Terminal Blocks

Terminal Blocks: Learn About Terminal Blocks Here > Terminal Blocks are modular, insulated blocks that secure two or more wires together and consist of an insulating body and a clamping device. Their flexibility allows wiring to be centralized and makes it easier to maintain complex control circuits. Today's control wiring systems are growing increasingly complex as equipment is confined to smaller and smaller spaces, and industry simultaneously demands more responsiveness and higher levels of automation. Within the control panel, these handy modular components can be snapped securely into place on a mounting rail.
When changes in the circuit need to be made, terminal blocks can be easily added or pulled off the rail without disrupting other wire terminations. Along with reducing complexity of control wiring, the plastic bodies of terminal blocks also prevent shorts and therefore provide greater safety to installers and maintenance crews.


Insulating Body
Mounting Block Construction
Current Carrying Parts
IEC vs. NEMA Classified Terminal Blocks
Selecting the Right Terminal Block


The insulating body houses the current carrying parts. The body insulates the wire termination to minimize heat generated when current passes through the wires. It also provides a base for the clamping mechanism and other parts such as switches and fuses. The body of the terminal block has specially designed holes for access to the clamping screws. The body also has a mounting foot that enables to terminal block to be snapped on and off the mounting rail with being weakened. The space efficient design of a typical modular block permits high-density circuits to fit in a standard control panel. A 5mm wide block can provide up to 60 terminations per linear foot. Standard 6mm single level blocks yield up to 50 terminations per linear foot; double level blocks can handle up to 100 terminations per foot.


Mounting block bodies can be constructed from Polyamide 6.6, Melanie 150, or Ceramic.

• Polyamide 6.6 6 is a rugged thermoplastic material, designed to work under any environment and remain elastic and fracture proof from -40 degrees Celsius to 105 degrees Celsius. This material offers high insulation, tracking resistance and a high flammability rating. The strength and ease of handling of this material makes it the most commonly used.
• Melanine 150 is a resin-based thermoset plastic with an organic filler. Although it is a far more delicate material than Polyamide 6.6, Melanine has a high resistance to heat radiation. It is recommended for applications where the continuous operating temperature is between 110 degrees Celsius and 140 degrees Celsius.
• Ceramic is a highly rugged material that can be used in very high temperatures approaching 250 degrees Celsius. Ceramic blocks can also function in industrial plants with heavy dust deposits and unusually corrosive atmospheres.

The current carrying parts include a current bar and a clamping device. The current bar, at the center of the insulating body, is made of copper or brass. The clamping device secures the wire in the terminal block and makes an electrically sound connection between the wire and the current carrying bar. The size of these current carrying parts differs in relation to the amperage/wire size and the configuration of the block itself. Depending upon terminal block design, wires can be clamped in place using screws, a combination of screws and pressure plates, wire cages or spring clamps. A screw is the simplest method of connection: A screw is used to tighten the wire against the current bar. With screws and a pressure plate, the wire is held in place with a metallic plate, which secures the wire when the screw is tightened. With a wire cage, when the screw is tightened, a cage pulls upward and pressed the conductor against the current bar. The large contact surface provides excellent contact properties. Because the screw does not come in direct contacts with the conductor, wire damage is avoided. Even fine stranded wire can be connected without the use of additional Ferrules or wire pins. Spring clamps need a tool to open. The clamp closes on the wire to provide dynamic clamping. This extra holding action withstands vibration.



Terminal blocks are classified as DIN (IEC) or NEMA, depending on characteristics, which include panel mounting method, standards and approvals and design features. DIN are IEC terminal blocks, which originated in Europe. DIN refers to the channel or rail that the terminal block mounts on. The design offers more terminations per linear foot, which meets industry demands to maximize the number of terminations in a control panel. There blocks are easier to install, wire and mark than NEMA style blocks. Rails are available in 15mm, 32mm and 35mm widths. DIN blocks are generally interchangeable between manufacturers because the rail is standard and can be designed to handle a wide variety of applications.

Nearly 60% of the terminal blocks used in the U.S. are DIN type. Most designers prefer them because they offer:

1. Space saving, small compact designs and more units per foot.
2. A wide range of block types for design flexibility.
3. A worldwide acceptance.

NEMA terminal blocks, known as American-style, have self-lifting pressure plates, binder head screws or box lug connectors. They are more commonly used for heavy-duty applications. However, NEMA blocks take up more panel space and provide fewer terminations per linear foot. Also, rail dimensions differ across manufacturers, restricting the user's choices.

NEMA terminal blocks are:

1. Cost-effective for some applications.
2. Capable of having more options for high current applications.
3. Open construction, which can allow easier wiring.

Choosing the correct terminal block is straightforward. Just follow the steps below:

1. Determine current, voltage and wire size (AWG) to be used for the individual sire runs in the control or power distribution application.
2. Define any special space constraints imposed by the application
3. Considering the first two steps, select the appropriate rated feed-through terminal types to satisfy the electrical and space requirements. Select block widths. Double to block level and wire size if necessary to meet space requirements.
4. Determine how many blocks (number of Poles) are needed, whether any blocks need to be jumped and how the jumper connections should be made (which place).
5. Select special function block (ground, disconnect, fuse, indicating types, switch) based upon application requirements.
6. At the end of each terminal block assembly, wherever a change in terminal block size occurs or when a side of a terminal block is not used, specify as end plate. Unused sides of terminal blocks must not be left uncovered and exposed.
7. Determine where isolation partitions are needed to provide visual separation or to contain creepage within the terminal assembly. To ensure safety, isolation partitions separate blocks with different amperage ratings or voltages to prevent current/voltage jumping from one to another. Partitions also indicate to maintenance crew or electricians that there may be a different voltage in each of the groupings of blocks.
8. For rail mounted blocks, select the type of rail mounting. Calculate the length required by totaling the individual widths (thicknesses) of the terminal blocks, end plates, isolation partitions and end stops.
9. Mark wire terminations using marking tags.