If cabinets are classified by switching equipment, they include load switch cabinets, circuit breaker cabinets, GIS (Gas Insulated Switchgear), etc.—there is no specific category called "ring main units". The term "ring main unit" is a conventional name; it originally referred to load switch cabinets used for ring network power supply, but now it is often used as a synonym for load switch cabinets, regardless of whether they are actually used in ring network power supply.
Load switch cabinets can be used for ring network power supply, tapping in medium-voltage distribution rooms, and power supply in medium-voltage terminal substations. Requirements for the specifications of transformers protected by load switches plus fuses vary among power supply departments in different regions. For example, in Beijing, the transformer capacity is generally required to be no more than 1000kVA, while in Shenzhen, it may be 1600kVA.
Load switch cabinets have a simple structure, low cost (though a few imported products are not cheap), and small size. Most can be wall-mounted, and they generally only have fuse protection without relay protection. The rated current-carrying capacity of the main bus of a load switch cabinet is usually ≤630A; the rated breaking current of the load switch is generally ≤630A (a few reach 1700A); the rated current (of the fuse) for transformer cabinets (outgoing cabinets) is generally ≤125A; and the transfer current of high-end load switches can reach 2800A. Some load switch cabinets can be equipped with dedicated vacuum circuit breakers, SF₆ circuit breakers, or use arc-extinguishing methods such as air-blast. Their short-circuit breaking capacity is close to or reaches the level of circuit breaker cabinets. Such cabinets, when equipped with relay protection, should be classified as circuit breaker cabinets.
Medium-voltage circuit breaker cabinets are well-known. They range from old fixed-type GG1a cabinets to current common withdrawable armored cabinets (floor-mounted and middle-mounted types). Circuit breakers have evolved from bulk oil, minimum oil to vacuum and SF₆ types; operating mechanisms include manual operation, electromagnetic operation, and motor spring energy storage; and operating power supplies are AC or DC. Whether relay protection uses GL shunt release, DL definite-time overcurrent protection, or current combined relay protectors, medium-voltage circuit breaker cabinets share common characteristics: complex structure, high cost, large size, heavy maintenance workload, and complete relay protection devices. They can be designed with different types of main bus wirings as needed, with short-circuit current breaking capacity up to 50kA or more, main bus current-carrying capacity up to 3150A or more, and the mechanical life of circuit breakers is generally longer than that of load switches.
From the above comparison, it can be seen that in conventional use, the main difference between load switch cabinets and medium-voltage circuit breaker cabinets lies in relay protection—and this difference determines the applicable scenarioses for each. Based on foreign operation experience, load switch cabinets can replace medium-voltage circuit breaker cabinets in most scenarioses and even have advantages. For example, the breaking speed of current-limiting high-voltage fuses is much faster than that of vacuum circuit breakers, making them more suitable for transformer protection. Domestic power supply departments are gradually accepting this view.
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