Ring main units protect medium-voltage feeders with a combination of switching devices. These include load break switches, fuses, vacuum circuit breakers and earthing switches. Understanding how these components work together is essential. It helps you specify the right protection for your network.

1. Load Break Switches

Typically, the load break switch (LBS) is the core switching device in most RMUs. It makes and breaks load current. This is the normal operating current of the feeder. However, it does not break fault current. This keeps the design economical and compact. In SF6 RMUs, for example, the LBS uses SF6 gas for insulation and arc extinction. An example is the FLN36-40.5 Indoor AC SF6 Load Break Switch.

2. Fuses

In fuse-switch combinations, the fuse provides short-circuit protection. The switch handles load breaking. For example, this arrangement is common for transformer protection in RMUs. As a result, it is simple, fast and inexpensive. When the fuse operates, a trip mechanism opens the switch. As a result, the switch isolates the faulty transformer.

3. Vacuum Circuit Breakers

Some RMUs need reclosing, remote control or full short-circuit breaking. In these cases, install a vacuum circuit breaker (VCB). Also, VCBs break fault current directly. They support multiple operations. This makes them the right choice for important feeders and automated networks. Combined RMU designs with VCB ways are used in primary substations and critical industrial supplies.

4. Earthing Switches and Interlocks

Earthing switches safely earth the circuit for maintenance. They are also a mandatory safety feature of RMUs. In addition, mechanical interlocks ensure the correct operating sequence. For example, the earthing switch cannot close while the load break switch is closed. This interlocking makes RMU operation safe even for less experienced staff.

5. How Ring Main Unit Protection Is Chosen

In practice, an RMU may combine all three options. Load break switches serve the ring ways. A fuse-switch or VCB protects the transformer way. Earthing switches protect every way. The GSRM6 Gas Insulated Ring Main Unit and the HXGN15-12 Ring Main Unit both support these configurations.

A Typical RMU Panel Layout

A standard three-way ring main unit often looks like this. Two ring ways carry load break switches and earthing switches. One transformer way carries a fuse-switch combination. Typically, each way has a position indicator, operating handle and cable compartment. For more critical installations, the transformer way uses a vacuum circuit breaker. This breaker also adds overcurrent and earth-fault protection. The unit can also carry a motor operator and remote control for automated networks.

Whichever layout you choose, confirm the fault rating of every way. It must match the upstream network. Also confirm that the earthing switch can make and break the specified conditions. This keeps maintenance safe.

In practice, protection coordination matters as much as the components themselves. The fuse, relay and upstream breaker settings must work together. Otherwise, a small fault can trip the whole ring instead of one section. Ask the manufacturer for coordination guidance and typical settings when you order. Modern RMUs with motor operators also support remote switching, which cuts outage times significantly.

6. Conclusion

RMU protection is about matching the switching and protection functions to the duty. Then match fuses, load break switches and circuit breakers to the fault level of each feeder. Always keep interlocks and earthing switches in perfect working order. For a related product, see the FLN36-12 Indoor AC SF6 Load Break Switch.

Electric-GS manufactures load break switches, vacuum circuit breakers and complete ring main units in Yueqing, Zhejiang, China. For a quotation or technical consultation, contact info@electricgs.com or WhatsApp +86 135 8897 8050.

Related Products

FLN36-40.5 Indoor AC SF6 Load Break Switch

GSRM6 Gas Insulated Ring Main Unit

HXGN15-12 Ring Main Unit Medium Voltage Switchgear