If you can't explain it simply, then you don't know it well enough. — Albert Einstein

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.

Radial, Parallel, Ring main and Interconnected Distribution Systems

An electric power distribution system can be classified according to its feeder connection schemes or topologies as follows -
  • Radial distribution system
  • Parallel feeders distribution
  • Ring main distribution system
  • Interconnected distribution
There are few other variations of distribution feeder systems, but we'll stick to these four basic and commonly used systems.

[Also read: Classification of distribution systems according to number of phases and wires involved.]

Radial distribution system

This system is used only when substation or generating station is located at the center of the consumers. In this system, different feeders radiate from a substation or a generating station and feed the distributors at one end. Thus, the main characteristic of a radial distribution system is that the power flow is in only one direction. Single line diagram of a typical radial distribution system is as shown in the figure below. It is the simplest system and has the lowest initial cost.
radial electric power distribution system
Image credit: Wikimedia commons
Although this system is simplest and least expensive, it is not highly reliable. A major drawback of a radial distribution system is, a fault in the feeder will result in supply failure to associated consumers as there won't be any alternative feeder to feed distributors.

Parallel feeders distribution system

The above-mentioned disadvantage of a radial system can be minimized by introducing parallel feeders. The initial cost of this system is much more as the number of feeders is doubled. Such system may be used where reliability of the supply is important or for load sharing where the load is higher. (Reference: EEP - Distribution Feeder Systems)
parallel feeders distribution system

Ring main distribution system

A similar level of system reliability to that of the parallel feeders can be achieved by using ring distribution system. Here, each distribution transformer is fed with two feeders but in different paths. The feeders in this system form a loop which starts from the substation bus-bars, runs through the load area feeding distribution transformers and returns to the substation bus-bars. The following figure shows a typical single line diagram of a ring main distribution system.
ring main distribution system
Ring main distribution system is the most preferred due to its following advantages.

Advantages of ring main distribution system

  • There are fewer voltage fluctuations at consumer's terminal.
  • The system is very reliable as each distribution transformer is fed with two feeders. That means, in the event of a fault in any section of the feeder, the continuity of the supply is ensured from the alternative path.

Interconnected distribution system

When a ring main feeder is energized by two or more substations or generating stations, it is called as an interconnected distribution system. This system ensures reliability in an event of transmission failure. Also, any area fed from one generating stations during peak load hours can be fed from the other generating station or substation for meeting power requirements from increased load.

Types of AC power distribution systems

As we all know, electrical power is almost exclusively generated, transmitted and distributed in it's AC form. A distribution system usually begins from a substation where the power is delivered by a transmission network. In some cases, the distribution system may start from a generating station itself, such as when consumers are located near the generating station. For larger areas or industrial areas, primary and secondary distribution may also be used.

Types of AC power distribution systems

According to phases and wires involved, an AC distribution system can be classified as -
  1. Single phase, 2-wire system
  2. Single phase, 3-wire system
  3. Two phase, 3-wire system
  4. Two phase, 4-wire system
  5. Three phase, 3-wire system
  6. Three phase, 4-wire system

Single phase, 2-wire distribution

This system may be used for very short distances. The following figure shows a single phase two wire system with - fig (a) one of the two wires earthed and fig. (b) mid-point of the phase winding is earthed.
single phase 2-wire power distribution

Single phase, 3-wire system

This system is identical in principle with 3-wire dc distribution system. The neutral wire is center-tapped from the secondary winding of the transformer and earthed. This system is also called as split-phase electricity distribution system. It is commonly used in North America for residential supply.
single phase 3-wire distribution system

Two phase, 3-wire system

In this system, the neutral wire is taken from the junction of two phase windings whose voltages are in quadrature with each other. The voltage between neutral wire and either of the outer phase wires is V. Whereas, the voltage between outer phase wires is √2V. As compared to a two-phase 4-wire system, this system suffers from voltage imbalance due to unsymmetrical voltage in the neutral.
two phase 3-wire distribution system

Two phase, 4-wire system

In this system, 4 wires are taken from two phase windings whose voltages are in quadrature with each other. Mid-point of both phase windings are connected together. If the voltage between the two wires of a same phase is V, then the voltage between two wires of different phase would be 0.707V.
two phase 4-wire distribution system

