The standard range of contact resistance between cold shrink terminals and cable conductors is determined based on multiple factors, including material properties, contact configuration, environmental tolerance, and long-term operational reliability. The core principle is to ensure manageable energy loss during current transmission by controlling the physical state, electrical performance, and mechanical stability of the contact surface, while also preventing insulation aging or equipment failure caused by local overheating.
Contact resistance is directly related to material properties. Cold shrink terminals typically use highly conductive metals (such as copper or tin-plated copper) as conductors. Their low resistivity reduces Joule heating during current flow. Contact surface treatment is also critical. Surface roughness must be controlled within a certain range. For example, the surface roughness Ra for medium and high current terminals is typically required to be ≤3.2μm to maximize the actual contact area and reduce shrinkage resistance. Furthermore, metal surfaces are prone to forming oxide films or adsorbing contaminants. The resistance of these films can be significantly higher than that of the metal itself. Therefore, plating (such as tin or nickel plating) or applying conductive paste to isolate the metal from the air is necessary to prevent the formation of oxide films.
The impact of contact configuration on resistance is reflected in the matching of contact pressure and contact area. According to an empirical formula, contact resistance is inversely proportional to the contact pressure raised to the power of m, where the value of m depends on the contact type: for point contact, m=0.5; for line contact, m=0.5–0.8; and for surface contact, m=1. Cold shrink terminals typically employ either line or surface contact designs, ensuring stable contact pressure through optimized terminal structure (such as anti-drop barbs or threaded fastening). For example, threaded fastening terminals require the use of 316 stainless steel self-locking nuts and a limited pre-tightening torque range to prevent excessive contact resistance due to insufficient pressure or damage to the contact surface due to excessive pressure.
Environmental resistance is a key consideration in determining the standard range for contact resistance. Cold shrink terminals must maintain stable performance over a wide temperature range. Their contact resistance increases with increasing temperature, with the temperature coefficient of copper conductors being approximately 0.00393/°C. Therefore, standards typically require that the rate of change in contact resistance does not exceed a certain percentage at extreme temperatures (e.g., -40°C to 125°C). Furthermore, corrosive environments such as salt spray and humidity accelerate metal surface oxidation, leading to increased contact resistance. For example, the coating must pass a 240-hour neutral salt spray test to ensure no discoloration or corrosion during long-term use in humid environments.
Long-term operational reliability requires that contact resistance remain stable throughout the device's lifecycle. This is verified through mechanical strength testing. For example, plug-in/out durability testing requires that the contact pressure decay by less than a certain percentage after hundreds of plug-in/out cycles. Vibration testing simulates the vibration environment experienced during device operation to ensure that contact resistance does not change significantly after vibration. For high-voltage or high-current applications, temperature rise testing is also required to ensure that the temperature rise caused by contact resistance does not exceed the maximum allowable temperature rise of the conductor (e.g., 60K for pure copper) to prevent degradation of the insulation material due to overheating.
In practical applications, contact resistance measurements require standardized methods to reduce errors. For example, a microohmmeter applies a constant DC current and calculates the resistance value by measuring the voltage drop. Temperature correction is required to eliminate the effects of ambient temperature. For special materials such as aluminum conductors, the creep properties of the material may cause the contact resistance to change over time, requiring an extended current stabilization time to ensure measurement accuracy.
The standard contact resistance range for cold shrink terminals and cable conductors is the result of a combination of these factors. It must meet both short-term electrical performance requirements (such as initial contact resistance ≤ 0.5mΩ) and long-term operational stability (such as resistance change rate ≤ 15% after temperature rise testing). Determining this standard range is essentially a process of balancing performance, cost, and reliability, aiming to provide safe and efficient connection solutions for power systems.
