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The Design of the Terminal Blocks
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The Design of the Terminal Blocks

Views: 221     Author: Site Editor     Publish Time: 2020-12-01      Origin: Site

As a kind of connector, the terminal block is an important component in the electrical industry and plays an irreplaceable and non-negligible role. Because engineers and technicians first start with interface inspection, that is, terminals, when overhauling, the design of wiring terminals is particularly important. The design of terminal block products is a comprehensive design integrating product standards, materials, structure, plating, certification, molds, and manufacturing processes.


1. Product standards play a general guiding ideology.

Almost all considerations are based on product standards. Our terminal design standards are first based on UL and CSA, but other standards can also be used as the basis for some electrical parameters, such as high and low-temperature tests.


2. The choice of material directly affects the performance of the entire product and is the key to the design.


Take plastic materials as an example: If the design is based on UL94 and V-0 flame retardancy, it is necessary to carefully review whether the technical parameters of each material property table can meet the product standards, such as whether the impact withstands voltage and aging resistance test can pass. In terms of hardware materials, the choice of TP sheet material is particularly important, because the sheet must meet not only a certain conductivity (current) but also have certain flexibility.


The choice of materials has brought difficulties to our engineers, and it is precisely in this respect that connector manufacturers are pursuing the trend of elastic components. Many manufacturers of hardware materials are working hard in this regard. The electrical conductivity directly affects the temperature rise and contact resistance, and the quality of elasticity is related to the chemical elements, elastic modulus, hardness, and tensile strength of the material. The elastic modulus is calculated using the fourth strength theoretical formula of material mechanics.


The greater the conductivity of the material, the lower the contact resistance and the lower the temperature rise. The relationship between the insertion force and the contact resistance is a quadratic curve. Contact resistance is mainly divided into: compression resistance, film resistance, volume resistance (the resistance of the conductor itself). Among them, the film resistance accounts for 70-80% of the total resistance, which is also the main factor affecting the life of the connector and should be given full attention. Just compare the terminals with gold plating and silver plating. Although the conductivity of silver is higher than that of gold, its chemical stability is not as good as gold, and the film resistance caused by it is much greater than that of gold plating. Therefore, understanding the relationship between the above has guiding significance for our selection of materials and is a prerequisite for product design.


3. The design of product structure is also crucial,


The product structure is entirely a matter of experience, and there is no shortcut. In this regard, each series has its own characteristics, such as: anti-falling screws, spliced products are arc-shaped front and back, deformation of long digits, etc., shrinkage deformation caused by uneven wall thickness.


There are mainly the following types of screws to prevent falling: three ribs to prevent falling, hoop mouth to prevent falling, neck mouth to prevent falling, and punching to prevent falling. Due to the influence of technology, the hoop mouth guard is not used much, and most of the neck mouth guards. The realization of the above structure is based on the premise that the plastic material is PA66, and the prevention of the neck opening needs to be explained here. Taking the screw as M3 as an example, the actual outer diameter of the M3 screw is φ2.90mm, so the size of the shell neck should be designed in: φ2.5~φ2.6, the thickness of the shell neck should be 0.4~0.5mm, and there should be a polished rod under the head of the screw which is smaller than the neck of the shell. Only in this way can the screw be smoothly screwed into the thread. The production process should also be adjusted accordingly. The following is a statistical and solution plan for the structural problems of each series of products.


terminal block application -1

1) The deformation problem of the growth digits of the splicing product portfolio.

The main reason for this is also due to the unreasonable structure of the two splices, which lead to unbalanced forces on the front, rear, up, down, left, and right. As shown in the structure design, the force and deformation direction of the splicing should be considered.


2) The problem of screws preventing falling.

In this regard, I recommend using the neck to prevent it from falling off as much as possible. Compared with the anti-dropping of the hoop, it avoids the troubles caused by the tolerance accuracy of the screw polished rod and the hoop, and the structure is reliable and the tolerance is easy to control.


3) The problem of salt spray test of screws and threaded steel parts after electroplating.

Because it is in accordance with the standard of the company's thread plating in 2012 (the coating is limited, but the thickness of the coating corresponding to the thread is still to be determined). There are two ways to solve this problem under the normal electroplating process, choosing the correct sealing agent and using the plating layer to electroplate multiple times. Both methods are to cover the holes created by the screws that are invisible to the naked eye. The cost of electroplating multiple times is higher, so the first method is recommended. The key to this method is the study of the sealant formulation.


4) The problem of plugging force.

There are many things involved in this aspect, which are related to materials, electroplating, structure, and the applied industry. It is an important mechanical performance requirement of the connector industry, especially plug-in terminals. The choice of material is limited by conductivity (current), contact resistance, and temperature rise in terms of insertion force. I won't give a detailed explanation here. Electroplating is mainly affected by the type of coating and whether it is pre-plated.


Generally speaking, the press plate after pre-plating under the same material and structure is more stable in insertion and extraction force than the press plate which is stamped and electroplated. Because in the post-electroplating process, the hardware material is hardened invisibly, and the prior stamping just eliminates this phenomenon. The main consideration in the structure is the contact between the press and the lead pin. The contact between the lead pin and the pressing piece is mostly lined contact, which causes the sudden large moment when the insertion occurs during mating, and then suddenly becomes smaller. This gives customers an extremely unstable feeling during use and affects their lifespan. If the contact between the pressing piece and the lead pin is changed to surface contact in terms of structure, the problem of insertion and extraction force is solved and the contact resistance and temperature rise are reduced.


