Unsoldered connections in wired alarm systems.

Solderless connections in wired alarm systems.

Solderless connections in wired alarm systems

Fire and security alarm systems use a large number of electrical connections, both detachable and non-detachable, both permanent and temporary. A significant part of the costs of installing a wired alarm system is the cost of mounting and connecting components into the system using electrical connectors and electromechanical joints of parts and units. And most of the damage to the system is usually caused by poor quality electrical connections. After all, as all electricians and electronics engineers say, in their business there can only be two faults: there is no contact where it should be, and there is contact where it should not be.

If in the components of alarm systems a significant part of all connections of radio components between themselves are soldered connections, then the construction of systems is carried out, for the most part, with the help of connections without heating — unsoldered or, in other words, cold connections. Such connections are made at room temperature, without heating the constituent materials. In this case, the stripped wires intended for connections are pressed against each other with such force as to ensure the largest possible contact area of ​​the contacting surfaces, and the conductors themselves are subject to deformation. As is known, the main requirement for such connections is that they have maximum and constant conductivity over time. To meet these requirements, it is necessary to ensure a number of conditions:

• use materials with the required specific conductivity for connectors;
• ensure maximum dimensions of contacting surfaces;
• ensure cleanliness and corrosion resistance of these surfaces;
• ensure appropriate contact forces.

As already noted, the use of a large number of solderless connections in alarm systems leads to a decrease in the operational reliability of such systems. The connection of conductors is an integral, but very vulnerable element of wired alarm systems. Therefore, the main task in the field of designing and manufacturing components of fire and security alarm systems and, in particular, elements of their external connections is the use of new technical solutions that lead to an increase in the reliability of such connections.

We can distinguish at least three groups of solderless connections that are widely used in the construction of alarm loops:

• connections of conductors to each other, when it is necessary to ensure the connection of several conductors into one circuit, or cables with paired connection of conductors, as well as connections of conductive buses, etc.;
• connections of conductors to elements installed on printed circuit boards — this is how the connection of alarm loops to control and monitoring devices and to some detectors is ensured;
• connections by which a wire can be mechanically and electrically attached to a connecting element, such as a contact on the base of a plug-in detector.

The traditional and, one might say, ineradicable method of ensuring contact between conductors in an alarm loop is twisting the ends of the wires with the insulation removed (Fig. 1). Since there is a relatively small compression force per unit of surface of the contacting conductors, and the voltage in fire alarm loops does not exceed 30 V, such connections do not provide high-quality contact for a long time. The oxidizing surfaces of tinned conductors are especially susceptible to corrosion.

The quality of electrical contact in twists can be improved using connecting insulating clamps (SIZ) — special metal bushings with internal conical thread and external insulating coating. The best performance is demonstrated by a SIZ containing a plastic tip with a pressed-in conical spiral made of phosphor bronze. Such SIZ, shown in Figure 2, is screwed clockwise onto a twist of two or more conductors, both single-core and multi-core. On the one hand, it increases the compression force of the conductors, and on the other hand, the conical spiral, cutting into the conductors, destroys the oxide layer, but due to the low compression force, the quality of such a connection is inferior to connections made with screw terminals. For normal use of SIZ, it is necessary to correctly select its standard size and strip the conductors of the OPS Fig. 1 to such a length that all the bare parts of these conductors are inside the insulating coating (Fig. 3).

Fig.1	Fig. 2	Fig. 3

In distribution boxes, screw connectors are widely used, which are no less traditional. With such connections, the compression force occurs when the screw is tightened due to the presence of threaded force. In order to ensure a reduction in transition resistance, it is necessary that the contacting surfaces are not smooth. It is also necessary to remember that when the screw is tightened, plastic deformation occurs in the wire, and it does not end immediately after the screw is screwed in. The flow of the wire material slowly continues further (especially with temperature fluctuations), which can lead to weakening of the connection. It is also known that the flow of tin is much greater than that of copper. This is why it is not recommended to tin conductors, especially multi-core ones, before installing them in screw terminals. When designing electrical terminals, it is necessary to strictly comply with international regulatory requirements, in particular, IEC 60999199, NFPA 72 [2, 3]. Thus, the examples in Figure 4 (Figure A.5.4.6(a) NFPA72) with correct and incorrect screw electrical connections in alarm loops clearly show typical errors in installing electrical connections. It is a pity that there are no such examples in the Russian set of rules for installing fire extinguishing and alarm systems.

Screw terminal strips have proven themselves well both in electrical engineering and in low-current alarm circuits. They are manufactured for different conductor cross-sections. Each contact of the strip is a metal sleeve enclosed in hard or flexible plastic, where the ends of the conductors with the insulation removed are clamped using two screws. Terminal strips allow both their dismemberment and unification into the required number of contact groups. An example of a distribution box with terminal strips for alarm loops is shown in Figure 5. In the center of this box there is a tamper contact — when the box housing is opened, the contact opens. The disadvantage of these connections is that the conductor is clamped between a stationary and rotating surface, which can lead to the dissection of thin multi-core conductors.

