A secondary distribution box should have appropriate circuit breakers, leakage protection, surge protection, and an enclosure rated for environmental conditions, typically IP55 or higher for outdoor or industrial use.

Electrical Protection

Secondary distribution boxes generally contain large three-phase circuit breakers to protect downstream equipment from overloads and short circuits, as well as leakage protection devices to prevent electric shock hazards . This forms part of a two-level protection system, where the upstream main distribution box provides primary protection, and the secondary box ensures safety for connected equipment and personnel .

Surge Protection

For sensitive or critical loads, it is recommended to install a secondary surge protective device (SPD) in the distribution box. While the primary SPD at the service entrance handles large transient surges, the secondary SPD protects downstream equipment and the distribution box itself from residual voltage spikes, ensuring full protection for UPS systems and other critical devices . The selection of SPD level (Type I or Type II) depends on the building's lightning protection level and the expected surge current, with Type I preferred for higher flow rates and longer service life .

Environmental Protection

The enclosure of a secondary distribution box should be chosen based on the installation environment. IP ratings indicate protection against dust and water ingress. For industrial or outdoor environments, an IP55 or IP65 rating is recommended, providing dust resistance and protection against water jets or heavy rain . For extremely dusty or wet conditions, IP66 or IP67 may be necessary to ensure long-term reliability .

Summary of Key Requirements

• Circuit Protection: Three-phase circuit breakers for overload and short-circuit protection.
• Leakage Protection: Residual current devices to prevent electric shock.
• Surge Protection: Secondary SPD (Type II) for downstream equipment; Type I may be used for higher lightning protection levels.
• Enclosure Rating: IP55 minimum for outdoor/industrial use; higher ratings (IP65–IP67) for harsh environments.
• Installation Principle: Follow “one machine, one switch, one leakage, one box, one lock” to avoid multiple devices being controlled by a single switch . By combining these measures, a secondary distribution box can provide reliable, safe, and durable protection for electrical systems in both commercial and industrial applications.