What is a terminal block? | Significance and Types

How do we connect two wires? By stripping the insulation at the ends and twisting them together? Yes, it works. But, is it safe? We can apply insulation tape over the joint or use a wire connector. But what if there are a number wires that need to be joint/connected near each other? Or, what if multiple outgoing wires are to be connected to a single incoming wire? Then this method will neither be safe nor be convenient anymore. Here we use terminal blocks.

What is a terminal block?

A terminal block (also called as connection terminal or terminal connector) is a modular block with an insulated frame that secures two or more wires together. It consists of a clamping component and a conducting strip. A typical simplest terminal block is as shown in the image below.
screw clamp terminal block
Image Credit: Wikimedia Commons

The insulating body of a terminal block houses a current carrying element (a metal strip or terminal bar). It also provides a base for clamping element. The body has a mounting arrangement so that the block can be easily mounted on or unmounted from a PCB or a mounting rail. Most terminal blocks are usually modular and mounted on DIN rail. That allows us to increase the number of terminals according to the requirements. Terminal blocks keep connections much more secure and wires well organized.

Types of terminal blocks

Electrical terminal blocks can be classified on the basis of structure, device type, termination options etc.
single level pass through or feed through terminal block
Single level pass through terminal block

Structure type

  • Single level pass-through terminal blocks: These are simply used to connect two wires together, i.e. wire-to-wire connection. These are also called as single feed terminal blocks. Single level terminal blocks are of the most simple type having one input contact and one output contact.
  • Dual level terminal blocks: These blocks have another level of connection terminal stacked on the first one. This arrangement is generally used to save space.
  • Three level terminal blocks: Just like dual level blocks, these have an extra level at the top. An advantage of using multilevel blocks is that multiple connections can be made in the same block.
dual level (duble decker) and three level terminal block
Image credits: Connectwell.com

Device type

ground terminal block and fuse terminal block
Image credits: Connectwell.com

- Ground terminal blocks

These blocks often look like a single level feed through terminals. The exception is that these blocks and the metal connection where the wire is terminated are grounded to the panel or DIN rail on which the block is mounted.

- Fused connection terminals

These are similar to the pass-through blocks with an exception of the metal connection strip is replaced with a fuse. Therefore, the wires will be connected through a fuse providing an added protection.

- Thermocouple terminal blocks

These are designed to accept thermocouple lead connections. Some thermocouple connectors essentially clamp the thermocouple leads together on both sides of the block, eliminating the metal connection strip inside the block. However, in some thermocouple blocks, the metal connection strip of the same metal as that of the wire may be present.

- I/O blocks and sensor blocks

I/O blocks are used to make a connection between a device and a controller. Whereas, sensor blocks handle three or four wire devices such as proximity sensors.

- Disconnect terminal blocks

These blocks allow wires to be easily disconnected just by lifting a lever or knife switch. They can be used for convenient disconnection and connection without removing the wires. They are also known as switch blocks.

- Power Distribution blocks

These blocks are used in electrical power distribution. An electric power distribution terminal block is a convenient, economical and safer way to distribute power from a single input source to multiple outputs. One large wire is connected to the input terminal of the block and multiple output terminals are provided at the output. This way, wires are well arranged in a control panel giving it a neat, clean and professional look.
electric power distribution terminal block
Image source: ABB e-library

Clamping options in terminal blocks

  • Screw terminal: Screw clamp terminals are the most common type of connection method. The wire or conductor is simply pressed against the conductor strip in the block by tightening the screw. Screw terminals accommodate a very wide range of wire or conductor sizes.
  • Spring clamp: These type of terminals use spring pressure to retain the wire clamped. Spring clamps are a newer alternative to screw clamps and are generally used for relatively small wires.
    spring clamp terminal block
    Image credit:C J Cowie | Altech Corp.
  • Push-in terminal blocks: Push-in terminals allow you to connect a wire simply by inserting it. Most push-in terminals require the use of a ferrule. A ferrule strengthens the end of the wire/conductor. However, some push-in terminal blocks allow to insert a solid conductor directly or a stranded conductor by inserting a screwdriver into the release hole.
  • Insulation Displacement Connector (IDC): These connectors do not require us to strip the insulation for contact. We simply need to insert the wire without stripping the insulation, and the two sharp metal blades inside the terminal will cut through it to the conductor making proper contact.
  • Barrier terminal block: These are used where vibration is an issue. A spade or ring terminal is attached to the wire and then inserted into a bolt and tightened with a nut on the terminal block. This prevents loosening of the wire due to vibrations.