Protection devices are devices that are designed to protect electrical circuits, electrical equipment, machines and other units from any threats that interfere with the normal operation of these devices, as well as to protect them from overloads. It is important to note here that they must be correctly installed, and operation must be carried out exactly in accordance with the instructions, otherwise the protection devices themselves can cause equipment failure, explosion, fire, and other things.
Basic requirements for fixtures
In order for the device to be able to operate successfully, it must meet the following requirements:
- Under no circumstances should protective devices be above their permissible temperature under normal load of the electric network or electrical equipment.
- The device must not disconnect the equipment from power during short-term overloads, which often include inrush current, self-starting current, etc.
When choosing fuse-links for fuses, it is necessary to be based on the rated current in the circuit section, which will protect this device. This rule of choice of protection devices is relevant in any case when choosing any device for protection. It is also important to understand that with prolonged overheating, the protective qualities are significantly reduced. This negatively affects the devices, since at the time of a critical load they may, for example, simply not turn off, which will lead to an accident.
Protection devices must necessarily disconnect the network in the event of prolonged overloads inside this circuit. In this case, an inverse dependence on the current with respect to the exposure time must be observed.
In any case, the protection device must disconnect the circuit at the end when a short circuit occurs (short circuit). If a short circuit occurs in a single-phase circuit, then disconnection should occur in a network with a grounded neutral. If a short circuit occurs in a two-phase circuit, then in a network with an isolated neutral.
The apparatus for protecting electrical circuits has a breaking capacity I pr The value of this parameter should correspond to the short circuit current that may occur at the beginning of the protected area. If this value is lower than the maximum possible short-circuit current, then the process of disconnecting a section of the circuit may not occur at all or may occur, but with a delay. Because of this, not only the devices connected to this network can be damaged, but also the apparatus for protecting the electrical circuit. For this reason, the breaking capacity factor must be greater than or equal to the maximum short circuit current.
Fuses
To date, there are several devices for protecting electrical networks, which are the most common. One of these devices is a fuse. The purpose of this type of protection apparatus is that it protects the network from current-type overloads and short circuits.
Today, there are single-use devices, as well as with interchangeable inserts. Such devices can be operated both in industrial needs and in everyday life. For this, there are devices that are used in lines up to 1 kV.
In addition to them, there are high-voltage devices used in substations, the voltage of which is more than 1000 V. An example of such a device can be a fuse on transformers for auxiliary needs of substations with 6 / 0.4 kV.
Since the purpose of these protection devices is to protect against short circuit and current overloads, they have received quite widespread use. In addition, they are very simple and convenient to operate, their replacement is also quick and easy, and by themselves they are very reliable. All this led to the fact that such fuses are used very often.
To consider the technical characteristics, you can take the PR-2 device. Depending on the rated current, this device is available with six types of cartridges, which differ in their diameter. In the cartridge of each of them an insert can be installed with the expectation of a different rated current. For example, a cartridge designed for a current of 15 A can be equipped with an insert for 6 A and 10 A.
In addition to this characteristic, there is also the concept of lower and upper test current. As for the lower value of the test current, this is the maximum value of the current, during the course of which the circuit section will not be disconnected for 1 hour. As for the upper value, this is the minimum current coefficient, which, when flowing for 1 hour in the circuit, will melt the insert in the protection and control device.
Circuit breakers
Circuit breakers play the same role as fuses, but their design is more complex. However, this is offset by the fact that the use of switches is much more convenient than fuses. For example, if a short circuit appears in the network due to aging of the insulation, the circuit breaker can disconnect a damaged section of the electrical circuit from the power supply. At the same time, the control and protection device itself is quite easily restored, after operation it does not require replacement with a new one, and after repair work it is again able to reliably protect the part of the circuit it controls. It is very convenient to use such switches if it is necessary to carry out any scheduled repair work.
