How to choose the distribution terminal?
The selection of distribution terminals needs to comprehensively consider many factors such as electrical performance, mechanical strength, installation environment, cost, etc. The following are detailed selection points and suggestions:
First, according to the electrical parameters selection
1. Rated voltage and current
Rated voltage: it is necessary to match the highest working voltage of the circuit (for example, the circuit voltage is 380V, and the rated voltage of the terminal should be ≥400V) to avoid insulation breakdown.Rated current: according to the maximum working current of the loop, the rated current of the general terminal should be more than 20% larger than the actual current to cope with overload.
For example, for a circuit with a long-term running current of 50A, terminals with rated current of 63A or 100A can be selected.
2. Contact resistance and temperature rise
Contact resistance: Choose terminals with high purity of conductive materials (such as copper alloy) and good surface coating (such as tin plating and silver plating) to reduce contact resistance and heat generation.
Temperature rise requirements: According to the national standard (such as GB 7251), the temperature rise of the terminal under rated current shall not exceed the allowable value (such as copper terminal is usually ≤60K).
3. Short circuit tolerance
For circuits with possible short-circuit current, it is necessary to select terminals with sufficient short-circuit tolerance (such as "short-time tolerance current" parameter) to avoid terminal burning or falling off in case of failure.
Second, according to the mechanical properties.
1. Terminal type and structure
By connection method:
Screw-type terminal: suitable for hard wire connection, and attention should be paid to screw torque (to avoid damage to wires due to too tight or poor contact due to too loose).
Spring-type terminal: suitable for soft and hard wires, with quick connection, but it is necessary to confirm whether the cross-sectional area of the wire is within the terminal adaptation range.
Crimping terminal: the conductor is fixed by crimping tool, which has reliable contact and is suitable for high current scenes (such as the main circuit of power distribution cabinet).
According to the structural form:
Single-layer terminal: simple structure, used for single circuit.
Double-layer terminal: the upper and lower layers are connected independently, which saves space and is suitable for complex circuits.
Terminal with protective cover: to prevent electric shock or foreign body invasion, suitable for outdoor or high dust environment.
2. Cross-sectional area adaptation of conductor
Terminals are usually marked with the range of adaptive wires (such as 1.5 ~ 16 mm), and it is necessary to ensure that the cross-sectional area of wires matches the terminals.
For example, the terminal marked "4 ~ 10 mm" should be selected for 6mm² conductor to avoid that the conductor is too thick to be inserted or too thin to be firmly contacted.
3. Mechanical strength and protection grade
Metal terminals (such as copper and iron) have high strength and are suitable for heavy vibration or heavy equipment; Plastic terminals are light and have good insulation, but the flame retardant grade (such as UL94 V-0) should be confirmed.
Terminals with protection grade IP44 or above (such as waterproof terminals with sealing aprons) should be selected for outdoor or humid environment.
Third, according to the installation environment selection
1. Ambient temperature and humidity
In high temperature environment (such as poor heat dissipation in the cabinet), it is necessary to select high temperature-resistant terminals (such as nylon material with temperature resistance ≥105℃, and metal terminals should consider the heat resistance of coating).
Damp-proof and corrosion-resistant terminals (such as nickel plating or stainless steel) should be selected in wet environment to avoid poor contact caused by oxidation.
2. Installation space and layout
Compact distribution cabinets need to choose terminals with small volume and high density (such as rail terminals, which can save space); Independent terminals can be selected for large power distribution cabinets, which is convenient for maintenance.
Pay attention to whether the terminal spacing meets the electrical safety distance (for example, the phase-to-phase distance is ≥20mm and the creepage distance is ≥25mm).
3. Anti-electromagnetic interference (EMC)
Terminals with shielding function (such as metal shell grounding terminals) should be selected for high-frequency circuits or sensitive equipment loops to reduce electromagnetic interference.
Fourth, other key factors
1. Certification and standards
Select terminals that have passed the national standard (GB), international standard (IEC) or industry certification (such as CE and UL) to ensure reliable quality.
For example, the terminal of low-voltage distribution cabinet should meet the GB 7251.1 standard, and the terminal of fire protection system should pass the fire protection certification.
2. Convenience of identification and maintenance
Terminals with marked strips (printed circuit numbers) are convenient for later maintenance and troubleshooting.
Priority is given to tools-free disassembly or terminals with quick release structure to improve maintenance efficiency.
3. Cost and cost performance ratio
High-quality metal terminals are selected for high-current and high-demand scenes (such as main incoming circuit); Low current control loop can choose economical plastic terminal.
Consider long-term reliability and avoid downtime loss caused by low-cost terminal failure.
Five, common mistakes and evasive suggestions
Wrong choice risk avoidance method
If the current capacity of the terminal is insufficient, it shall be selected as more than 1.2 times of the rated current if it burns out and causes fire.
Mismatch between lead and terminal, poor contact and fracture. Strictly check the cross-sectional area of lead and the adaptation range of terminal.
In wet environment, ordinary terminals are used for oxidation corrosion, and waterproof and nickel-plated terminals are selected for insulation degradation.
Without considering the short-circuit withstand fault, the terminal damage expands the accident. Check the short-circuit withstand current parameters of the terminal.
Ignoring the protection level, foreign body intrusion leads to short circuit, and selecting the terminal with appropriate IP level according to the environment.
Summary: Terminal Selection Process
Determine the loop voltage, current, frequency and other electrical parameters;
Define the cross-sectional area of the conductor and the connection mode (hard wire/soft wire);
Analyze the installation environment (temperature, humidity, vibration, protection requirements);
Select terminals that meet the standards, are marked and are easy to maintain;
Check the cost and cost performance, and give priority to reliability.
Through the comprehensive evaluation of the above dimensions, it can ensure the safe and stable operation of distribution terminals and reduce the risk of electrical failure.
