Terminal blocks and terminal blocks for connecting wires

Recommendation points

• What are the types of terminal blocks
• About resistance to loads
• Advantages of terminal connections
• Scope of use of terminal blocks
• Selection of quality products
• Rules for making quality connections

Terminal connections in electrical installations are considered the best way to ensure reliable contact between conductors. We will tell you about the types of terminal blocks, the advantages and the scope of their application, and most importantly – the correct implementation of the connecting part of the electrical installation.

What are the types of terminal blocks

In the range of consumables for electrical installation, terminal blocks represent one of the most extensive product classes. In addition to the fact that these connections are completely different in design, the products of individual manufacturers can vary greatly in price, quality and usability..

Screw terminal connections have become very popular and are therefore presented in an extensive range. There are three main categories of screw terminals:

• checkpoints;
• barrier;
• terminal.

The main difference lies in the shape of the “cage” – a conductive metal platform. For feed-through terminal blocks, it is sleeve-type, while the barrier connections are based on contact plates. End or dead-end terminal blocks look like a cap, and are designed to crimp several twisted conductors.

Push terminal blocks are arranged differently. In them, the cable core is clamped inside a collet or wedge-shaped plate. The clamping force is provided by a lever on the terminal block body, thus the connection is quick-disconnect. Pads of this type can have a water-resistant design, which is ensured by a tight fit of the insulating band in the inlet.

Spring-loaded terminal blocks are very similar to push-type ones, but in essence they are permanent connections. The wire is fixed inside a wedge-shaped cut or a tapered collet, that is, the terminal block works on the principle of a trap. However, for some products, manufacturers provide for an opening procedure, but subsequently it is not possible to assess the safety of the contact pad, therefore, spring-loaded terminal blocks are rarely used again.

In addition to the method of contact fixation for terminal blocks, there are differences in the location of the terminals, the method of installation, the row, the material of the case, the diameter of the wires and the permissible load. One of the more specific features is the presence of internal filler. So, certain types of spring terminal blocks can be filled inside with conductive paste or special grease that prevents metal from contact with moisture and oxygen.

About resistance to loads

Each terminal connection is designed to withstand a strictly defined current strength. However, there is some confusion on this issue: it is not always possible to focus on the permissible load declared by the manufacturer. For example, for through-type screw terminal blocks, the maximum current value is indicated, while their size should be selected according to the cross-section of the cable used. The structure and principle of operation of most terminal connections are described in the technical documentation, so you can easily assess the resistance to loads by a number of signs.

It is quite obvious that the maximum load on the terminal block is determined by its conductivity, that is, by the material and section of the conductive stand. But this is far from the only and not even the most important parameter: the conductivity of the block goes to the last plan, if the fixation mechanism has significant transition resistance, that is, does not provide a sufficiently high contact area and clamping force.

The tight fit of the core to the contact pad is ensured in different ways. This can be a partial indentation of the end of the clamping screw into the body of the conductor, or coating the site with a plastic viscous alloy, due to which deformation of one of the contact sides occurs in the contact zone and an increase in the contact area. Also, a common technique can be called the manufacture of a substrate for a core from hard alloys, followed by grinding the surface, thus, due to the pressure, the material of the core itself is deformed.

In general, the following can be said about the current-carrying capacity: it simply must be sufficient and not decrease over time, redundancy is even harmful here. On the Internet, you can find a lot of reviews of modern terminal blocks with stress tests under high load. However, in modern electrical installation, the operation of the power grid at currents exceeding the permissible values ​​for this type of conductor is not allowed. The cable line is protected by a circuit breaker, but if the protection is out of order, a terminal connection with a strictly defined load capacity will play the role of a weak link by analogy with a fuse.

Advantages of terminal connections

Of course, terminal blocks are not the only way to connect wires. However, the practice of electrical installation for several decades shows that it is they that are best suited for household and small industrial power grids, for which there are a number of reasons.

First and foremost is the tolerance for errors when assembling the schematic. When using soldering and welding of wires, the slightest oversight leads to the need to cut the tails of the conductors inside the installation and junction boxes, thus a maximum of one or two reconnections are allowed. If you intend to declare that you need to do everything right at once, we answer: even hardened electricians make mistakes in the wiring diagram, no one is immune from this.

Another advantage of terminal blocks is their availability and ease of installation. Soldering and welding not only require special equipment that needs a temporary power supply. Such connection methods are not particularly convenient, moreover, they are very demanding on the qualifications of the employee, who is obliged to avoid the so-called “cold” joints and wire burns during welding. At the same time, the cost of terminal blocks is quite low – on average, 10–15 rubles for one group of contacts. Given that this method of connection almost completely excludes the influence of the human factor, the choice becomes quite obvious.

Scope of use of terminal blocks

Terminal blocks of different types are designed for use in various nodes of the electrical network. All kinds of “bosses” of the spring and push type are mounted in a hinged way, due to which it is convenient to use them for connecting wires in branch boxes. The absence of the need to attach the block allows you to easily group wires and organize the internal space, which is almost always very limited.

