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Electric supply (added)

In this additional text I would like to discuss the topic of electric power in all aspects, but without relating to small and obvious things like the fact that we shouldn’t wrap together copper and aluminum cable, etc. At first I dint even planned to write this text because it is the subject of electric power is away from the topic of this site. At the time I was intensely interested in this subject and I didn’t noticed that I have assembled quite interesting and fresh information about the problems regarding electric power, ways of solving them, network filters, stabilizers, grounding, grounding allocation, etc.. Unfortunately regarding the subjects like the stabilizer and grounding there are many incredible myths manufactured from the minds of basic technical illiteracy of people, at the level of elementary school physics. Because of that, the material is totally simple and at the same time includes theory and practical advices.

Network filters

Get into the technical details of everyday devices, such is network filter, even long before the idea of this site, made me one time problem with electricity. Earlier I had indifferent approach regarding network filters and I was convinced that they are, in fact, extension cables, which can solve almost any problem with electricity. At home I had once a voltage misbalance, precisely voltage started to increase and I couldn’t measure it but intensive light color of light bulb indicated that. And why should i worry about? I had a device that was supposed to protect my equipment. But suddenly something exploded in my TV. It turned out that special protective element, which saved my TV, burned out. How could that happened? The unit that supposed to protect the device didn’t react, but the device protected itself and in much better way? Is that protective device necessary and for what purpose? Then I started to read various manuals, browse the Internet regarding theories and the best of filter design, including finished industrial samples. But they were all similar as two drops of water and were represented by a simple construction of a Varistor.

Principles of protection of voltage jumps

In the home electrical network normal voltage is 220 V, (-/+10%). But, it's meaning, because of the simplicity was equalized with direct current. Maximum or minimum allowed sinusoid spacing will be in range 280 to 341 V DC (direct current). Such figures are available, when multiplied the value of voltage to and the square root of No. 2. No filter can be protected from voltage reduction, because it is a task of another device that has transformers (UPS and stabilizers). However, it has to protect from increase of voltage! That is a task of any filter, but it only can be done properly by few models. For protection of high voltage mainly are used semiconducting components, based on the diode - varistors. But besides varistors, there are two more devices used for full protection from voltage jumps: gas contact breakers and protection (TVS) diode. They differ in reaction time and the maximum voltage, which they are able to stabilize. The principles they are based on is the same. If the voltage in the network exceeds a critical level (usually 250 V), through the protective element current is circulating, which causes the voltage drop to acceptable level. But that is in the theory. The practice leads to the following situations:

1) Voltage significantly exceeds the critical level: strong current circulates trough the element and produces a popping out of safety fuse within fuse box. It also provides protection from high voltage. Electricity is turned off by safety slot. The same situation is at short-circuiting.

2) Voltage significantly exceeds, or not, the critical level, but in very small intervals (not more than a few tens of seconds) through the current, which passes through the protective element and then it decreases its value to the critical meaning for the fuse: It comes to high voltage surge filtration. Electricity is not shut off.

3) The current flow, which slightly or significantly exceeds the critical value, during long time intervals in a small voltage, which passes through the protective element, the voltage, which can pass through the slot: It secured the filtration of high voltage jumps and increase of voltage during few seconds. Further, protective element overheats and its semiconducting structures are melting, leading to a short circuit. Electricity is shut off by protective automation. Protective element must be replaced.

Different network voltage jumps are caused by following situations:

1) Constant high voltage, from the substation it’s a problem of electric distribution and the reason for law suit from your point of view. Also increased voltage is possible when the majority of consumers were shut off.

2) Lightning, carries out the power of about 200 kA. During the struck it comes to the air ionization, in the atmosphere, that conducts electricity. That is a powerful source of electromagnetic radiation. The duration of impulses is 100-500 microseconds and the voltage is 100 kV

3) The transitional process which occurs during the change of regime and voltage in the work of different devices like welding devices, heater plugs, refrigerators and other electronics, to the ordinary lamps. They are more often than lightning. For example, in the normal electrical network with alternating current (AC) in the regime change or a short circuit, an extremely fast change in current where time of impulse front increase is less than a few microseconds. In systems with a reactive load that causes transitional processes, the overload is produced in the form of high-frequency electric and high voltage surge.

the complex protection of the thunderbolt, high voltage and transitional processes

Working principle of protection elements

Gas switch. Contains a tube filled with argon or neon, which has electrodes made of special alloys. All this is housed in glass or ceramic housing. When high voltage impulse, with speed of 1 kW / microsecond enters that type of device, it generates electricity. When the speed of increasing front is lower, it is necessary to increase the voltage in addition to initialize the electricity within the device. Through such a device can pass current with the intensity up to 100 kA. Besides that the great ability to reduce the voltage, gas switch has a response time of several hundred nanoseconds to several microseconds, which is few dozen times slower comparing to the metal oxide varistors. The application of these devices is available only in a open rural environment, where is a possibility of direct lightning strike in the cables.

varistors. This electric set is in fact resistor with resistance that depends on the supplied voltage. They are manufactured from zinc oxide and have a disk shape. Increasing the voltage in relation to normal rapidly reduces the resistance of varistors within 25 nanoseconds, thus limiting the interference to the size of voltage stabilization. Such elements are able to work with the current of 40-80 A. The main disadvantage of the resistors is the lifetime of several years, which reduces after each new response, or overheating.

