According to the installation and operation manual of the Chint NG7-12/24 RMU, before each operation you check that the SF6 gauge is in the green zone, above 1.4 bar. A load-break switch function (C or F) is de-energized by opening the load-break switch and then closing the ground switch; a circuit breaker function (V), by opening the breaker, then the disconnector, and closing the ground switch. The load-break switch and the ground switch are mechanically interlocked, but the interlock does not see the other end of the cable: that verification is up to the operator.
The Chint NG7 RMU and its functions
The NG7-12/24 is Chint’s ring main unit for medium-voltage distribution networks: a common SF6 tank with one to five functions, extensible with an extension busbar when more are needed. The manual lists fifteen modules. Those found in a typical ring station or in a solar plant transformer station are:
| Module | What it is | Typical use |
|---|---|---|
| C | Three-position load-break switch: closed, open and ground | Ring in and out |
| F | Fused load-break switch | Protection of a small transformer |
| V | Vacuum circuit breaker with a three-position disconnector | Transformer protection with a relay |
| D / De | Direct cable connection, without or with ground switch | Cable entry with no switching device |
The family and its variants are described in the sheet Chint KYN28A switchgear and NG7 RMU.
What the interlocks block
The manual declares three protections against improper operation:
- Load-break switch and ground switch. A mechanical interlock prevents operating one while the other is closed: the load-break switch opens and closes only with the ground switch open, and the ground switch operates only with the load-break switch open.
- Padlock. Each handle has its padlock hasp, which prevents operating that device.
- Cable compartment. The door is interlocked with the ground switch: it can be removed only with the ground switch closed.
These are the interlocks that IEC 62271-200 asks of metal-enclosed switchgear. The principle common to all brands is in interlocks in medium-voltage switchgear and RMUs.
Sequences from the manual
Before energizing or de-energizing any function, the needle of the SF6 density gauge must be in the green zone, above 1.4 bar.
- Check the gauge. Needle in the green zone, above 1.4 bar
- De-energize. Open the load-break switch and then close the ground switch
- Energize. Open the ground switch and then close the load-break switch
- Check the gauge. Needle in the green zone, above 1.4 bar
- De-energize. Open the breaker, then the disconnector, and close the ground switch
- Energize. Open the ground switch, close the disconnector and finally the breaker
The handles are operated with the manufacturer’s lever, following the front mimic diagram, which shows the position of each device. The load-break switch turns one way to close and the other to open; the F function and the vacuum breaker close after the spring is charged, with the close button, and open with the open button. To operate the breaker with its motor operator from the front, the manual says to set the local/remote selector to local.
Grounding a cable between two RMUs
The interlock of a unit only knows its own device. To work on a cable that joins two RMUs, both ends must be open, locked and grounded:
- Check the gauge on both units.
- Open the load-break switch of the end that keeps the cable energized. If the other end is the normally open point, confirm that it is still open.
- Padlock both load-break switches.
- Verify the absence of voltage on the cable, on the voltage presence indicators of the units and with the voltage detector rated for that voltage level.
- Close the ground switch at both ends. With the ground switch closed, the cable compartment door can be removed.
Steps 3 and 4 are not written by the switchgear manual: they come from the five safety rules of EN 50110-1 and from the local safety rules and the facility operator’s procedure. If the ground switch is closed with the cable energized from the other end, the interlock does not prevent it. In a solar plant, a station with inverters can also feed the cable through its transformer; that part is covered in switching an MV collector circuit.
Restoration
The order is reversed: the cable compartment door is replaced, the padlocks are removed once the tagout is returned, the ground switches at both ends are opened and the load-break switch is closed. Before closing, confirm that no ground is left at the other end: closing onto a forgotten ground is one of the classic causes of failure on restoration.
Maintenance the manual asks for
The manual describes the NG7 as maintenance-free under normal conditions, with an expected life of 30 years. It asks for inspection of the tank pressure, the lubrication of the mechanisms and corrosion, and for one close-and-open cycle of the load-break switch and the vacuum breaker at least once every three years. At the end of its life, the SF6 is recovered according to IEC 62271-303. Tests of the unit itself are in tests of a medium-voltage ring main unit (RMU).
We test your switchgear
Our team tests ring main units of any brand at receiving, commissioning and maintenance: insulation, contact resistance, mechanisms and interlocks, voltage presence indicators and the transformer function relay, in substations, industry and solar plants, in Colombia or elsewhere on request. See testing and commissioning or write to us through contact.
Related guides
- Chint KYN28A-12, KYN28A-24 switchgear and NG7 RMU
- Switching an RMU: opening, grounding and restoration
- Interlocks in medium-voltage switchgear and RMUs
- Switching an MV collector circuit in a solar plant
- Tests of a medium-voltage ring main unit (RMU)
- Open medium-voltage ring: operation, faults and RMUs
References
- CHINT Electric, NG7-12/24 (RMU): installation and operation manual, Russian edition: §1.1, §3.2, §4.2.2 and §5.1, pp. 1, 9, 13–17 and 18.
- IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
- IEC 62271-303, Use and handling of sulphur hexafluoride (SF6).
- EN 50110-1, Operation of electrical installations – Part 1: General requirements.
Frequently asked questions
What do the letters C, F and V mean on a Chint NG7 RMU?
They are the RMU modules according to the Chint manual: C is a three-position load-break switch (closed, open and ground), F is a fused load-break switch to protect a transformer, and V is a vacuum circuit breaker with a three-position disconnector. A typical ring station is C-C-V or C-C-F: two cable functions and one transformer function.
Can the ground switch of a C function be closed with the load-break switch closed?
No. The Chint manual declares a mechanical interlock between the load-break switch and the ground switch: each is operated only with the other open. What the interlock does not check is whether the cable is energized from the other end; that is why that end is opened as well and the absence of voltage is verified before closing the ground switch.
What is checked before operating a Chint NG7 RMU?
The manual asks you to check that the needle of the SF6 density gauge is in the green zone, above 1.4 bar, before energizing or de-energizing the unit. It also says to operate the handles with the manufacturer's lever following the front mimic diagram, and to use the padlock of each handle to prevent an improper operation.