4. Product application

Terminals are mainly used in lighting, communications, security, and other industries. The application industry of plug-in terminals is the most extensive, such as frequency conversion, vandal-proof, numerical control panel, access controller, sensor, PLC, instrumentation, power supply, servo drive, etc. The common requirements of the above-mentioned application industries for plug-in terminals are: the plugging force must be stable, the contact resistance must be small, and certain life and fatigue must be met. Therefore, pluggable terminal blocks have higher requirements for hardware shrapnel materials. This requires product designers to have certain experience in structure and material selection.


Barrier terminal blocks are mainly used in power supplies, relays (especially double-layer terminals), frequency converters, elevator industry (less), air conditioners, etc. The requirement of the barrier terminal blocks is: when crimping, the screw torque is higher. The distance from the screw to the highest position is greater than the maximum crimping range. For long-digit terminals, there should be no warping and deformation, and the corrosion resistance of the screws is higher.


5. International and national regulations

With the beginning of China's 3C certification system, the country has put forward higher requirements for the safety and electromagnetic compatibility of electrical products. Related standards include GB16836-2003 "General requirements for the safety design of protective devices for measuring relays"; GB/T14598.3-1993 "Insulation test of electrical relays"; JB/T 9568-2000 "Relays, protection and General Technical Requirements for Automatic Devices"; International Standard IEC60255-27 (CD:2002) "Product Safety Requirements for Measuring Relays and Protection Devices". Among them, the safety items involved in GB16836-2003 mainly include mechanical structure, enclosure protection, insulation coordination, fever, fire test, electric shock prevention, safety signs, and safety instructions.

1) Safety requirements for mechanical structure

The terminal block shall ensure personal safety in terms of chemical and structural materials and live moving parts. The edges and corners of the housing, the burrs of the hardware, the connection strength of various parts, and the stability of the product under sports and other conditions are all requirements of the mechanical structure of the power industry on the terminal.


2) Protection requirements of the shell

The housing of the terminal block mainly needs to meet the requirements of JB/T 9568-2000 for the protection level. It is mainly divided into two types, one is the protection level of solid foreign objects, and the other is the harm caused by water to equipment and products.


3) Insulation coordination requirements

According to the materials used, the working voltage, and the environmental pollution level, the electric clearance and creepage distance are selected to protect the safety of workers and products. At the same time, the terminal will not be damaged due to overvoltage.


4) Requirement of tracking index

During the use of the terminal, the surface of the solid insulating material forms a conductive path under the action of the electric field and the electrolyte, which reduces the insulation performance of the housing insulating material and affects the safety of the product. If the creepage tracking index is high under the same insulation voltage level, the creepage distance of the product can be reduced. If the product enters the power industry, please pay full attention to this aspect.


terminal block application -2

5) Thermoplastic materials require deformation during work

The temperature rise generated by the terminal during operation and the high temperature caused by the contact resistance causes the shell to deform and soften, thereby causing an electric short circuit and causing a serious accident. Therefore, the selection of the current-carrying component material of the terminal is obviously very important.


6) Anti-electric shock requirements

In terms of structure, operating elements should not be charged. The internal connection terminals should not be accessible. This aspect is mainly the requirement for the reliability of screw torque. The screws and wires of accessible terminals should be prevented from accidentally loosening and short-circuiting. Such as the requirements of the ground terminal: the grounding resistance of Class I safety products should not be greater than 0.5Ω.


6. Mould

At least in the connector industry, almost all important parts are realized through molds, with high efficiency, good precision, and easy maintenance. The main surface of the terminal block is mainly plastic mold and continuous hardware mold.

Plastic mold: mainly in design. First of all, the selection of the parting surface of the product is very important. It determines the design of the mold's own structure, as well as the design of the side core (or four sides) inlet and direction. The die system is also a very important item. The design of some structures will be affected by the type of plastic material, and the uniformity of the insulating shell structure will affect the mold. The inserts on the front mold and the back mold should avoid the phenomenon of mating.


As a product design engineer, if you only consider the functions and standards of the product, it will affect the normal production and appearance of the mold. Therefore, it is also necessary to take into account the mold manufacturing process. Also need to consider the molding process of the injection molding machine, including injection temperature, molding time, clamping force, injection speed, etc.

The influence of the molding process of the injection molding machine is also very important.


7. Other manufacturing processes

The manufacturing processes of other parts used in the terminal include instrumentation, casting, heat treatment, etc. It is not only to understand the above process, but more importantly, to accumulate experience, summarize in time, and make progress gradually. Therefore, in terms of research and development, greatly improving the quality of product project engineers and increasing the management system of project engineers are the keys to complete product development.


As a professional China terminal block connector manufacturers, Sun possess development division which has extensive engineering knowledge to design high quality and durable pluggable terminal block connector. Our laboratories are regularly verifying compliance with specifications that our products have to meet for approvals and operation. Of course, the respective test reports and declarations of conformity are available for our customers. If you have any needs, please contact us!

Sun Electric Co., Ltd
Founded in 2011, Sun Electric Co., Ltd specializes in the production of terminal blocks and industrial electrical connection products.

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