News Detail

1.Ring Main Unit (RMU) Definition

Ring Main Unit cabinet or medium voltage cabinet is integrated electrical equipment to connect, measure, and integrate fixed type breaker withtransformerprotection function.

RMU cabinets are small in size, highly reliable, safe, easy to maintain, easy to replace, and expand. Depending on requirements, the RMU is available in different voltages suitable for both indoor and outdoor installations.

2. Structure of a Ring Main Unit (RMU)

The RMU cabinet is connected by output and input cable compartments by isolating breakers and cables out to transformers using load breakers and protective fuses.

In general, the switch element (including load breaker, cutter machine, and ground knife, busbar, and terminals) is placed in an SF6 gas tank made of a substance that is particularly resistant to the environment, such as dirt, oil grease, not rust. 

RMU cabinets are divided and connected by separate compartments, with RMU standards with two compartments, four compartments, six compartments, depending on customer requirements with functions of them.

In the past, most protection schemes for medium / low voltage transformers in distribution grids used fuse combined with a cutter knife commonly. It’s a simple and cost-effective solution. On the other hand, circuit breakers in the past were often expensive and used in high/medium voltage (HV / MV) stations instead of medium / low voltage (MV / LV) stations.

Recently, mid-voltage cabinet manufacturers such as Schneider Electric design a compact RMU cabinet breaker that incorporates SF6 gas insulators, self-powered relays, and optimized current transformers at a total cost of ownership the equivalent of a mid-voltage fuse-cutter solution.

3. Purpose of using RMU

The essential functions of the switchgear are:

Circuit control: It controls the circuit to switch functions.

Isolation: It isolates the faulty equipment from the rest of the circuit

Protection: It protects the circuit from ground-fault current, short-circuits current fault, and overload.

Compared with medium voltage fuses, the circuit breaker combined with relays will enhance the ability to protect selectively; Avoid incorrect operation of the transformer‘s surge current; Detecting phase-to-phase fault currents and small earth intensity.

RMU electrical cabinets are used with many different applications in the secondary circuit distribution system, with the function of input switching for transformer stations,integrated transformer stations, etc., in many fields of business.