Common Faults Of Terminal Blocks
The plastic insulating material and conductive parts of Cable Lugs are directly related to the quality of the terminal, which determine the insulation and conductivity of the terminal respectively. Any failure of the terminal will lead to the failure of the entire system engineering. The painful lessons learned at home and abroad in this regard are very profound.
Prevention is the purpose, and analysis is the basis. In a sense, preventing failure is more important than analyzing failure. It has a more practical significance for ensuring the quality and reliability of the terminal.
From the perspective of use, the function that the terminal should achieve is: the contact parts that should be conductive must be conductive and the contact is reliable. The insulation parts that should not be conductive must be insulated reliably. There are three common fatal failure forms of the terminal:
1. Poor contact
The metal conductor inside the terminal is the core part of the terminal, which transmits the voltage, current or signal from the external wire or cable to the contact corresponding to the connector. Therefore, the contact must have an excellent structure, stable and reliable contact retention and good conductivity. Due to unreasonable contact structure design, wrong material selection, unstable mold, over-tolerance of processing size, rough surface, unreasonable surface treatment process such as heat treatment and electroplating, improper assembly, bad storage and use environment and improper operation and use, poor contact will be caused in the contact and matching parts of the contact.
2. Poor insulation
The function of the insulator is to keep the contact in the correct position and to insulate the contact and the contact, and the contact and the shell from each other. Therefore, the insulating parts must have excellent electrical properties, mechanical properties and process molding properties. In particular, with the widespread use of high-density and miniaturized terminal blocks, the effective wall thickness of the insulator is getting thinner and thinner. This puts more stringent requirements on insulating materials, injection mold accuracy and molding processes. Due to the presence of excess metal on the surface or inside of the insulator, surface dust, flux and other pollution and moisture, organic material precipitates and harmful gas adsorption films merge with the surface water film to form an ionic conductive channel, moisture absorption, mildew, aging of insulating materials, etc., short circuit, leakage, breakdown, low insulation resistance and other poor insulation phenomena will occur.
3. Poor fixation
Insulators not only play an insulating role, but also usually provide precise centering and protection for the extended contact parts. They also have the functions of installation positioning and locking and fixing on the equipment. Poor fixation will affect the reliability of contact and cause instant power failure in the mildest case, and the product will disintegrate in the most serious case. Disintegration refers to the abnormal separation between the plug and the socket, and between the pin and the socket caused by the unreliable structure of the terminal due to materials, design, process and other reasons when the terminal is in the plugged state, which will cause serious consequences such as interruption of power transmission and signal control in the control system. Poor fixation will be caused by unreliable design, wrong material selection, improper molding process selection, poor process quality such as heat treatment, mold, assembly, welding, and inadequate assembly.
In addition, poor appearance caused by plating peeling, corrosion, bruises, plastic shell flash, cracks, rough processing of contacts, deformation, etc., poor interchangeability caused by positioning and locking fit size tolerance, poor processing quality consistency, and excessive total separation force are also common and frequently occurring diseases. These types of faults can generally be discovered and eliminated in time during inspection and use.
Reliability screening test to prevent failure
In order to ensure the quality and reliability of the terminal blocks and prevent the occurrence of the above-mentioned fatal failures, it is recommended to study and formulate corresponding screening technical requirements according to the technical conditions of the products, and carry out the following targeted reliability tests to prevent failures.
1. Prevent poor contact
1) Conductivity test
In 2012, there was no such item in the product acceptance test of general terminal block manufacturers, and users generally need to conduct continuity test after installation. Therefore, it is recommended that manufacturers should increase 100% point-by-point continuity test for some key models.
2) Momentary disconnection test
Some terminal blocks are used in dynamic vibration environments. Experiments have shown that only checking whether the static contact resistance is qualified cannot guarantee reliable contact in dynamic environments. Because connectors with qualified contact resistance often still experience momentary power failure when conducting vibration, impact and other simulated environmental tests. Therefore, for some terminal blocks with high reliability requirements, it is best to conduct 100% dynamic vibration tests to assess their contact reliability.
