Problems and Precautions in the Operation of Distribution Boxes

Outdoor low-voltage power distribution boxes (hereinafter referred to as "distribution boxes") are low-voltage distribution equipment used in 380/220V power supply systems to receive and distribute electrical energy. They are generally installed at locations such as the low-voltage side of distribution transformers. The interior is typically equipped with protective devices like fuses, leakage protectors, and surge arresters; control devices like contactors, circuit breakers, load switches, and disconnectors; metering devices like current transformers and energy meters; and compensation equipment like capacitors. With the implementation of urban and rural power grid construction and renovation projects, the widespread use of distribution boxes, and the continuous increase in social electricity consumption, various operational problems have emerged successively, which require attention.

1. Excessive Temperature Reducing the Service Life of Electrical Equipment inside the Distribution Box

The maximum ambient temperature around electrical equipment designed and manufactured according to national standards should not exceed 40°C during operation. However, for distribution boxes operating under the scorching summer sun, due to direct sunlight, heat reflection from cement ground, and heat generated by the equipment inside, the temperature inside the box can sometimes exceed 60°C. Such high temperatures can easily cause insulation aging and breakdown burning of electrical coils and leads. High temperature also increases the contact resistance of electrical contacts, which in turn加剧 heating, creating a vicious cycle that ultimately leads to contact burning. Furthermore, excessive temperature affects the stability of protection characteristics, the reliability of operation, and the accuracy of metering. Therefore, it is recommended:

(1) Choose distribution boxes with louvered vents on both sides and an incomplete internal partition to facilitate air convection for heat dissipation.

(2) The box body should preferably be made of natural-color stainless steel, which is less prone to corrosion and reflects heat. If heat-insulating coatings can be applied periodically to reduce heat radiation, the effect would be even better.

(3) Besides ensuring ventilation, the box should be positioned to avoid direct midday sunlight, and the ground beneath should preferably be non-gravel.

(4) Avoid overloading equipment during high-temperature seasons and minimize heat generation from devices inside the box.

2. Lightning Protection Limited by Installing Surge Arresters Only on the Incoming Line Side

Typically, fuses or other devices are installed between the incoming/outgoing lines and the busbar inside the distribution box. If an outgoing line is struck by lightning, causing the incoming line fuse to blow first, the entire box loses lightning protection. Many distribution boxes are damaged by lightning strikes every year. It is recommended to install zinc oxide surge arresters on all incoming and outgoing line sides of the distribution box.

3. Use of Inappropriate Products Increasing the Failure Rate of Distribution Boxes

It is advisable to select high-quality, low-resistance products (e.g., low-resistance fuses), which can not only reduce losses but also decrease heat accumulation within the box, extending equipment service life. Additionally, the safety margin for some components should be appropriately increased. Due to the high internal ambient temperature, the current-carrying capacity margin for conductors should be increased by at least one specification. Without changing the rated current of the fuse element, selecting a slightly larger physical size for the fuse holder can reduce the probability of its base burning out.

4. Improper Installation Techniques Causing Overheating and Burnout of Connections

Some electricians, when replacing leads, fail to use crimped lugs, instead twisting stranded wires to form a lug for screw connection, leading to burned-out leads shortly after replacement. In boxes produced by some manufacturers, branch lines are overlapped and screw-connected directly onto the main bus, leading to poor heat dissipation and frequent failures under heavy loads. It is recommended to add a distribution block on the load side of the main bus, with branch lines connected from this block. This improves heat dissipation, appearance, clarity, and facilitates secure wiring.

5. Commissioning Without Inspection, Creating Safety Hazards

Although products provided by manufacturers undergo strict factory inspections, transportation bumps and handling vibrations can cause some connection bolts to loosen upon arrival. This leads to overheating of wire connections shortly after operation. It is recommended to perform inspection and re-tightening before commissioning.

6. Other Issues

• Improper Installation Location: Improper placement affects the urban  landscape and makes the box susceptible to external damage. Choose an appropriate location considering all factors.

• Inadequate Grounding System: Some TN-C systems (protection neutral connection) still use      the three-phase four-wire supply method. The neutral wire in the low-voltage network is often long with significant impedance. Under  unbalanced three-phase loads, zero-sequence current flows through the neutral. Furthermore, due to environmental factors, conductor aging, and moisture, leakage currents can also create a loop through the neutral,causing it to carry a potential, which is detrimental to safe operation. Adopting a TN-S system (three-phase five-wire supply) is recommended.      Here, the working neutral and protective earth conductor are separate,effectively isolating the hazardous voltages possible in the TN-C system and keeping equipment enclosures at "earth potential," thereby eliminating the risk.

• Inadequate Spacing and Features: Insufficient clearance between devices and between phases, sometimes without visible disconnection points, poses risks for electricians and prevents live replacement of fuses during rain or fog.

• Lack of Phase-loss Protection: The absence of phase-loss protection leads to motor  burnouts due to single-phasing.

• Use of  Non-electronic Meters: Some boxes lack electronic energy meters,preventing remote centralized meter reading.

• Lack of  Maintenance: Some boxes remain closed year-round without routine inspection and maintenance.

The author believes that in locations requiring high power supply reliability and/or having poor environmental conditions, the distribution box specifications should be appropriately increased to facilitate maintenance; forced cooling measures or high-temperature resistant electrical components should be used where necessary to reduce failure rates; and intelligent equipment should be installed for remote monitoring and dynamic management to achieve safe, high-quality, and reliable power supply.