Basic Composition of Household Photovoltaic Distribution Boxes
Typically, the household distribution box consists of a knife switch, a self-resetting over and under voltage protector, circuit breakers, surge protectors, and backup circuit breakers for surge protectors.
Circuit Breakers
Circuit breakers (air switches, miniature circuit breakers) mainly provide overload and short circuit protection in circuits, and are also used for infrequent circuit disconnection under normal conditions. Their key technical parameters are rated current and voltage. The rated current is 1.2 to 1.5 times the maximum output current of the inverter’s AC side, with common specifications including 16A, 25A, 32A, 40A, 50A, and 63A. Rated voltages are single-phase 230V and three-phase 400V.
Self-Resetting Over and Under Voltage Protectors
These are commonly used protection switches in low-voltage distribution systems. They automatically disconnect when over-voltage or under-voltage exceeds specified values and automatically close when the voltage returns to normal. They provide dual protection with the inverter’s automatic over and under voltage protection, with common models rated at 20A, 25A, 32A, 40A, 50A, 63A (rated current ≥ main circuit breaker’s rated current).
Surge ProtectorsAlso known as lightning arresters, surge protectors divert surge currents or voltages caused by external disturbances in electrical or communication circuits, thus preventing damage to other devices in the circuit. Selection criteria include maximum operating voltage Uc > 1.15U0 (U0 being the nominal voltage between phase and neutral in low-voltage systems, i.e., 220V phase voltage). For single-phase, 275V is typically chosen, and for three-phase, 440V. The nominal discharge current should be In=20kA (Imax=40kA).
Fuse
If the surge through the surge protector exceeds its Imax, causing the protector to fail and create a short circuit fault, a circuit breaker or fuse is required to interrupt the fault. Each lightning strike can age the surge protector. If leakage current persists for a long time, the protector can overheat and age rapidly. In such cases, the thermal protection system of a circuit breaker or fuse should act to disconnect the surge protector before it reaches its maximum tolerable heat. For Imax>40KA, 40-63A fuse is suitable; for Imax<40KA, 20-32A fuse is appropriate.
DC Isolator Switch
Mainly used for infrequent manual connection and disconnection of AC and DC circuits or as an isolator switch, creating a visible disconnection point for safety indication. The rated current of the DC isolator switch should be ≥ the main circuit breaker’s rated current, with common models being 16A, 32A, and 63A. A physical isolator is essential in distribution boxes to create a visible disconnection point for safety during maintenance and repair, with the DC isolator switch mainly providing overcurrent and short circuit protection.
AC Side Cable
When selecting AC cables, soft copper wires are preferred due to their low resistivity, minimal loss, high current carrying capacity, and avoidance of copper-aluminum chemical corrosion. The rated current of the cable is generally 1.25 times the maximum continuous current calculated for the cable.
Energy Meter
Generally, energy meters in photovoltaic power stations are installed together with the distribution box. In some places, meters are installed separately from the distribution box, but it is advantageous to install them together for proximity, reduced line loss, savings on an extra box, and ease of inspection and maintenance. In China, household photovoltaic power stations are provided and installed with meters by the power supply bureau. To prevent unauthorized tampering with the meter settings, the door of the distribution box housing the meter should have a safety device and be accessible only by the power supply bureau.
Typical Household Distribution Box Design
When choosing a box, metal boxes are preferred over plastic ones, with stainless steel being the best among metal options. In metal boxes, galvanized steel sprayed with plastic offers good value for money, providing secondary corrosion protection. Regardless of whether your photovoltaic distribution box is installed outdoors or indoors, its dust and waterproof specifications should be considered. For outdoor installations, use IP65 rating; for indoor, use IP21. In coastal or severe salt mist environments, it is crucial to choose galvanized steel sprayed with plastic, aluminum-zinc plated steel sprayed with plastic, 304 stainless steel, or higher specification boxes for corrosion protection.