Leakage current protection
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network
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Developed by the author many years ago and described in the article "Protection against current" ("Modelist-constructor", 1981, No. 10, pp. 29, 30), the protective-switching device worked when a voltage of more than 24 V appeared on the ungrounded metal case of the protected device relative to earth. Today, grounding instrument cases has become mandatory and it seems more correct to control the current in the ground wire. In the event of a violation of the insulation between the case and the network, the permissible value of this current (4 ... 10 mA) will be exceeded, which will serve as a signal to disconnect the faulty device from the network.
Fig. 1
A diagram of a protection device operating according to this principle is shown in Fig. 1. The XP1 plug is inserted into a mains socket equipped with a grounding contact. A three-pin power plug of the protected electrical appliance is connected to the XS1 socket. The electronic unit of the protective device is powered from the mains through a step-down transformer T2 and a bridge rectifier on diodes VD2-VD5. The supply voltage of the DA1 timer chip and the VT1 transistor amplifier is stabilized using the VD6 zener diode.
The primary winding of the current transformer T1 is included in the break in the wire connecting the grounding contacts of the XP1 plug and the XS1 socket (PE circuit). A voltage proportional to the current flowing through it is allocated to the resistor R1 and, after being rectified by a half-wave rectifier on the diode VD1, through a DC amplifier on the transistor VT1, it enters the input S of the DA1 timer.
If there is no leakage current, the voltage at the collector of the transistor and at the input of the timer is high, and at the output of the timer (pin 3) the logic level is low. If the leakage current increases beyond the permissible value, the high voltage level at the VT1 collector will change to a low one, which will allow the operation of the DA1 timer. Pulses of positive polarity will appear at its output, the first of which will open the trinistor VS1. Relay K1, opening the contacts, will disconnect the load from the network. The flashing of the HL1 LED will show that the protection has worked. The blinking frequency (1 ... 5 Hz) depends on the values of resistors R7, R8 and capacitor Sat.
After the leakage is eliminated, the trinistor VS1 will remain open, and the contacts of relay K1.1 will remain open. In order to apply mains voltage to the load, the protection device must be returned to its original state: turn it off for a while by pressing the SB1 button, and turn it on again by releasing it.
Capacitors C1 and C4 eliminate false alarms from short-term interference in the network. The R6C5 circuit prevents the timer from starting as a result of power-on transients. The R9C8VD7 circuit suppresses switching voltage surges on the winding of relay K1.
Fig. 2
The printed circuit board of the protection device and the location of parts on it are shown in fig. 2. The KT3102A transistor can be replaced by another of the same series or KT312, KT315 series. Import analogues of the timer KR1006VI1 - NE555 and many others with the numbers 555 in the designation. The trinistor KU101B in the device under consideration can be replaced by one of the KU201, KU202 series.
Relay K1 - RES47 version RF4.500.407-01 (winding resistance - 160 ... 180 Ohm). With a load power of more than 1 kW, it must be switched using a relay with more powerful contacts, and the relay K1 installed on the board should be used as an intermediate one.
The current transformer T1 is made of a matching transformer from a broadcast loudspeaker. The magnetic core of the transformer is steel Ш8х10. The winding with a smaller number of turns is removed, and three turns of insulated wire with a diameter of about 2 mm are wound in its place - this is the primary winding of the current transformer. The former primary winding of the matching transformer now becomes the secondary. Its conclusions are connected to the resistor R1. Power transformer T2 - any step-down with a primary winding of 220 Vis with two secondary windings connected in series at 9 V, 100 mA or with one secondary at 15 ... 18 V. The value of the protection operation current should be in the range of 4 ... 10 mA. This is achieved by selecting the resistor R2, and, if necessary, by changing the number of turns of the primary winding of the current transformer T1. A leakage of 10 mA can be simulated by connecting the primary winding of the T1 transformer to the 220 V network through a 22 kΩ resistor with a power of at least 5 W.
Author: V. Konovalov, Irkutsk; Publication: radioradar.net
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