Protective TVS-diode. varistor capacity is less than 1000 pF which restricts filtering the impulses with a frequency above 1000 MHz. In such cases the best solution is fast suppressor-diode. They operate on the principle of stabilitron (bidirectional diodes), but they differ from it with the speed of reaction, which is measured picoseconds. These elements allow that the stabilized current, in range of 4-15 A, can pass through itself. They are rarely used in the electric power networks, because such small impulses can suddenly disappear in network filters or blocks of electric equipment supplying. Such diodes are most commonly used for protection of information line of high frequency street antenna, local network etc.

Let's look the example, when an impulse with amplitude 10 kV and speed of the growing front of 1 kV / microseconds, enters the plate shown on the picture and it causes the induction of the gas switch which reduces the impulse amplitude to 600-700 V. Varistor decreases the voltage to level close to the voltage stabilization. Passing through a suppressor-diode, the amplitude is reduced exactly to the required voltage stabilization. The reaction order of these devices should be determined by inductivity, which brings time delay to the distribution of pulses (interference). If the front at the entrance of the system is small and its growing speed is less than 1 kV / microseconds, the high voltage Impulse is shut off by the following levels of protection - varistor and (or) suppressor-diode.

(prevod slike-the switch, varistor, diode)

We came to the point of temptation to restrict the use of only one TVS diode. Theoretically this is OK: that element reacts much faster than all the previous elements of protection. But every next element is more sensitive than the previous one. In the case of thunderbolt varistors and diodes will burn up or smash to pieces.

At the protective disk shaped varistors there are two basic sizes: classification voltage (after the letter K) and size of the casing (after the letter S). Classification voltage is stable voltage at which electricity of 1 mA, flows through the element. At the further increase of voltage, varistor begins to heat, which eventually leads to a crack of a thick layer of paint, and the element changes to a short circuit mode and the space around the element, within a radius of 10 cm, is covered by a thin layer of grime. If the element has low hookup legs, when heated it can melt the contacts, printed circuit board or to jump out of its slot. Its housing, when overheated, should not touch the network cables or any other fusible or burning materials and surfaces. Usually, for the protection of home devices can be used varistors with the following classification voltages: 390, 430 and 470 V, which corresponds to a voltage of alternating current of 250, 275 and 300 V. An exception is the Epcos varistor, where the maximum allowed voltage of the alternating current is printed on the housing. Practice has shown that the voltage of 250 V is dangerous for the devices, although most of these devices, particularly those who have digital blocks of power supply, can work at 270-280 V with a certain overheating of the blocks. If the voltage crosses the limit, the device failure is simply inevitable. Strange, but true is the fact that the majority of network filter has varistors at 300 V (471 V voltage classification) but that is the voltage at which most of the techniques burns out. The best is to choose the elements by its size (S14-20), because the bigger ones can support large loads, but the varistors that have a diameter of less than S10-14 can easily explode when overheated, during the voltage jumps or when we turn on the protective device after the malfunctioning of the element itself. We should not forget the maximum voltage tolerance within the varistors can react and response can be +/- 10%, depending on the party. That means that in real situation there is no guarantee that the varistor will react accurately on 250 V, but in the range of 225-275 V. Because of the greater security it is recommended that simultaneous use of several elements of the same type, from different parties of delivery or different manufacturers. It's the kind measure of duplicated protection.

All protective elements are gathered parallel to power supply line after the safety appliance, which has to be doubled, in order to be able to shut down phase and null. It is recommended to use appliance C-Class, to fight against false activation during the stabilization of smaller voltage jumps. It is highly recommended the usage of protective appliance in order to prevent the overheating of varistors which can work with the electric current of 10-16 A. in that case it is recommended to use the appliances with nominal power, which only slightly exceeds the electricity used by the device which is attached to it. It is also necessary to bear in mind the possibility of overheating and fire protection elements. Because of this it is recommended the appliance of its output to the line, not by soldering, but with rail link or contacting cable joints, which significantly simplifies the replacement of blown out parts. Elements should be included in a special small metal housing, which is near the base or in a separate part of the base case that does not include cables or other flammable materials. Varistors carriers necessarily have to be at the layout, which is made of materials that are not flammable, to avoid their possible melting.


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