What Is A Ring Main Unit Used For?

What is a ring main unit used for?

1.What is a ring main unit used for?

A Ring Main Unit (RMU) consists of individual modular units. It connects to a ring circuit that can be supplied from one or both ends by single or dual power sources. This configuration creates redundancy for the electrical load. If one power source fails, the load can still be supplied from the other end, thereby significantly improving power supply reliability.

 

2.How is the product model named?

Initially launched as "SafeRing," this product featured predefined, fixed configurations like CCV and CCVV, which were not expandable. In response to market needs for scalability, ABB introduced "SafePlus," building upon SafeRing to add extension capabilities and additional units. Consequently, the product line is now collectively known as SafeRing/SafePlus.

3.What units does the product have?

SafeRing/SafePlus has many standard unit, but a few of the most common unit is C, F, V and De

Available Modules:

C - Cable Switching Module (Load Switch)

De - Direct Cable Connection Module with Earthing

D - Direct Cable Connection Module

F - Switch-Fuse Module

V - Vacuum Circuit Breaker Module

V20/V25 - High-Capacity Vacuum Circuit Breaker Module (20/25 kA)

Be - Busbar Earthing Module

Sl - Busbar Sectionalizer, Load Switch Module

Sv - Busbar Sectionalizer, Vacuum Circuit Breaker Module

Sv20/Sv25 - Busbar Sectionalizer, Vacuum Circuit Breaker Module (20/25 kA)

CB - Circuit Breaker Module

M - Metering Module

Mt - Metering Module with Tariff Functionality

 

 

Typically, the C module is used for the incoming feeder in an RMU, while F and V modules are used for the load or T-off feeders. This is primarily because the C module is lower in cost and does not include protection functions. In contrast, both F and V modules provide protection; the F module relies on fuses, and the V module utilizes a circuit breaker, making it the higher-cost option.

Currently, in some overseas regions with lower levels of automation and higher cost sensitivity, the F module remains quite common. However, in more developed markets (e.g., Saudi Arabia, Europe), the F module is being phased out as it cannot support distribution automation. Modern eco-friendly gas-insulated RMUs generally do not offer the F module.

It is important to note that the C/F/V terminology was originally introduced by ABB. Other manufacturers like Schneider Electric and Siemens have their own naming conventions for RMU modules. Nonetheless, because many Chinese manufacturers base their designs on ABB's concepts, the C/F/V designations have become customary in the industry.

4.What combinations are there?

 

Generally, for companies like ABB and Schneider Electric, highly customized requests from customers can significantly increase product costs. Therefore, they typically predefine a range of standard configurations. If a customer requires a highly customized solution, such projects may be assigned to authorized manufacturers to handle the engineering design work.

However, in China's power industry, Ring Main Units (RMUs) generally accommodate a wide variety of customization requests. Common examples include adding potential transformers (PTs) on the incoming line side or installing PTs directly on the busbar.

5.Technical parameters

ABB's overall parameters are well-balanced, though some are specified conservatively low. For instance, the mechanical endurance for the load switch and earth switch is typically rated for only 1,000 operations.

It is important to note that the SafeRing/SafePlus platform is a single product line covering 12kV, 17.5kV, and 24kV ratings. This means that even a unit specified for 12kV is designed to meet the technical requirements for 24kV, such as the 125kV lightning impulse withstand level. However, due to differences in manufacturing processes, many domestically produced imitations often fail to achieve the 24kV insulation performance in their 12kV-rated products.

Similarly, regarding internal arc containment, ABB's design is such that the full 25kA/1s rating is only achieved when an upward arc venting channel is installed. With cable trench pressure relief alone, the rating is 20kA/1s.

Even this 20kA/1s internal arc rating is something many manufacturers in China struggle to meet in practice.

6.Technical Strengths and Weaknesses

Overall, this product features a well-balanced design and low cost, which are the key reasons for its widespread popularity in China. However, with the emergence of competing products, a few of its parameters have become less competitive. For example:

The current rating for the F (Switch-Fuse) unit is limited to 125A, whereas competing products typically offer 200A.

The cable installation height is only 595mm. Other brands generally provide at least 700mm, which offers significant practical advantages for on-site cable bending.

It lacks a dedicated cable test port. To perform cable testing, the cable compartment door must still be opened, and a specific cable bushing (bushing) is required. In contrast, most competitors offer specialized cable test adapters, making the operation much more convenient.