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Secondary switch in distribution box

Secondary switch in distribution box

Secondary distribution boxes typically contain large three-phase circuit breakers, ring main units (RMUs), or overhead line switches to manage medium-voltage power distribution.Overview of Secondary DistributionA secondary distribution box serves as an intermediate stage between the primary distribution cabinet and the final tertiary distribution to individual units or appliances . It is commonly used in buildings, construction sites, or localized networks to handle three-phase electricity and medium-voltage loads, usually ranging from 11 kV to 24 kV . Unlike primary distribution, which handles high-voltage transmission, secondary distribution focuses on localized, reliable power delivery to multiple end points.Types of Switches in Secondary DistributionLarge Three-Phase Circuit BreakersThese are the main protective devices in secondary distribution boxes.They interrupt fault currents and protect downstream equipment from overloads or short circuits .Ring Main Units (RMUs)RMUs are factory-assembled, metal-enclosed switchgear used in ring-type distribution networks.They typically include two switches connecting both sides of the load to the main conductors and a fusible switch or circuit breaker to feed a distribution transformer .RMUs are compact, modular, and suitable for urban or confined spaces.Overhead Line SwitchesUsed in overhead medium-voltage networks, these switches can be air break disconnectors or load break switches insulated with air or SF6 gas .They provide reliable switching and protection for overhead lines, especially in rural or cost-sensitive installations.Function and ProtectionSecondary distribution switches are designed to:Control and isolate power to specific buildings or floors.Protect equipment from faults using circuit breakers or fuses.Enable operational flexibility, such as switching between feeders in secondary selective service networks . These switches form part of a three-level protection system, ensuring that each machine or load has its own dedicated switch and protection device, following the principle of "one machine, one switch, one protection" .SummaryIn summary, the secondary distribution box contains large three-phase circuit breakers, RMUs, or overhead line switches, depending on whether the network is underground or overhead. These switches manage medium-voltage power, provide protection, and ensure reliable distribution to downstream loads such as buildings or construction sites .

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