Barrier type screw blocks are ideal for assembling a group of connections in cabinets and switchboards. Due to the rigid attachment to the mounting panel, such connections ensure high-quality organization of the conductor routing. Do not think that barrier terminal blocks are designed exclusively for connecting high-power networks; they can just as well be used for organizing complex control circuits. At the same time, the presence of a cover and partitions allows the joint laying of circuits of both types in one housing: in an accident, sparking and ionization will not affect the operation of sensitive devices in any way.

Through-type screw strips can be fitted with a common strip of 12 terminal blocks, making them an excellent replacement for barrier connections. Also, the bar can be easily divided into groups with an arbitrary number of contact groups and used in installation and distribution boxes or as temporary connections.

It remains only to clarify what the terminal connections are not intended for. It is strictly forbidden to use them to connect hidden wires. For these purposes, it is allowed to use only hermetic heat-shrinkable couplings. It is also undesirable to use terminal connections in cavities of false walls and suspended ceiling structures. However, there is a small caveat here: terminal blocks can be used to connect, for example, spotlights that do not have standard contact connectors, for example, LED with a monolithic body.

There is also a specific category of DIN rail mounted terminal blocks. It can be both screw and spring contact blocks, push-type terminal blocks are the rarest. These types of connections are intended for the assembly of control and monitoring circuits; this method of organization allows you to quickly reconnect a large number of electrical devices. Separately, it is worth mentioning cross-modules – massive open-type pads with several screw clamps, which are designed to assemble a group of connections with a high load. There are both single cross-modules and blocks of group installation in a common case, they are mainly used as common zero, protective and phase buses.

Selection of quality products

There are several recommendations on such an issue as the reliability of electrical products for making permanent connections. The products on the market differ greatly in quality, so the mere fact that the electrical network is assembled on terminal connections is actually not enough..

For through-type screw terminal blocks, the wall thickness of the bushing and, in particular, its threaded part is of critical importance. It is very common for the thread to break or the bottom of the sleeve to burst when trying to provide sufficient torque. The type of terminal blocks is preferred, the screws of which are designed for a flat-head screwdriver.

Barrier terminal blocks generally have fewer rejects. It is imperative to pay attention to the quality of the stamping of the screws, sometimes due to the insufficient depth of the punching, the edges lick off, which does not allow tightening with the proper effort. Terminal blocks for connecting conductors with a cross section over 16 mm² should preferably have Allen head screws. As for the material of the housing of the barrier terminal blocks, we can recommend ABS-plastic and fluoroplastic. Bakelite bodies are too fragile, they often break during installation, especially with significant tightening forces.

Plug-in push-type and spring-type terminal blocks are the least problematic type of product, mainly due to the fact that they are designed mainly for connecting small-section conductors. For spring pads, it is desirable to have a transparent rear part of the body, which makes it possible to assess the reliability of fixing the conductor in the cage. And, of course, it is always better to give preference to more or less well-known trademarks instead of nameless Chinese manufacturers..

Rules for making quality connections

Although the use of terminal blocks does not require special qualifications of workers, there are still some basic installation rules. Before describing them, we note that the internal mechanism of any terminal is a strict mechanical model, so the installation must be carried out in the way intended by the product developers..

First of all, we draw attention to the fact that absolutely all terminals are designed to connect the conductor cores of a strictly cylindrical shape. It is for this reason that the edge of the core must be carefully leveled before stripping, after which the insulation must be removed with a special tool – a stripper. With regard to the use of plug-in terminal blocks, you can only limit yourself to the correct preparation of the core.

Feed-through screw terminal blocks are more specific in use. Contrary to popular belief, the presence of two screws does not imply individual clamping of each core. Cleaning should be done to a depth of 1–2 mm less than the length of the conductive sleeve. When the screws are loosened, the stripped ends are inserted into the block from opposite sides and placed in parallel, then tightening is performed. End-type screw terminals are designed for crimping twists, that is, the cores are pre-twisted with a pigtail, after which the screw, cutting into the central part, prevents the twisting of the twist under the influence of residual elastic forces.

Barrier terminal blocks have contact pads with specially prepared surfaces. They should not be cleaned even before reuse, the presence of microscopic scratches from the abrasive greatly reduces the contact area. Oxidation traces can only be removed with a regular eraser. Also, screw terminals are an exception in terms of core preparation. Ideally, the edge of the conductor is twisted into a ring to prevent skewing of the washer. You can deviate from this rule if the veins are wound up under the bolt in pairs from different sides. For barrier terminal blocks, the issue of controlling the tightening force is especially acute, therefore, when making critical connections, it is better to get a torque screwdriver.

Screw terminal blocks, in principle, are not intended for connecting stranded conductors, however, they can be used for this purpose, provided that the nominal load on the conductor is significantly lower than the permissible one. Beforehand, the ends of the cores must be crimped with bushings. It is strictly forbidden to use plug-in terminals to connect stranded conductors, even if they are crimped.