Ring Main Unit Cable Connection: Interface Data to Include in an RFQ

A ring main unit cable connection is normally made inside a dedicated cable compartment, where each incoming and outgoing medium-voltage cable lands on an equipment bushing through a separable connector or termination. Because the bushing, the connector, and the cable must match one another, the connection is an interface package  not a single part.

Two ring main units with the same voltage class can still be incompatible at the cable compartment. An RFQ that states the bushing interface expectation, the connector class and supply scope, the cable construction data, and the access and testing provisions removes the most common source of installation delay.

Part 1. What does a ring main unit cable connection involve?

An RMU is a compact, sealed assembly of switching devices for medium-voltage distribution, and its cables are commonly attached with plug-in cable ends such as elbow connectors or T-body connectors. The technical overview of ring main units in secondary substations describes exactly this arrangement: switch, fuse-switch, or breaker ways above, and a cable compartment below where the network physically enters the unit.

Assemblies of this kind fall under the metal-enclosed switchgear scope of IEC 62271-200, which covers prefabricated switchgear for rated voltages above 1 kV up to and including 52 kV with air-insulated or fluid-filled compartments. The insulation concept  SF6 gas, solid, or air  changes the compartment layout, so the cable interface always has to be read from the offered design rather than assumed from a product category.

For buyers, the practical starting points are a unit-level shortlist and the interface data package. Unit selection is covered in the high voltage ring main unit selection guide; this article covers the data package. Where the network also uses street-level cable nodes, an outdoor cable distribution box carries a related but separate interface definition, so keep the two requests distinct.

Part 2. Which bushing interface types shape the connection?

Bushing interfaces are standardized so that connectors and equipment from different suppliers can mate. EN 50181 defines the essential dimensions of plug-in bushings above 1 kV up to 52 kV, from 250 A up to 2 500 A, so that a separable connector of equivalent rating fits any conforming bushing. Outside-cone interface types carry letter designations, while inside-cone systems use numbers.

In distribution practice, three outside-cone types dominate. An OEM cable-accessory reference for XLPE-insulated medium voltage cables summarizes them as follows:

Treat this table as scope framing rather than a configuration rule. Which interface sits on which way of an offered unit is an OEM design decision, and the same functional way can carry different interfaces across product families. An RFQ should therefore state the expected interface type per way and ask the supplier to confirm the interface actually provided. Air-insulated ring switchgear families follow the same logic with their own compartment geometry, as shown in the fixed type metal ring main switchgear guide.

Important: Interface letters describe mating dimensions, not switching capability or fault ratings. Confirm electrical ratings separately from the mechanical interface. Source context: EN 50181 scope description.

Part 3. Which cable data must travel with the interface request?

Connector selection starts from the cable, not from the switchgear. A separable connector body must seal onto the cable insulation within a defined diameter window, and its conductor fitting must match the conductor size and material. Missing cable data is the usual reason a correct bushing still ends up with an unusable connection kit.

Cable data the RFQ should state per way

Where two cables per phase, surge arresters, or future tee-offs are expected, say so in the same table. Those choices change the connector geometry and may exceed the space that a compact cable compartment provides.

Part 4. How are connectors and terminations verified and supplied?

Cable accessories carry their own type-test framework, separate from the switchgear assembly tests. IEC 60502-4 specifies the type-test requirements for accessories  terminations, joints, and separable connectors  used on extruded-insulation cables rated from 3,6/6 (7,2) kV up to 18/30 (36) kV, with test methods drawn from IEC 61442. Asking for the accessory’s type-test evidence alongside the switchgear documentation keeps both halves of the interface accountable.

