RMU (ring main unit) testing confirms, before energizing, that the tank still holds its gas, that the switching devices operate and interlock as they should, that the insulation and cable connections are sound after installation, and that the transformer protection, by fuses or by a breaker and relay, clears faults as designed. Acceptance criteria come from the factory test reports, the manufacturer's manual and IEC 62271-200, not from generic values.
What an RMU is and what gets tested
An RMU (Ring Main Unit) is a compact medium-voltage assembly, almost always gas-insulated, with cable functions for the incoming and outgoing ring and a transformer function. The cable functions have three-position switch-disconnectors (closed, open and grounded); the transformer function has either a fuse-switch combination or a circuit breaker with a relay. The enclosure follows IEC 62271-200, the switch-disconnectors IEC 62271-103 and the combinations IEC 62271-105. The principle of each device is covered in medium-voltage switch-disconnector and RMU.
Testing happens at three moments: at the factory, as routine tests; at commissioning, with the cables connected and before energizing; and after a fault, a high number of operations or a gas event. It applies to secondary-distribution RMUs, which manufacturers offer up to 36 kV and, in some ranges, 40.5 kV, and to those that are part of compact substations, buildings, industry and solar plants.
The tests and what each one detects
| Test | What it detects | Criterion |
|---|---|---|
| Inspection and gas | Transport knocks, covers without connectors, a tank losing gas | Indicator in the green zone; enclosure grounded |
| Operation and mechanisms | Stiff mechanisms, incomplete travel, false position indicators | Full operation of every device, local and remote |
| Interlocks | Ground switch that closes with the device closed, covers that open while live | Every lock in the manual works |
| Insulation resistance | Moisture, contamination or damage in connectors and terminations | High, stable and even across phases; manufacturer’s minimum |
| On-site withstand voltage | Dielectric defects from installation | Withstands the value and the time without discharge |
| Contact resistance | Contacts with low pressure or poorly assembled | Close to factory values and even across phases |
| Breaker timing | Slow mechanism or uneven poles | Within the manufacturer’s range |
| Transformer protection | Wrong fuse, relay without settings, trip that does not arrive | Clearing as set in the protection study |
| Voltage indicators | Indicator that does not respond | Passes the manufacturer’s function test |
| Telecontrol and signals | Crossed positions and alarms in SCADA | Match the real state |
Gas, operation and interlocks
The first check is the gas pressure or density indicator. The operating manual of the Lucy Electric SabreEcoTec RMU, for example, requires the needle to be in the green zone before any operation and forbids energizing the unit if it is in the red. In any sealed-tank RMU, an indicator outside its zone is referred to the manufacturer before operating.
Next, each device is operated several times through its full travel, with the handle and, if motorized, locally and remotely, checking indicators and auxiliary contacts. Interlocks are tested by attempting the forbidden operation: closing the ground switch with the switch closed, opening the cable cover without the grounding function, removing a fuse while live. What each one must block is covered in interlocks in medium-voltage switchgear and RMUs.
Insulation and on-site withstand voltage
Insulation resistance is measured phase to ground with the other phases grounded, and across open contacts, with the cables disconnected or open at their far end. The value is compared between phases and against the factory.
IEC 62271-200 provides for dielectric tests after on-site installation, at a fraction of the factory withstand voltage. The value, the waveform (AC or very low frequency) and the connection point are set by the RMU manufacturer, and beforehand the voltage transformers, surge arresters and any elements the manufacturer names are removed. It is a pass or fail test; the general criterion is in on-site withstand voltage test.
- Does the RMU manufacturer accept that type and value of test voltage?
- No: Do not apply it; test the cable separated from the switchgear or agree the value with the manufacturer
- Yes: Is the function grounded and are the manufacturer's test points and adapters used?
- No: Do not connect the source; first ground the function and access through the intended points
- Yes: Remove VTs, arresters and the indicated elements, ground the neighboring cables and test
Medium-voltage cables
The RMU cables are tested without opening the tank. Depending on the model, access is through the plug-in connectors or through test bushings. On the Lucy Electric SabreEcoTec, the cable-test access cover opens only with the function in the ground position, and the function cannot leave that position until the cover is closed. Before connecting the source, absence of voltage is verified with the switchgear indicators, now standardized in IEC 62271-213, and after the test the cable is discharged to ground. The cable tests themselves are the usual cable diagnostics.
