cables, an earthing system, ventilation and fire protection. The utility usually owns the RMU and transformers, and the building owns the LV board, so the room has to work for both: utility access from outside, and safe operation for both teams.
PractitionerCheck the utility's standard room drawing first. It fixes the room size for each transformer combination, the door positions, trench layout, louvre sizes and equipment plinths. Designing your own room and asking the utility to accept it later is the usual cause of redesign.
What clearances do MV and LV equipment need?
- Operating aisle in front of the RMU and LV board, so an operator can stand clear and the door or panel can open fully. IEC 61936-1 gives minimum aisle widths around 0.8 m, and utilities often require 1.0 to 1.5 m.
- Around transformers, room for cooling air and maintenance access, often 0.6 m or more to walls and between units.
- Delivery route: door and route sized for the largest transformer, with lifting or skating space.
- Cable trench deep enough for the MV cable bending radius (often 12 to 15 times the cable diameter).
How do you size transformer room ventilation?
For natural ventilation, a common first-pass formula from manufacturers' MV design guides is inlet area S = 0.18 P / √H, with the outlet about 10% larger. P is the total transformer losses in kW and H is the height between the centres of the inlet and outlet louvres in metres. It assumes about a 15 K rise in air temperature. For forced ventilation, the airflow is Q = P / (1.2 × 1.005 × ΔT) m³/s. In the GCC, outside air at 45 to 50 °C leaves little headroom, so fans, sand-trap louvres and sometimes cooling are common.
Oil containment and fire
An oil-filled transformer can leak or burst. The room needs a bund, pit or drain to a tank that can hold at least the full oil volume, plus allowance for fire water where the code asks for it. Inside buildings, oil-filled transformer rooms are usually fire-separated (often 2 hours) and may need detection or suppression, which is a big reason cast resin units are used in towers.
Regional notes
GCC
Follow the utility's substation room standards (DEWA, ADDC/AADC, SEWA, KAHRAMAA, SEC and others) and the civil defence fire code. Design ventilation for the real outside temperature and dust, and agree access for utility staff.
Australia
AS 2067 covers substation design, including clearances, fire segregation and oil containment. Network operators publish their own substation manuals, and pad-mount kiosks are very common.
New Zealand
Lines companies set the kiosk and room standards. Seismic restraint of equipment and plinths is designed to NZS 1170.5 and NZS 4219.
Asia
Utilities such as TNB and SP Group publish substation room requirements, including room sizes for each transformer arrangement.