Supply scope deserves equal attention. In many RMU designs the connectors are not included with the switchgear and are ordered separately against the confirmed cable data. One published OEM application guide for a compact RMU family also limits connector dimensions inside its compartments and requires externally screened connector types at its higher voltage levels. Practices like these are design-specific, which is exactly why the RFQ should ask three questions in writing:

• Who supplies each connector or termination kit  the switchgear supplier, the cable contractor, or the buyer?
• Which connector families and maximum dimensions has the switchgear supplier validated for the offered compartment?
• Does the offered design require screened (externally earthed) connector construction at the project voltage?

Screened, dead-front construction keeps outer surfaces at earth potential, which practitioners value in compact compartments where clearances to covers and steelwork are tight.

Part 5. Which compartment access and clearance details matter?

Compact units win on footprint and lose on forgiveness. Field discussions about T-body connections in switchgear repeatedly come back to one theme: limited termination space drives the connector choice, and a kit that fits on a drawing can still be impossible to dress and bolt in a shallow compartment.

State the physical boundary conditions in the RFQ rather than discovering them during installation:

• Cable entry direction (bottom, top, or rear) and the civil interface  trench depth, floor opening, or gland plate.
• Minimum bending radius of the actual cable, checked against the space below the bushings.
• Number of cables per phase and any parallel-cable or surge-arrester hardware sharing the same compartment.
• Door swing, cover removal, and the working space in front of the unit for connector installation and later testing.
• Environmental sealing needs at the entry  outdoor kiosks and damp basements need a stated ingress expectation, not an assumption.

A useful acceptance question for the supplier: which connector kits, at which maximum dimensions, have already been installed in this compartment on reference projects? The answer converts a drawing review into installation experience.

Part 6. How should earthing and cable-test access be recorded?

Cable works do not end at energization. Over the unit’s life, crews will prove dead, earth the cable, test insulation, and locate faults  and each of those tasks passes through the cable connection. Ring main unit ways therefore commonly integrate an earthing switch, and the compartment arrangement defines how a test set reaches the cable.

Record three provisions in the RFQ:

1. Earthing: which ways include an integrated earthing switch, and how the interlocking sequence releases access to the cable compartment.
2. Voltage indication: whether voltage presence indication is provided per way, and whether separable connectors should carry a capacitive test point for circuit checks without breaking the bolted connection.
3. Test access: how the offered design applies a cable test  through the connector system, through dedicated test bushings, or by disconnecting  and what outage that method implies.

Practitioners on engineering forums specifically recommend capacitive test points on insulating plugs because they allow a circuit condition check without disturbing the connection. Whether that option applies is, again, a property of the offered design and the selected accessory family.

Part 7. Ring main unit cable connection RFQ checklist

Send the interface data as one structured package. A supplier who receives the full set can confirm compatibility in a single pass instead of a chain of clarification emails.

RFQ input list

• System voltage, highest voltage for equipment, system frequency, and neutral earthing practice.
• Way configuration of the unit  ring switch ways, fuse-switch or breaker ways  with the expected bushing interface type per way, to be confirmed by the offered design.
• Cable type, cores, conductor material, conductor cross-section, insulation diameter, and cable screen construction per way, plus cables per phase.
• Connector class expectation (plug-in or bolted; elbow or T-body) and the required supply scope for each connector or termination kit.
• Screened or dead-front construction expectations, with surge-arrester or parallel-cable hardware named where planned.
• Compartment constraints: entry direction, bending radius, connector dimension limits, door and working space, ingress expectation at the entry.
• Earthing switch positions, interlocking expectations, voltage indication, capacitive test-point preference, and the intended cable-test method.
• Documentation set: accessory type-test evidence per IEC 60502-4 scope, switchgear documentation per the applicable IEC 62271 series parts, drawings showing the cable compartment, and installation instructions for the validated connector kits.
• Commercial data: delivery expectations, spare connector policy, and the contact for interface clarifications.

The checklist deliberately excludes ratings and test values. Those come back from the supplier as the offered design’s documentation, which the project engineer then reviews against the network study.

Part 8. Which JUBANG ring main unit fits this discussion?

