Understanding Terminal Blocks for Safe Connections
Learn about the key electrical and mechanical considerations, safety ratings, and more in greater detail to aid in the terminal block selection process.
Consisting of a modular housing and insulated body, terminal blocks are used to secure two or more wires together in electrical systems in need of safe connections. Also known as screw terminals, terminal connectors, or connection terminals, terminal blocks offer engineers semi-permanent wire connections that provide organized and simplified inspection or repair capabilities in the field.
Although a widely used component in numerous applications, it is still important to have a basic understanding of common terminal block types and specifications before making a final selection. This article will cover topics including key electrical and mechanical considerations, safety ratings, and more in greater detail to aid engineers in the selection process.
Common Types of Terminal Blocks
Three of the most common terminal block types used in designs are PCB mount, barrier strips, and feed-through. PCB mount terminal blocks operate by inserting bare wires into the module where a clamp secures the wire in the housing. This housing is then soldered to a PCB, which is why they are commonly called wire-to-board terminal blocks. Also referred to as Eurostyle terminal blocks, they can be found in single-, dual-, or multi-row configurations.
Barrier strips are often used where vibration is a factor. They feature a screw down terminal that can accept either a bare wire or a wire that has a ring or spade attached to its end. The wire is simply placed onto the screw and tightened into the housing for a secure connection. Barrier strips are also offered in multiple configurations including single or double row.
Lastly, utilized for wire-to-wire connections, feed-through terminal blocks contain one input and one output contact where two separate wires are inserted into opposite sides of the housing to connect the two wires together. Feed-through terminal blocks, similar to PCB mount types, are available in single, dual, or multi-row modules.
Key Electrical Specifications
Although an engineer will likely know their overall system voltage and current requirements when implementing a terminal block, there are still other factors that are important to note during the design phase. They are as follows:
Current rating: Typically, the most crucial specification to consider in terminal block designs, current rating is based on three areas: the conductivity of the terminals, cross-sectional area, and the corresponding heat rise. Overheating and damage to a terminal block can occur when operating at too high of a current, which can present critical safety concerns. In general, it is recommended to select a terminal block with a current rating of at least 150% of the system’s expected max current.
Voltage rating: Like the current rating, a terminal block’s voltage rating must be greater than the maximum system voltage and should also take into account any voltage surges that may be present in the end system. The voltage rating is partially determined by the dielectric strength and pitch of the terminal block housing.
Pole Count: The pole count is used to specify the number of individual circuits that the terminal block will house based on the application need. Terminal blocks usually offer pole counts from a single-pole up to as many as 24 poles.
Pitch: Defined as the center distance from pole to pole, the pitch is represented in values such as 2.54 mm, 3.81 mm, 5.0 mm, and so on. Pitch is determined by the terminal block’s overall rating where factors like creepage, voltage/current, and clearance are concerned.
Wire Size/Type: To ensure wires will physically fit into the terminal block housing, terminal blocks list the size or gauge of wire that the module can accept, which is specified in units of American wire gauge (AWG) in North America. Each terminal block will generally be able to accept a flexible range of wire sizes, for example, 26~12 AWG. In addition, the type of wire used should also be considered based on the module type. Single-core wire is typically paired with push-in style terminal blocks, while stranded or multi-core is ideal for screw terminals.
Key Mechanical Specifications
Along with electrical specifications, there are several mechanical factors that can have an impact on a terminal block’s footprint, orientation, and accessibility of the connections within the design. These mechanical specifications can include the following:
Wire-Entry Orientation: The three most common terminal block orientations are horizontal, vertical, and 45°. Horizontal and vertical can also be specified as 90° and 180°, respectively. Choosing a specific orientation will ultimately come down to overall design requirements and which option provides the best fit and accessibility.
Figure 1. Common terminal block orientations
Wire-Securing Method: As with orientations, there are three main types of wire-securing methods for terminal blocks: screw terminal, push-button, or push-in. True to their name, screw terminal or screw-type terminal blocks utilize a screw that tightens a clamp to secure the wire in place. Push buttons operate by opening a spring clamp when the button is pressed, allowing the wire to be inserted and clamped to the conductor once the button is released. Finally, push-in terminal blocks support the direct insertion of wires without the need for a screw or button to open the connection point.
Figure 2. Common wire securing methods
Interlocking vs. Single-Piece: Terminal blocks can be constructed in interlocking or single pieces. Interlocking terminal blocks are typically available in 2 or 3 pole versions, which allow designers flexibility in achieving varied pole counts or intermixing different colors of the same module type by simply snapping units together like building blocks. On the other hand, as the name suggests, single-piece modules contain all poles in a single housing. This adds a bit more rigidity and ruggedness in higher current or higher temperature designs.
Figure 3. Interlocking vs. single-piece modules
Wire-to-Housing Method: As seen throughout this article, the most common terminal block connectors have wires inserted directly into their housing. However, pluggable terminal blocks are an alternative option for designs where frequent connecting and disconnecting of the main connection occurs. Pluggable terminal bocks operate by inserting wires into a modular plug housing that then connects to a fixed receptacle housing on the PCB. This eliminates the need to handle individual wires each time disconnecting is required.
Figure 4. Example of a pluggable terminal block
Safety Ratings and More
Most terminal blocks will be certified and/or designed to UL and/or IEC safety standards. Each agency uses a different standard that a terminal block must adhere to, so engineers will often find both ratings on a datasheet with differing values. It is important for engineers to know their overall system safety requirements in order to select a terminal block that will be in compliance.
Other design considerations can include color customization of a terminal block’s housing or buttons. This can help to differentiate between various terminal block connections in complex systems, making it easier to diagnose connection points in the field.
As demonstrated, terminal blocks come in all shapes and sizes to meet the needs of a variety of complex electrical system connections. Taking note of key electrical and mechanical specifications as well as different module types can allow engineers to find the right terminal block for the job. CUI Devices makes this selection process easier with a range of terminal blocks in multiple color options and configurations.
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An important distinction not mentioned here, is the difference between screw type terminal blocks in which the screw pushes down a deformable metal tongue into the strands pushing them apart, and the type where the screw pulls up a metal block, strangling the strands into a tight bundle. The latter can easily be opened and reused, while the first type tends to let go more easily and where after unscrewing it is necessary to push up the metal tongue using a screwdriver.