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As a common connection component for all engineers, terminal blocks have been used for many years to provide semi-permanent secure wiring for a variety of applications. A terminal block, also known as a terminal block, terminal connector, or threaded terminal, consists of a modular housing and an insulator that connects two or more wires together. Because the connection is semi-permanent, the terminal block helps simplify the field inspection and repair process. Although it is a relatively simple component, but before the selection of terminal block and its specifications have a basic understanding or good.

This discussion will cover common terminal block types, key electrical and mechanical considerations, and provide some further details to help engineers with selection.

Common configuration

PCB mount type, fence type and straight-through type are the three most common terminal block types in design. The following table lists the three different types and their rationale, installation, and configuration.

Important Electrical specifications

There are a number of key electrical specifications to consider during the design phase, covering common terminal block types. Specifically include:

Rated current. In general, the specification that needs most attention in junction box design is the rated current. This is based on three aspects: the electrical conductivity of the terminals, the cross-sectional area and the corresponding temperature rise. When selecting terminal blocks, it is recommended that the rated current be at least 150% of the maximum expected current of the system. If the rated current of the terminal block is incorrect and the operating current is too high, the terminal block may overheat and be damaged, resulting in serious safety problems.
Rated voltage: The rated voltage portion of the terminal block is affected by the spacing and dielectric strength of its housing. In the same way that the rated current is selected, the rated voltage of the terminal block must be greater than the maximum voltage of the system, taking into account any voltage surges that may damage the connection.
Number of poles: The number of poles is a common way of expressing the number of independent circuits contained in a terminal block. This specification generally varies from unipolar to 24.
Spacing: The spacing is defined as the center distance between adjacent poles, which is determined by the overall rating of the terminal block and involves factors such as creepage distance, voltage/current, and clearance. Some common examples of spacing include 2.54mm, 3.81mm, 5.0mm, etc.
Wire size/Type: In North America, wire acceptable for terminal blocks is in American Wire gauge (AWG), which specifies the wire size or gauge acceptable for the module to ensure that the wire physically fits into the housing. Fortunately, most terminal blocks have tolerances that can accommodate a range of wire sizes such as 18 to 4 or 24 to 12AWG. In addition to the wire gauge, consider the wire type depending on the type of module selected. Twisted or multi-core wires are ideal for threaded terminals, while single-core wires are usually paired with push-in terminal blocks.
Important mechanical specifications

Next comes the mechanical specification, which relates to the size of the terminal block, orientation, and ease of handling of the connection in the design. Important mechanical factors include:

Wiring directions: Horizontal (90°), vertical (180°) and 45° are the three most common terminal block directions. This choice depends on the layout of the design and which direction is most suitable and convenient for wiring.
Figure 1: Typical terminal block orientation (Image source: CUI Devices)

Wire fixation: Similar to the orientation, there are three common ways of wire fixation for terminal blocks: threaded terminals, push-buttons, or push-in. All three of these categories are fairly worthy of the name. A threaded terminal or screw-type terminal block contains a screw that, when tightened, closes a clamp to secure the conductor to the conductor. The button function is very simple, just press a button, open the clip to allow the wire to insert, release the button and close the clip to clamp the wire. For push-in terminal blocks, the wire can be inserted directly into the housing and a connection can be established without a screw or button to open the clamp.
Figure 2: Typical wire fixation method (Image source: CUI Devices)

Interlock type and single type: terminal block can be interlock type or single type housing. Interlocking terminal blocks are usually available in 2 – or 3-pole versions, allowing engineers to quickly achieve different numbers of poles or connect different colors of the same module type together. Monomer terminal block is undoubtedly all the poles are contained in a module, according to the design requirements, so that it has higher rigidity and robustness.
Figure 3: Interlocking versus monomer terminal blocks (Source: CUI Devices)

Wire-to-shell: Plug – in terminal blocks are a good choice for frequent connection and disconnection of the main connection. These are done by inserting the wire into a modular plug and then connecting the plug to a fixed socket on the PCB, making it easy to disconnect without having to deal with individual wires.
Figure 4: Plug and socket connection of plug and plug terminal block (Image source: CUI Devices)

Safety levels and other considerations

UL and IEC are the main safety bodies for certifying terminal blocks. UL and/or IEC safety standards are usually listed in terminal block specifications, and parameter values often vary. This is because each mechanism uses different test standards, so engineers must understand the safety requirements of their overall system to select the appropriate terminal blocks.

While some elements may be an afterthought in many designs, it pays to color customize the housing or buttons of the terminal block. By choosing unique colors for terminal blocks, engineers can more easily connect points in complex systems without misconnecting them.

Finally, in environments or applications dealing with extreme temperatures, terminal blocks with higher temperature grades can also be selected.