Three phase, 3-wire distribution system

Three phase systems are very widely used for AC power distribution. The three phases may be delta connected or star connected with star point usually grounded. The voltage between two phases or lines for delta connection is V, where V is the voltage across a phase winding. For star connection, the voltage between two phases is √3V.
three phase 3-wire distribution system

Three phase, 4-wire distribution system

This system uses star connected phase windings and the fourth wire or neutral wire is taken from the star point. If the voltage of each winding is V, then the line-to-line voltage (line voltage) is √3V and the line-to-neutral voltage (phase voltage) is V. This type of distribution system is widely used in India and many other countries. In these countries, standard phase voltage is 230 volts and line voltage is √3x230 = 400 volts. Single phase residential loads, single phase motors which run on 230 volts etc. are connected between any one phase and the neutral. Three phase loads like three-phase induction motors are put across all the three phases and the neutral.
three phase 4-wire distribution

Classification on the basis of connection scheme

Distribution system can be classified according to its connection scheme or topology as follows -
  1. Radial system
  2. Ring main system
  3. Interconnected system
You can learn more about these here.

DC Power Distribution Systems

At the end of 19th century, when Edison built the first electrical distribution networks, they were based on DC technology. However, with the invention of transformers, AC system proved to be much more superior to DC system at that time and AC systems were universally adopted for power generation, transmission as well as distribution.

Why DC now?

Electrical power is almost exclusively generated, transmitted and distributed in AC form. However, for certain applications, DC supply is absolutely necessary. For example, variable speed machinery incorporating DC motors, critical areas where storage battery reserves are necessary. Following are some points that make us think about dc power distribution.
  • Advancements in Power electronics have made it possible to transform DC voltage levels and conversion between AC and DC efficiently. It is now possible to replace existing AC distribution network with DC distribution network.
  • Distribution generation from solar and wind energy is increasing rapidly and both of these sources are intrinsically DC.
  • A large number of office and household appliances internally require low voltage DC. These appliances are fed with AC supply and then transformed to lower voltage and converted into DC by an internal circuitry.
  • Harmonic issues, phase balancing problems, skin effect etc. are not present in DC systems.
  • DC energy can be stored easily in batteries and fuel cells. Such backup batteries can be utilized easily in case of supply failure.

Types of DC power distribution

Wherever DC power distribution is required, AC power from the transmission network can be rectified at a substation using converting equipment and then fed to the dc distribution system. AC consumers can also be connected to DC system using a DC to AC inverter. A low voltage DC distribution system is of two types.

Unipolar DC distribution system (2-wire DC system)

As the name suggests, this system uses two conductors, one is positive conductor and the other one is negative conductor. The energy is transmitted at only one voltage level to all the consumers using this system. A typical unipolar dc power distribution system is as shown in the following figure.
unipolar 2-wire dc power distribution system

Bipolar DC distribution system (3-wire DC system)

This is basically a combination of two series connected unipolar DC systems. It consists of three conductors, two outer conductors (one is positive and the other is negative) and one middle conductor which acts as neutral. This system leaves following connection choices to a consumer -
  1. between positive conductor and neutral
  2. between negative conductor and neutral
  3. between positive and negative conductor (double voltage)
  4. positive to negative with neutral connected
bipolar 3-wire dc power distribution system
The above figures of unipolar and bipolar dc distribution system suggest that, DC to DC converter or DC to AC inverter can be installed at the consumer's premises according to consumer's or load's requirement. Consumers can also be directly connected to the DC distributors if the distribution voltage level is similar as per their requirement.

Types of DC distributors

DC distributors are usually classified on the basis of the way they are fed by the feeders. Following are the four types of DC distributors.
  1. Distributor fed at one end
  2. Distributor fed at both ends
  3. Distributor fed at center
  4. Ring distributor

Distributor fed at one end

In this type, distributor is connected to the supply at one end and loads are tapped at different points along its length. The following figure shows the single line diagram of a distributor fed at one end. It worth to note that -
dc distributor fed at one end
  • The current in various sections of the distributor away from the feeding point goes on decreasing. From the above figure, the current in section DE is less than the current in section CD and likewise.
  • The voltage also goes on decreasing away from the feeding point. In the above figure, voltage at point E will be minimum.
  • In case of a fault in any section of the distributor, the whole distributor will have to be disconnected from the supply. Thus, continuity of supply is interrupted.