Terminal blocks, in which the conductor is clamped by two plates under the action of a pressing force, can improve the quality of electrical connections. An example of using such terminal blocks in a distribution box is shown in Figure 6.

Fig. 5	Fig. 6

The second group of electrical connections ensures the connection of conductors with elements installed on printed circuit boards. The most common in this group are terminal blocks with screw connectors that are soldered into printed circuit boards, as well as combined ones — screw and detachable connectors. The latter have an obvious advantage with a large number of connected conductors. If, when designing a printed circuit board of a product, such blocks are combined in pairs into groups of 6 contacts, then during maintenance or repair, a group of terminal blocks with conductors connected to them can be disconnected from the printed circuit board like a regular connector. Examples of screw and combined connectors used in fire alarm control devices are shown in Figures 7 and 8. Obviously, the combined connectors shown in Figure 8 have obvious advantages over the previous model, which are manifested during installation and dismantling of the blocks.

Fig. 7	Fig. 8

A typical screw-type electrical connection is a connector in which the conductor is pressed against the conductive surface using a square nut. Such connectors are used both in distribution boxes (Fig. 9) and in fire alarms (Fig. 10) [4]. In such devices, the conductors of the alarm loop are clamped between the conductor of the printed circuit board and the square nut. The direction of movement of the square nuts is ensured by niches in the plastic housing of the device.

A particular variety of electrical contact connectors is observed in the connection units of fire detectors, both removable and non-removable. None of the current standards that provide the main definitions and terms in fire alarms, neither the Ukrainian DSTU 2273, nor the Russian
GOST R53325, nor the European EN 541, ISO 72401, ISO 84213, contain either names or definitions for the parts of a removable detector. On the other hand, DSTU EN 547 uses the concepts of «base» and «detector head», but without definitions. I believe it is possible to offer the following definitions:

• active part– a removable component of a fire alarm that can be separated from the base for inspection, maintenance or replacement;
• base – a removable component of a fire alarm that is used for mechanical fastening at the installation site of the active part and serves to match and electrically connect it to the fire alarm loop. A fire alarm may not contain a base if fastening, matching and connection to the loop are carried out directly on it.

The bases of removable detectors use various screw connections to connect the loop conductors to the base contacts, which in turn provide a detachable connection with the active part.

Fig.9	Fig.10	Fig.11

Initially, a single base was offered for different smoke fire alarms, which were made removable. It was with this base that the first fire alarms were produced in Obninsk and Saratov, Vinnitsa and Chernivtsi.

To connect the loop conductors to the base contacts, either simple washers installed under the screw head or square washers with a «trunk» bent beyond the edge of the contact were used. This «trunk» was supposed to prevent the washer from turning when connecting the conductors and elements of the fire alarm loop. Often, such a washer was made of a different, more durable material than the base contacts. These materials were not always electrochemically compatible, which led to corrosion of the metals. What such a base looks like after several years of storage in an office closet can be seen in Figure 11.

The technical solution under the patents for invention UA85211 and utility model RU67783 allows improving the quality of the electrical connection of the fire alarm loop conductor with the base contact. Figure 12 shows the base contact of a removable fire alarm with one limiter, which is made of the base contact material as one of the solutions under the cited patents. Figure 13 shows a section of such a contact along line AA. The loop conductor is clamped with a screw connector between the base contact and the square washer. The limiter, made on the base contact, ensures the space limitation for the loop conductor, and also prevents the square washer from turning.

Another technical solution for screw connectors according to these patents was implemented in the design of the next fire alarm base, shown in Figure 14. This product implements two more inventions according to patents UA83277 and UA87554, as well as according to patents RU2317620 and RU23164941. The first of them allows to significantly reduce the consumption of non-ferrous metal used in the base contacts due to the shape of the contact petal. The second invention allows not only to reduce the weight of the base, but also to create a new base for a two-point fire alarm (Fig. 15), which becomes possible when using base contacts in the shape of a hockey stick. In this case, the hole in the center of the base is significantly expanded, and it allows the upper sensor of the two-point alarm to be passed through it without obstruction.

Fig.14	Fig.15

The problem of improving the quality of the electrical connection of the conductor contact of the base is also the subject of the invention under patents UA43096 and RU67784. This technical solution is implemented in the new design of the fire alarm base, which is shown in Figure 16. In this design, the loop conductor is clamped between the base contact and the square nut. The base itself contains additional 5th, and if necessary, 6th screw clamps, which eliminate the need to twist the connections of conductors and loop elements.

Fig. 16	Fig. 17

In recent years, the fire alarm markets have increasingly shown a tendency to use fire alarm bases with screwless connection of conductors and alarm loop elements. An example is the technical solutions under the patents for inventions RU2314612 and RU2314613, implemented in the bases shown in Figures 17 and 18, respectively.

