A terminal block consists of a modular housing with an insulated body that secures two or more wires together. Terminal blocks—also referred to as terminal connectors, connection terminals, or screw terminals—are used in a wide array of applications where electrical systems need to be safely connected. They are ideal for designs requiring secure, well-organized, and semi-permanent wire connections that can be easily swapped out for inspection or repair in the field.
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There are various types of terminal blocks that can be used in a design. These are some of the most common:
Often referred to as Eurostyle or wire-to-board terminal blocks, PCB mount terminal blocks work by inserting bare wires into the module where a clamp secures the wire in the housing. The housing is then soldered to a PCB in common footprints. PCB mount terminal blocks can be single, dual, or multi-level modules.
These terminal blocks have a screw down terminal where a ring or spade terminal is attached to the wire and then inserted onto the screw and tightened into the housing. Barrier strips are commonly used where vibration is a concern.
Feed-through terminal blocks are used to connect two wires together for wire-to-wire connections. This terminal block type has one input and one output contact where two distinct wires are fed into opposite sides of the housing. Like PCB mount versions, these can also be single, dual, or multi-level modules. The feed-through style is most commonly found in DIN rail mount configurations. DIN rail terminal blocks are typically offered as a feed-through type for wire-to-wire connections or ground type, which visually looks the same as a feed-through style, but instead of connecting the incoming wires, it grounds them to the DIN rail or panel.
When designing a system that will utilize a terminal block, you will likely know your overall system voltage and current requirements. While these are very important, there are other factors to consider in your design as detailed below.
The current rating is often the most important parameter to consider in terminal block designs. The current rating is based on the conductivity of the terminals, cross-sectional area, and the corresponding heat rise. Operating at too high of a current can cause overheating and damage of the terminal block, leading to major safety concerns. It is best practice to use a terminal block that is rated for at least 150% of the max current that is expected in the system.
The voltage rating is in part determined by the dielectric strength and pitch of the terminal block housing. The maximum system voltage of the application must be less than the voltage rating. Any voltage surges in the system should also be evaluated when selecting the terminal block.
The number of individual circuits within the terminal block is also known as the pole count. This can be as few as a single pole, and as high as 24 poles and beyond, based on how many individual circuits are needed in the specific application.
A terminal block’s pitch is defined as the center distance from one pole to the next. The pitch of the connector is often determined by the overall rating of the terminal block where factors like voltage/current, creepage, and clearance are considered. Common industry pitches would be, but are not limited to, 2.54 mm, 3.81 mm, 5.0 mm, and 7.62 mm.
The minimum and maximum wire size that the terminal block can accept would be a function of the voltage/current rating. In addition to making sure the wires used will physically fit into the terminal block, the type of wire should also be considered. Stranded or multi-core wire is typically used for screw terminals, and single-core is typically used for push-in style terminal blocks. In North America wire size is specified in units of American wire gauge (AWG). Wire size can also be specified as mm2.
In addition to system electrical factors, mechanical restrictions and limitations need to be considered in terminal block design. The key features to consider are any mechanical restrictions on the housing of the design that may impact overall footprint, orientation, and accessibility of the connections. Mechanical variations to the housing can include, but are not limited to, the following.
Terminal blocks come available in three common orientations: horizontal, vertical, and 45°. Horizontal and vertical are also known as 90° and 180°, respectively. The wire orientation can often be determined by physical restrictions in the overall design that make one option more feasible than the other.
The method in which the wires are secured in the terminal block housing are typically accomplished by three main types: screw terminal, push button, or push in.
Terminal block modules can be constructed in pieces that are interlocked with one another, or in single housings. Interlocking terminal block modules are typically constructed in 2 and 3 pole versions. With these two sizes, a designer can achieve any pole count that is needed by just snapping the modules together. DIN rail terminal blocks operate in much the same way by sliding individual units together on the metal rail. Once the desired configuration and number of poles is achieved, they are then finished with a compatible end cap to protect the housing on the outer most units.
Single-piece terminal blocks are constructed in a way that all poles are contained in a single housing. These are typical for pluggable housings where it would not be feasible to have multiple pieces. Other uses include high current or high temperature versions where a single housing would be stronger.
While most terminal block connectors have wires inserted directly into the housing, there are some variations that utilize mating pairs that can be plugged together. These pluggable terminal blocks still have the wires inserted into a housing, but with the additional benefit of a modular housing that can be plugged directly into a fixed housing on a PCB. These simplify designs where the main connections need to be disconnected frequently without having to disconnect all individual wires.
Terminal blocks are typically certified and/or designed to UL and/or IEC safety standards and are composed of materials that have a flammability rating meeting UL94V-0. Both UL and IEC ratings can appear on a datasheet with differing values. This is because each agency uses a different standard and requirements that a terminal block must meet. When selecting a terminal block, you will want to know your overall system safety requirements, and make sure the terminal block ratings comply with them.