Distributor fed at both ends

In this type, the distributor is connected to supply at both ends and voltages at feeding points may or may not be equal. The minimum voltage occurs at some load point which is shifted with the variation of load on different sections of the distributor.
dc distributor fed at both ends
  • If a fault occurs at any feeding point, continuity of the supply is ensured from the other feeding point.
  • If a fault occurs on any section of the distributor, continuity of the supply is ensured on both sides of the fault with respective feeding points.
  • The conductor cross-section area required for a doubly fed distributor is much less than that required for a distributor fed at one end.

Distributor fed at the center

As the name implies, the distributor is supplied at the center point. Voltage drop at the farthest ends is not as large as that would be in a distributor fed at one end.
dc distributor fed at the center

Ring main DC distributor

In this type, the distributor is in the form of a closed ring and fed at one point. This is equivalent to a straight distributor fed at both ends with equal voltages.
ring main dc distributor

Electric Power Distribution System Basics

Electrical power is dominant as it is relatively much easier to transmit and distribute than other forms of energy such as mechanical. Imagine transmitting mechanical energy to just 20 feet of distance. Isn't it much easier to use wires instead of belts, chains or shafts?
We have seen how electrical energy is generated in generating stations and how it is transmitted over long distances through transmission networks. Now, let's see how electrical power is distributed to the consumers.

Power Distribution System

A distribution substation is located near or inside city/town/village/industrial area. It receives power from a transmission network. The high voltage from the transmission line is then stepped down by a step-down transformer to the primary distribution level voltage. Primary distribution voltage is usually 11 kV, but can range between 2.4 kV to 33 kV depending upon region or consumer.
A typical power distribution system consists of -
  • Distribution substation
  • Feeders
  • Distribution Transformers
  • Distributor conductors
  • Service mains conductors
Along with these, a distribution system also consists of switches, protection equipment, measurement equipment etc.
Distribution feeders: The stepped-down voltage from the substation is carried to distribution transformers via feeder conductors. Generally, no tappings are taken from the feeders so that the current remains same throughout. The main consideration in designing of a feeder conductor is its current carrying capacity.
Distribution transformer: A distribution transformer, also called as service transformer, provides final transformation in the electric power distribution system. It is basically a step-down 3-phase transformer. Distribution transformer steps down the voltage to 400Y/230 volts. Here it means, voltage between any one phase and the neutral is 230 volts and phase to phase voltage is 400 volts. However, in USA and some other countries, 120/240 volts split-phase system is used; where voltage between a phase and neutral is 120 volts.
Distributors: Output from a distribution transformer is carried by distributor conductor. Tappings are taken from a distributor conductor for power supply to the end consumers. The current through a distributor is not constant as tappings are taken at various places throughout its length. So, voltage drop along the length is the main consideration while designing a distributor conductor.
Service mains: It is a small cable which connects the distributor conductor at the nearest pole to the consumer's end.
simple radial AC power distribution
The above figure shows a simple radial AC power distribution system. The figure does not show other equipment like circuit breakers, measuring instruments etc. for simplicity purpose.

Primary distribution

It is that part of an AC distribution system which operates at somewhat higher voltages than general residential consumer utilization. Commonly used primary distribution voltages in most countries are 11 kV, 6.6 kV and 3.3 kV. Primary distribution handles large consumers such as factories and industries. It also feeds small substation from where secondary distribution is carried out. Primary distribution is carried out by 3-phase, 3-wire system.

Secondary distribution

This part directly supplies to the residential end consumers. Domestic consumers are fed with single phase supply at 230 volts (120 volts in USA and some other countries). Three phase supply may also be provided at 400 volts for big properties, commercial buildings, small factories etc. Secondary transmission in most countries is carried out by 3-phase, 4-wire system.

Classification of power distribution systems