Translate the network into named functions
A request for a “three-way RMU” leaves several questions unanswered. Three ways can serve different combinations of cable connections, transformer branches and switching duties. Suppliers also use different letters to identify their functions, so a short configuration code should always be accompanied by a legend and single-line diagram.
Name each circuit by destination. Identify the ring cable from the previous station, the cable to the next station and each transformer branch. Then state which devices perform normal switching, isolation, earthing and fault interruption. A label such as “incoming” does not prove that the circuit includes a fault-interrupting device.
Build a function schedule
| Function | Operating information | Rating or interface to review |
|---|---|---|
| Ring cable A | Normal state and alternative feeding direction | Through-current, cable termination and switching duty |
| Ring cable B | Open-point responsibility and restoration role | Transfer loading and the approved earthing arrangement |
| Transformer branch | Transformer identity and protection boundary | Primary load current, fault duty and coordination inputs |
| Metering function | Exact energy or voltage measurement point | Instrument-transformer configuration and space |
| Reserved function | Installed spare or future addition | Real primary interface and available enclosure arrangement |
Keep the topology decision separate from the review of load switching and fault-interruption duties. A switch-fuse arrangement also requires coordinated fuse selection; these protection decisions cannot replace a complete function schedule.
Distinguish transformer current from ring current
For an illustrative 1,000 kVA three-phase transformer at 11 kV, rated primary current is approximately 52.5 A, using I = S / (√3 × V). This calculation describes the transformer branch at its stated rating.
The ring cable ways may carry additional downstream stations, especially after a network reconfiguration. They must be assessed using the relevant network load flow, rather than the local transformer's current. Conversely, a cable way's current rating does not establish the required protection behavior of the transformer feeder.
These numbers are an example only, not a statement of a Jinxing model's available ratings or a recommended protective setting.
Write the normal and contingency states
A ring-shaped cable route can be operated with an open point. Document where that point lies and which organization controls it. Also describe permitted restoration states after a cable section is isolated. Closing a ring or connecting two sources may change fault levels and protection requirements; it cannot be assumed permissible because two cable ways exist.
A practical review scenario is a station normally fed from cable A, then temporarily fed from cable B. Ask whether the transformer protection remains selective and whether the metering boundary still measures the intended circuit. The project engineer should resolve those operating assumptions before the equipment configuration is finalized.
Check the assembled arrangement
Request the order of functions, cable entry positions, operating access and secondary compartment needs on one arrangement drawing. Where a metering function or spare branch is added, reconfirm the complete dimensions and the relevant verification evidence. Avoid assuming that another unit's function code implies identical construction.
For an RFQ, provide the network diagram, function schedule, transformer data, minimum and maximum source fault levels, earthing arrangement and approved operating states. Start with ring main units and send the documents through project enquiries. Availability, insulation technology and the final assembled configuration require a specific proposal; component descriptions do not imply separate supply.