Buyers preparing this data package can apply it directly to JUBANG’s ring main unit families. The GT-SRM-12 SF6 gas insulated ring main unit page is the entry point for compact gas-insulated ring networks, and the GT-HRM-12 environmental protection gas insulated ring main unit page covers the environmentally focused variant of the same duty. Metal-clad withdrawable switchgear is deliberately not recommended here: that family answers a different feeder-level requirement, not the compact ring interface this article covers.

Fit Boundary

• A good fit: distribution utilities, EPC contractors, and industrial buyers who hold the cable data and way configuration for a 12 kV-class ring network and want the interface confirmed against a specific offered unit.
• Not the right path: projects that cannot yet state cable construction data or name a responsible engineering authority, and requirements outside compact ring main unit practice, such as large withdrawable feeder line-ups.
• Confirm first: the bushing interface per way, the validated connector families and dimension limits, the connector supply scope, and the documentation list  all in writing from the offered design.

Nothing in this article assigns an interface type, connector class, cable range, test result, or compliance status to any JUBANG model; the offered design’s documentation governs those values.

Next step: send the cable interface data package  system data, way configuration, cable table, access constraints, and documentation expectations  and ask for the offered interface confirmation for your ring main unit project.

FAQ

What is a ring main unit cable connection?

It is the interface between the medium-voltage network cables and the RMU: the equipment bushings in the cable compartment, the separable connectors or terminations on the cables, and the earthing and test provisions around them. It is specified as a package because all three layers must match.

What is a bushing interface type on an RMU?

It is a standardized set of mating dimensions for the plug-in bushing on the switchgear, defined so that separable connectors of equivalent rating from different suppliers fit the same bushing. Outside-cone types carry letters; inside-cone systems use numbers.

Are separable connectors included with a ring main unit?

Often they are not. Many designs treat connectors as separately ordered accessories selected against the confirmed cable data. State the required supply scope in the RFQ and record in writing who provides and installs each kit.

Which cable data decide the connector selection?

Cable type and insulation, number of cores, conductor material and cross-section, insulation diameter, and screen construction. The insulation diameter is critical because each connector body seals only within a defined diameter window.

Do screened separable connectors matter above 12 kV?

Some compact switchgear designs require connectors with an external earthed screen at their higher voltage levels because clearances inside the compartment are limited. Treat it as a design-specific requirement and ask the supplier to state what the offered unit needs.

How are RMU cable accessories type tested?

Separable connectors and terminations for extruded-insulation medium-voltage cables are type tested under the IEC 60502-4 framework, with methods from IEC 61442. Request that evidence together with the switchgear documentation.

What cable compartment details belong in an RFQ?

Entry direction, bending space, connector dimension limits, cables per phase, door and working space, ingress expectations, earthing switch positions, voltage indication, and the intended cable-test method.

What happens if the cable connector does not match the bushing interface?

The cables cannot be terminated as planned: the kit may not mate mechanically, may not seal on the insulation, or may not fit the compartment. The result is re-ordering, site delay, and sometimes a compartment or accessory change  which is why the interface data belongs in the RFQ.

References

1. Metal-enclosed switchgear assembly scope: IEC 62271-200:2021, AC metal-enclosed switchgear and controlgear
2. Plug-in bushing interface dimensions: EN 50181:2010, plug-in type bushings above 1 kV up to 52 kV
3. Cable accessory type tests: IEC 60502-4:2023, test requirements on accessories for cables from 6 kV up to 30 kV
4. Interface types in accessory practice: Suedkabel cable accessories for XLPE-insulated medium voltage cables
5. RMU role and plug-in cable ends: Ring main unit as part of secondary distribution substations, Electrical Engineering Portal
6. Compact RMU connector practice: Eaton Xiria application guide
7. Practitioner discussion of T-body connectors and test points: Tee connectors in switchgear, Eng-Tips forum
8. Practitioner discussion of load-break and dead-break elbows: Dead break elbows, Mike Holt forum