In order to make assembly or maintenance easier for the user, housing color or special markings can be modified to help differentiate various terminal block connections. This is especially useful for more complex systems that utilize multiple wiring and circuits housed in a centralized location.
Operating temperature of the end system also needs to be considered to know whether a high temperature rated terminal block would be needed.
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Terminal Block is a modular connector engineered for a reliable connection that secures two or more wires together, and manages complex electrical circuits. They are also used for connecting wires to ground or earth connections. Outer bodies of Terminal Blocks are ideally made up of melamine or polyamide material.
Polyamide is a thermoplastic material with excellent electrical and mechanical characteristics. This material is known for high insulation, tracking resistance, and a high flammability score.
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Melamine is a thermoset material with dielectric properties. It has a high level of resistance to hot, cold and damp environment; retaining its electrical and mechanical stability. Also, it has excellent insulation properties and high resistance on flashover.
Ceramic is a sturdy material that can be used in extremely high temperatures. Ceramic blocks are preferred in industrial plants with excessive dust deposits and abnormally eroding environments.
Terminal Blocks majorly uses a copper alloy to ensure very low contact resistance. The wires are securely connected to current bars with the help of clamps; these clamps can be screw clamps, spring clamps, nut & bolt or studs. There are different types of lugs or ferrules available for connecting the wires to Terminal Blocks for e.g., pin-type, tubular, hook type, fork type and ring type lugs depending on the area of application, current & voltage ratings. There are a variety of Terminal Blocks used in the electrical industry as per application and circuit requirements like Standard Feed through Terminals, Multiple connection Terminals, Double Level & Triple Level Terminal Blocks, Micro Terminal Blocks, and Sensor Terminal Blocks, CT connection Terminals, Fuse Terminals, Bus bar Terminals and most important Grounding / Earth Terminal Blocks. These Terminal Blocks are mostly DIN Rail and can be mounted on standard DIN32 & DIN35 rails which are widely used in panel boards for mounting other electrical equipment like MCB’s and MCCB’s etc.
Spring-loaded Terminal Blocks are preferred for connections that adhere to safety requirements of the Electric Supply Industry standards (ESI).
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Certification:
Terminal Block design and development is mainly driven by standards to ensure quality performance, safety and universal acceptance of products. Standards from UL and IEC are utilized for design and development, while third party approvals like UL, CSA, VDE, CE, ATEX etc. from reputed organizations ensure that manufacturers adhere to the standards.
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Types of connection Methods:
Screw Clamp connection:
Screw Clamp Connection is the most popular method of wire termination. It offers distinct advantages over the other wire termination methods such as (i) Suitable for all types of wires, (ii) Wires can be connected without any special preparations (iii) Simple connection and disconnection of wires with the aid of an ordinary screw driver.
The steel clamping screw produces high contact force while the steel clamping yoke transmits this force by pressing the conductor against the current bar, which is made up of electrolytic copper. When the clamping screw is tightened, the clamping yoke gets pulled upwards, pressing the wire against the current bar. A flat clamping area ensures safe grip of wires of smaller cross sections.
Spring Clamp connections:
Screw-less Spring Clamp connection is as versatile as Screw Clamp connection. In this type of connection, the wire is held against the electrolytic copper current bar directly by a pre-stressed spring clamp. The spring is operated by using a screwdriver to provide access to the wire through the opening in the spring clamp. The inserted wire gets clamped on to the current bar on the removal of the screwdriver. The high-quality stainless steel clamp ensures a good connection of the wire with minimal contact resistance. The features of Spring Clamp connection are (i) Easy operation and time-saving connection method, (ii) Vibration proof connections (iii) Fail proof safe and maintenance-free connections.
Cable Lug and Bus bar Connection:
This method of wire termination is preferred for connections that are subject to very severe vibrations. The wire is crimped to ring or fork type lug and is screwed on to the flat current bar in the Terminal Block. This type of connection is also preferred for wires of larger cross-section; the conductor is fitted with a ring lug and bolted on to a flat current bar.
Tab Connection Method:
Tab connections are preferred in applications where the connected wire needs to be frequently connected and disconnected. Tab sleeves with the crimped wires are pushed on to the Terminal Block. These are simple connections and allow connection and disconnection of wires in a circuit with ease.
Solder Type Connections:
Solder connections are suitable for wires up to 2.5 sqmm. In this type of connection, the wire is soldered to a solder lug. These are firm and permanent type connections and cannot be easily removed.
Push-in Type Connections:
Push-in Type connection is a next-generation connection method and offers tool-free direct push-in type connections. Solid conductors can be directly used whereas stranded wires are lugged and pressed inside the clamp. The clamp is automatically opened and the wire gets locked inside the Terminal. Some of the features of this connection method are (i) Time-saving and fool-proof connections (ii) Convenient wire insertion without using any tool (iii) The wires can be easily released when required.
Plug and Socket Connection:
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