Contacts and timing
Contact resistance is measured with four wires on each closed pole of the switch-disconnectors and the circuit breaker, through the connection points the design allows. On the circuit breaker of the transformer function, opening and closing times and pole simultaneity are also measured and compared with the manufacturer’s range.
Transformer protection: relay or fuses
Relay with auxiliary supply. It is tested by secondary injection of phase and ground overcurrent, with a real breaker trip. The relay of this function is described in transformer feeder relay.
Self-powered relay. Many RMUs use relays that take their energy from dedicated current transformers and trip a low-energy coil. Primary injection is the test that covers that whole chain and confirms ratio and polarity; secondary injection is used only with the adapter the manufacturer indicates. External trips reaching the switchgear are tested as well, such as the transformer’s own protection or an emergency stop.
Fuse combination. Verify that the fuses are of the type and rating on the manufacturer’s list and that the striker of any one of them opens all three phases. The sequence of tests and energization of the complete station is in commissioning a solar skid, and the sequence for energizing the switchgear in energizing a compact substation.
If you also need the relay settings reviewed or loaded, we also configure them remotely.
What not to do
- Operate or energize with the gas indicator outside the green zone.
- Apply test voltage without confirming with the manufacturer the type and value the switchgear admits.
- Leave voltage transformers or surge arresters connected during the dielectric test.
- Test a self-powered relay by secondary injection only.
- Hand over without leaving the ground switches in the agreed position or recording the serial numbers of each function.
Equipment for this test
These tests are done with equipment from recognized manufacturers; these guides explain what each one measures. In the quotation we recommend the test and the equipment your case needs.
- Metrel MI 3252 micro-ohmmeter: contacts at 100 A
- Metrel MI 3201 and MI 3210 megohmmeters: insulation at 5 and 10 kV
- OMICRON CIBANO 500: circuit breaker testing
We test your RMUs
Our team tests RMUs and medium-voltage switchgear at commissioning, after a fault and during maintenance, in compact substations, industry and solar plants, using the protocol for each function. Field work is done in Colombia, or elsewhere on request. Learn about circuit breaker testing or write to us with the make, model and configuration of the RMU.
Related guides
- Solar transformer station (skid): what it is and its parts
- Commissioning a solar skid: tests and energization
- Energizing a compact substation: sequence
- Transformer feeder relay: self-powered or auxiliary
- Medium-voltage compact substation: parts, IAC and tests
- Medium-voltage switchgear maintenance: what is checked
- Schneider RM6 RMU (NE-IQI), FBX and Ringmaster
- Medium-voltage open ring: operation, faults and RMUs
References
- IEC 62271-1, high-voltage switchgear and controlgear: common specifications.
- IEC 62271-103, switches for rated voltages above 1 kV up to and including 52 kV.
- IEC 62271-105, alternating current switch-fuse combinations.
- IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV (tests after installation on site).
- IEC 62271-213:2021, voltage detecting and indicating systems.
- IEC 60282-1, high-voltage fuses: current-limiting fuses.
- Lucy Electric, SabreEcoTec Operation & Maintenance, LIOM19-003, edition 2, July 2024.
Frequently asked questions
How do you test the self-powered relay of an RMU?
By primary injection, because the relay draws its power from the same current transformers that feed its signal. That tests the whole chain of CT, relay and low-energy trip coil, and confirms ratio and polarity. Secondary injection is used only with the adapter or auxiliary supply the relay manufacturer specifies.
What if the gas indicator of the RMU is in the red zone?
The unit is not operated and not energized. The operating manual of the Lucy Electric SabreEcoTec, for example, says so explicitly, and with the indicator outside the green zone the manufacturer is consulted. Many RMUs are sealed-for-life tanks that are normally not refilled in the field.
Can medium-voltage cables be tested from the RMU?
Yes, through the test points the manufacturer provides, which depending on the model are the plug-in cable connectors or test bushings reachable with the function grounded. The manufacturer's adapters are used, the test voltage stays within what the switchgear admits, and the tank is never opened.