3) Single-hole separation force detection
Single-hole separation force refers to the separation force when the contact piece in the plugged state changes from static to dynamic, which is used to characterize that the pin and the socket are in contact. Experiments show that if the single-hole separation force is too small, it may cause a momentary interruption of the signal when subjected to vibration and impact loads. It is more effective to use the method of measuring the single-hole separation force to check the contact reliability than to measure the contact resistance. When the inspection finds that the socket with a single-hole separation force that exceeds the tolerance, the measured contact resistance is often still qualified. For this reason, in addition to developing a new generation of flexible plug-in contact with stable and reliable contact, the manufacturer should not use an automatic plug-in force tester for multi-point simultaneous testing for key models, and should conduct 100% point-by-point single-hole separation force inspection on the finished product to prevent the signal from being interrupted due to the looseness of individual sockets.
2. Prevention of poor insulation
1) Insulation material inspection
The quality of raw materials has a great influence on the insulation performance of the insulator. Therefore, the selection of raw material manufacturers is particularly important. Don't blindly reduce costs and lose material quality. Choose materials from reputable large factories. And carefully check the batch number, material certification and other important information for each batch of incoming materials. Make good traceability data for material use.
2) Insulation resistance inspection of insulators
Until 2012, some manufacturers' processes stipulated that the electrical performance should be tested after assembly into finished products. As a result, the insulation resistance of the insulator itself was unqualified, and the entire batch of finished products had to be scrapped. A reasonable process should be 100% process screening in the state of insulator parts to ensure that the electrical performance is qualified.
3. Prevent poor fixation
1) Interchangeability inspection
Interchangeability inspection is a dynamic inspection. It requires that the same series of matching plugs and sockets can be plugged and connected to each other, and find out whether there are reasons such as insulators, contacts, etc. that cannot be plugged in, positioned and locked due to the size tolerance, missing parts or inadequate assembly, or even disintegrated under the action of rotational force. Another function of the interchangeability inspection is to timely detect whether there are any metal excesses that affect the insulation performance through plug-in connections such as threads and bayonet joints. Therefore, 100% of the terminals for some important purposes should be inspected for this project to avoid such major fatal failure accidents.
2) Torque resistance inspection
Torque resistance inspection is a very effective inspection method for assessing the structural reliability of terminal blocks. For example, the US military standard MIL-L-39012 stipulates that according to the standard, samples should be sampled for torque resistance inspection in each batch to detect problems in a timely manner.
3) Pass test of crimped wires
During electrical installation, it is often found that individual core crimped wires cannot be delivered in place, or cannot be locked after being delivered in place, and the contact is unreliable. The reason for the analysis is that there are burrs or dirt stuck on the screw threads of individual mounting holes. In particular, when the factory has already installed the last few mounting holes of a plug socket, after discovering this defect, the crimped wires of other holes that have been installed have to be removed one by one and the plug socket is replaced. In addition, due to improper selection of wire diameter and crimping hole diameter, or due to errors in crimping process operation, accidents of loose crimping ends may also occur. For this reason, the manufacturer must test all installation holes of the delivered plug (socket) samples before the finished product leaves the factory, that is, use loading and unloading tools to simulate the wires crimped with pins or sockets to check whether they can be locked. According to the product technical conditions, the pull-off force of each crimped wire is checked.
Without reliable terminals, there is no reliable system engineering. Failure and reliability are two aspects of a contradictory body that are corresponding and interrelated. Through the reliability screening of terminals, various failure modes and failure mechanisms can be discovered, which can lead to a large number of lessons and eliminate various hidden dangers, providing a scientific basis for improving design, process, inspection and use. It is also an important basis for revising and formulating technical conditions for terminals. Finding measures to prevent failure and realizing the transformation from failure to reliability is the ultimate goal of failure analysis.
