Open the door of any typical ring main unit and you'll find three compartments arranged side by side, each called a "cell." Each cell has a name, a function, and its own cable — and understanding this trio is half of understanding the entire unit.
The Three Cells in Detail
| Cell | Function | Its Cable |
|---|---|---|
| Incoming Cell | Receives the supply from the nearest medium-voltage line — a nearby overhead line may transition underground to enter it | Incoming cable — initiates the supply |
| Outgoing Cell | Usually located in the middle; current leaves from here to a more distant ring main unit or continues its path through the distribution system | Outgoing cable — distributes to neighboring areas |
| Transformer Cell | Feeds the neighborhood transformer from which household meters are supplied; contains the transformer's protective fuse | Transformer cable |
These three cable names are important for job interviews — memorize them by function, not as abstract names.
How Does Power Flow Through the Cells?
- Medium voltage (11 kV for example) enters from the incoming cell to the unit's busbars.
- From the busbars, two paths branch out: toward the transformer cell (through a load-break switch and fuse) to feed the neighborhood transformer, and toward the outgoing cell (through a load-break switch) to continue the ring.
- Each cell, with its switch, can be isolated and earthed independently — this is the essence of operational flexibility.
The Number of Cells May Increase
The three-cell model is the common one, but you may find additional cells depending on site requirements: a second transformer cell, an extra outgoing cell, or a metering cell. However, anyone who understands the three-cell model can easily read any larger configuration — the same structure repeats. See Internal Components for what resides inside each cell.
Sample answer: The incoming cell receives the supply from the nearest medium-voltage line — and an overhead line may transition to an underground cable to enter it. The outgoing cell, usually located in the middle of the unit, sends current out to another, more distant ring main unit or continues its path through the distribution system. The transformer cell feeds the neighborhood transformer connected to the houses and contains its protective fuse. Each cell has its own cable named for its function: incoming, outgoing, and transformer cable.
Assuming the direction of flow is always fixed (incoming flows in, outgoing flows out). In a ring network, the direction can reverse during isolation switching and restoration from the other end — the naming is functional/design-based, while actual operation may temporarily reverse it.
Want to understand substations step by step?
Follow trainer Fahad Refai's Substations and Electrical Maintenance courses — a practical walkthrough from maintenance fundamentals to SCADA systems.
Browse Fahad Refai's Courses