Protects the machine (RCD). Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Current Devices

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With all the desire it is difficult to imagine our life without electricity. But it is not only a faithful assistant, but also a terrible enemy - in case of violation of elementary safety rules. Of course, the latter most often occurs due to the personal negligence of people and due to damage to the insulation of live parts of electrical equipment.

Statistics show that cases of single-pole contact with live parts, leading to death, are 92-95%. In this case, a person, as a rule, stands on a conductive floor (wet asphalt, tiles, soil). Or - a slightly different situation, when the victim touched grounded technological equipment (water pipes, heating pipes, gas pipes) with his second hand. The transition resistance between the point of contact with the phase wire and the grounded base (floor), taking into account the resistance of the shoe, can vary from hundreds of megaohms (beyond the sensation of current exposure) to 1000-800 Ohms (fatal). With a current of up to 10-12 mA, an adult is able to free himself from its effects. That is why such a current is sometimes called "letting go". At 20-30 mA, a "grasping" effect occurs, when the affected person's hand cannot be unclenched by sheer force of will. A current of 50-100 mA and a duration of exposure of a few seconds causes a sharp arrhythmia - ventricular fibrillation, suffocation and death. Of course, if special measures are not taken. It should also be remembered: under all aggravating circumstances, the current flowing through the human body (on a 220 V, 50 Hz network) cannot be more than 300 mA.

The problem of protecting people from injury when they accidentally touch live parts became especially acute for electricians immediately after the Second World War (due to a sharp increase in power consumption and the development of household electrical equipment). A promising way to solve it was proposed in 1949 by Austrian engineers, using the principle of a differential transformer as a convenient "detector" of current that can flow through the body of a person who has come under high voltage.

The principle of operation of a differential transformer

What is the point here? And in the fact that in the "waiting" (normal) mode, the load current creates in the magnetic circuit of such a transformer T two equal opposite magnetic fluxes. This means that there will be practically no voltage in the additional winding II.

But then an emergency occurred, and the person, as they say, came under stress. Then the current branching into the ground through the human body will create its own magnetic flux, which will induce voltage in the winding II. Moreover, the value of the latter, as studies have shown, depends on the ratio of the number of power turns (to the load) of the wires and winding II, as well as on the geometric dimensions of the magnetic circuit and the material from which it is made. On the principle of a differential transformer, residual current devices (RCDs) were created. Until now, they are nothing more than the most advanced technical means of protecting people from electric shock. RCDs are produced in mass quantities by the largest electrical companies in the world (Siemens, AEG, Thomson-Brandt, Toshiba, Mitsubishi, etc.). The widespread use of these devices, as evidenced by statistics, has made it possible to drastically reduce fatal electrical injuries and minimize damage from fires caused by electric current.

In the former USSR, the serial production of RCDs could only be started in 1966. But the bulk of these devices were sent to construction and agriculture (as the most disadvantaged sectors of the country). Since 1982, the domestic industry has taken up the production of RCDs for household use. But not as installation on the input distribution board, but in the form of portable products for effective protection of a person during the breakdown of power tools or electrical appliances with cable power. These RCDs have a sensitivity of 10mA "release" current and are called "personal protection". This is RCD-10.2.010. P. UHL2 of the Vladikavkaz plant "Binom", made with two built-in sockets for 6A; as well as UZOSH-10.2.010 UHL4 of the Gomel plant "Electroapparatura", designed for load current up to 10A; UZOV-6, 3.2.010UZ of the same plant, produced in the form of a "plug" with a load current of up to 6,3 A.

The electrical circuit of one of the serial RCDs for personal protection (see Fig.) is not so complicated. The “heart” here is the amplifying-converting organ “A” circled by a dotted line. Its power source is a half-wave rectifier on a VD6 diode with a voltage divider on resistors R10, R11 and a smoothing filter C3. Voltage stability is provided by the Zener diode VD5.

The electrical circuit diagram of the device is wired (click to enlarge)

The circuit works as follows. The terminals XI are supplied with a mains voltage of 220 V. When the button SB1 is pressed, the operational amplifier DA1 is powered by 15 V. Due to the selected operating point, a high voltage level (+6 V) is set at output 12. Through the diodes VD3 and R12, it is applied to the control electrode of the thyristor VD10, which opens. Relay K1 is immediately activated, connecting the load (protected electrical appliance) to the network, and blocks the contacts of the SB1 button. The VD8 LED, connected in series with the relay, lights up, signaling the on state of the RCD.

If a person touches the conductive elements or the insulation of the phase wire is damaged, a voltage will appear at the terminals 5,6 of the transformer T1, approximately proportional to the leakage current. It will immediately go to the non-inverting input 2 of the amplifier and transfer DA1 from one stable state (open) to another (closed). The voltage at pin 6 will drop sharply. The VD5 zener diode will close, followed by the VD10 thyristor.

The circuit VD3, R9, C2 fixes the off state of the amplifier DA1, and the relay releases the armature, breaking all its contacts. Elements C1, R2 weaken the influence of interference on the input circuits of the RCD. As for the diodes VD1, VD2, they protect the circuit from high-amplitude impulse noise. Resistors R3-R5 form a voltage divider for the inverting input 3 of the amplifier. A R8 creates a bias to input 2 and causes the RCD to trip if the winding circuit II of the transformer T1 is open.

Resistors R6, R7 create a circuit for setting the RCD to trip when the "appearance of a leakage current to the ground", 10 mA. The R13, C4 circuit protects the VD10 thyristor from interference. With the SB2 button, when the RCD is on, a mode is created that simulates a leak of 20-25 mA to check the performance of the RCD.

To assemble the circuit, it is better to use a printed circuit board made of 1,5 mm foil fiberglass (see Fig.). But you can opt for a "hinged" installation.

Printed circuit board with circuit elements located on it

The differential transformer T1 is made on a magnetic circuit made of 79NM strip permalloy with a thickness of 0,1-0,15 mm. But the K28x18x9 ring made of ferrite 3000NM1 is also quite suitable. In this case, winding II is wound around the circumference of a well-insulated ring with PEV-2-0,1 mm wire, the number of turns is 1500. The presence of short-circuited turns, of course, is not allowed here. The power turns are made with an insulated flexible wire of the MGSHV, NV brands, with a cross section of 0,75 mm2. The winding is carried out in two wires. The number of turns is 2x5.

As a switching device K1, it is better to use a relay with high-current contacts. The most suitable can be considered a relay type PP-21 with 3 groups of contacts for switching and a coil rated for 110 V DC

Resistors R1, R10 and R11 are taken of the MLT-0,5 type. As R7 used variable resistance SP3-38. The remaining resistors are the most common in modern equipment of the MLT-0,125 series. Capacitors C1 and C2 are of the K73-17 type, and C3 and C4 are of the K50-35 type. The name and type of the remaining radio elements used are indicated on the circuit diagram.

In conclusion, it should be emphasized: RCDs are devices of a special class, they are to protect human life. As a result, it is impossible, apparently, to rely especially on handicraft RCDs. Another thing - factory products. They are carefully checked parameters for compliance with international standards and specifications.

Last year, our industry mastered the production of a new class of devices designed to work with increased reliability during long-term operation. This is a bipolar circuit breaker UZO-20. Such a device will reliably protect a person when working with electrical units with a load current of up to 32 A. It is the best suited for installation in an entire apartment, house, garage, etc. A new "plug" UZO-2 (UZO-2.6.010) is also being produced. .2.V8UZ), designed to operate with a load of up to 135 A (refrigerators, washing machines, pumps, etc.). Its mass does not exceed XNUMX g.

Author: Y. Vodyanitsky, Moscow

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