To enable power outage maintenance for the incoming lines (ring network power supply lines) and outgoing lines (user power supply lines) of the cable distribution box, a load switch shall be installed inside the box. The load switch shall be capable of interrupting active load currents ≤ 600A, small capacitive currents of unloaded cable lines, small inductive currents of unloaded transformers, and transfer currents; closing short-circuit currents of 50kA (peak value); and withstanding thermal withstand currents of 20kA and dynamic withstand currents of 50kA (peak value). It can be of types such as SF₆ load switches or compressed-air load switches with metal enclosures or insulated enclosures.
A commonly used cable distribution box equipped with a metal-enclosed SF₆ load switch and shielded separable connectors (see Figure 3) consists of a grounded stainless steel enclosure filled with low-pressure SF₆ gas, which houses the load switch and earthing switch (with interlocking to prevent incorrect operation between them). The enclosure is provided with epoxy resin-cast "bushing" outlets, whose interfaces can mate with those of shielded separable connectors. This type of distribution box is maintenance-free, resistant to short-term flooding, and compact in size but relatively expensive.
The cable distribution box with an insulated-enclosed SF₆ load switch and shielded separable connectors has its load switch and earthing switch assembled in an epoxy resin-cast insulated enclosure. These switches feature three positions (closed, open, earthed) and also use low-pressure SF₆ gas as the insulating and interrupting medium, boasting excellent sealing performance, moisture resistance, and dustproofing. The load switch and earthing switch are operated by two manual operating mechanisms respectively, with interlocking to prevent incorrect operation. This maintenance-free and compact product can support up to 8 branches (2 incoming and 6 outgoing lines), is cheaper than the aforementioned type, has various classifications, and is suitable for any occasion in ring network power supply lines and radial power supply lines.
The distribution box adopting compressed-air load switches features mature technology, eliminates concerns about SF₆ gas leakage, can be equipped with fuses, and is more cost-effective. However, its overall dimensions are relatively large.
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