In order to connect the cable conductor to the contact of the first base, it is necessary to bend the plastic spring on the base using a special tool, a lever, and insert the bare conductor into the resulting gap between the metal contact and the plastic spring. Such a connection cannot be called reliable, since the clamping force during operation will be significantly reduced when pressing the conductor into the plastic, especially at elevated operating temperatures.

Fig.18	Fig.19

The base shown in Figure 18 also has several significant drawbacks:

• narrow range of cross-sectional area values ​​of the conductors used;
• the bending of the conductors of the loop inserted into such a screwless clamp occurs between the plastic and metal plates;
• the terminals of the elements and the conductors of the alarm loop come out towards the tray of the active part, interfering with high-quality electrical contact between the base and the active part of the fire alarm.

The technical solutions for the above patents became an analogue and prototype of a new invention patented in the patent offices of Russia and Ukraine.

The peculiarity of this invention is that the clamp itself, shown in Figure 19, consists of only three parts:
1 – flat contact;
2 – insulating base;
3 – shaped lever.

On flat contact 1 (Fig. 20), elements 4 for fastening contact 1 in insulating base 2, as well as element 5 for electrical connection of contact 1 are made by stamping. Insulating base 2, which is shown in Fig. 21, there is a groove 6 for inserting electrical conductors 16 with
previously removed insulation. Groove 7 is used to place axis 8 of figured lever 3 (Fig. 22) in it.

Fig. 20	Fig. 21	Fig. 22	Fig.23

Elements 9 are intended for reliable fixation of contact 1 on plane 11 of base 2. Additional groove 10 is necessary for rotation in it of U-shaped bend 14 on axis 8 of shaped contact 3. L-shaped channel 12 with latch 13 is used for placement and fixation of handle 15 of shaped lever 3.

The clamp operates as follows.In the initial state, when the handle 15 of the shaped lever 3 is perpendicular to the plane of the contact 1, the U-shaped bend 14 on the axis 8 opens the channel 6 for inserting the conductors. After the conductors are inserted to the stop, they are fixed. After turning the shaped lever 3 around its axis 8 and fixing the handle 15 in the L-shaped channel with the help of the latch 13, the U-shaped bend presses the conductors to the flat contact 1. The clamp in the state of fixing the conductors 16 is shown in Figure 23.

Like any new solution, this clamp was tested according to the technical requirements imposed by the relevant regulatory documents, in particular, the international standard [7], for this type of connection. Special test rigs were developed and manufactured (Fig. 24 and Fig. 25) to check the actual parameters of the clamp.

Fig. 24	Fig. 25

The first test setup, shown in Figure 24, was used to test the static clamping forces. For the selected cross-sections of conductors used in fire alarm systems, namely up to 1.5 mm2, the force is provided to be at least 40 N according to the specified regulatory document.

The second setup was used to test dynamic effects with a rotating conductor. With a given conductor tension and «attack» angle, the clamp provides at least 150 revolutions at a speed of (10±2) min 1. Not every screw clamp can withstand such a test, since due to the sharp change in the forces applied to the conductor in the screw clamp, it can simply break. The proposed design without a screw clamp is free from this drawback, which is confirmed by the corresponding tests.

Fig.26	Fig.27

The magnitude of the contact resistance in the electrical connection of a flat contact with a cylindrical conductor pressed to the contact plane at one point depends on the compression force. The theory of electrical connections states that the constriction resistance is inversely proportional to the cube root of the compression force. This means that in order for the resistance of the contact point to double, for example, from 0.01 Ohm to 0.02 Ohm, the compression force must decrease EIGHT times, i.e. from 40 N to 5 N. Since the stroke with the force of the lever handle is proportional to this force, it becomes obvious that with a static compression force, the contact contact resistance will practically not increase noticeably even during long-term operation of the clamp.

To date, a fire alarm base has been designed using the declared screwless clamp. The design of the base in section is shown in Figure 26, and its photograph in Figure 27.

Literature:

1. Frolikh Ya. Solderless connections in electronics, trans. from Hungarian. Moscow: Energia, 1978. – P. 11.
2. GOST R 51686.1 2000 (IEC 60999 1 99) Connecting devices. Safety requirements for contact terminals. Requirements for screw and screwless contact terminals for connecting copper conductors with a nominal cross-section from 0.2 to 35 mm2.
3. NFPA 72 National Fire Alarm Code 2002 Edition.
4. Bakanov V. Ways to solve problems in fire alarm loops. //F+S: Security and fire protection technologies. – 2009. – No. 4 (40). – P. 54.
5. Neplokhov I. Basic element //Hidden camera. – 2004. – №2 (22). – P. 22.
6. Maslov I. Contact? There is contact! For how long… //BDI. – 2005. – №1 (58). –P. 17.7. GOST R 50043/3 2000 (IEC 60998 2 2 91) Connecting devices for low-voltage circuits for household and similar purposes. Part 2 2. Additional requirements for screwless contact terminals for connecting copper conductors.

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V. Bakanov, Chief Designer of Arton PE