As for the production of these devices, the main indicator is the rated current for which the device is designed. In this regard, there is a huge selection that allows you to choose the most suitable device for each circuit. If we talk about the operating voltage, they, like fuses, are divided into two types: with a voltage of up to 1 kV and high-voltage with an operating voltage of more than 1 kV. It is important to add here that high-voltage devices for protecting electrical equipment and electrical circuits are made vacuum, with inert gas or oil-filled. This design allows at a higher level to carry out the uncoupling of the chain when such a need arises. Another significant difference between circuit breakers and fuses is that they are manufactured for operation not only in single-phase, but also in three-phase circuits.
For example, in the event of a short circuit to the ground of one of the cores of an electric motor, a circuit breaker will turn off all three phases, and not one damaged one. This is a significant and key difference, since if you turn off only one phase, the engine will continue to operate in two phases. This mode of operation is emergency and greatly reduces the life of the device, and can even lead to an emergency failure of the equipment. In addition, circuit breakers of the automatic type are manufactured for operation with both alternating and constant voltage.
Thermal and current relays
Today, among the devices for protecting electrical networks, there are many different types of relays.
A thermal relay is one of the most common devices that can protect electric motors, heaters, any power devices from such a problem as overload current. The principle of operation of this device is very simple, and it is based on the fact that an electric current is capable of heating the conductor through which it flows. The main working part of any thermal relay is a bimetal plate. When heated to a certain temperature, this plate bends, which breaks the electrical contact in the circuit. Naturally, heating of the plate will occur until it reaches a critical point.
In addition to thermal, there are other types of protection devices, for example, a current relay, which controls the amount of current in the network. There is also a voltage relay that will respond to voltage changes in the network and a differential current relay. The last device is a protection device against leakage currents. It is important to note here that circuit breakers, like fuses, cannot react to the occurrence of current leakage, since this value is quite small. But at the same time, this value is quite enough to kill a person in contact with the body of the device, prone to such a malfunction.
If there is a large number of electrical devices that need to connect a differential current relay, then combined automatic machines are often used to reduce the dimensions of the power shield. Such devices were devices that combine a circuit breaker and a differential current relay - differential protection machines, or diflavomaty. Using such devices not only reduces the size of the power shield, but also greatly facilitates the installation of the protection device, which, in turn, makes them more economical.
Thermal Relay Features
The main characteristic for thermal relays is the response time, which depends on the load current. In other words, this characteristic is called time-current. If we consider the general case, then before the load is applied, a current I 0 will flow through the relay. In this case, the heating of the bimetallic plate will be q 0 . When checking this characteristic, it is very important to consider from which state (overheated or cold) the device is triggered. In addition, when checking these devices, it is very important to remember that the plate is not thermally stable when a short circuit current occurs.
The selection of thermal relays is as follows. The rated current of such a protective device is selected based on the rated load of the electric motor. The selected relay current should be 1.2-1.3 of the rated motor current (load current). In other words, such a device will work if within 20 minutes the load will be from 20 to 30%.
It is very important to understand that the operation of a thermal relay is significantly affected by the ambient temperature. Due to the increase in ambient temperature, the response current of this device will decrease. If this indicator will differ too much from the nominal, then you will need to either carry out additional smooth adjustment of the relay, or buy a new device, but taking into account the real ambient temperature in the working area of this unit.
In order to reduce the influence of ambient temperature on the amount of current triggering, it is necessary to purchase a relay with a large nominal load value. In order to achieve the proper functioning of a warm device, it should be installed in the same room in which the controlled object is located. However, it must be remembered that the relay responds to temperature, and therefore it is prohibited to place it near concentrated heat sources. Such sources are considered boilers, heating sources and other similar systems and devices.
Device selection
When choosing equipment for the protection of power consumers and electric networks, it is necessary to be based on the rated currents for which these devices are designed, as well as on the current supplying the network where such units will be installed.
When choosing a protection device, it is very important to keep in mind the occurrence of such abnormal operating modes as:
- interphase type short circuits;
- phase closure to the housing;
- a strong increase in current, which can be caused by an incomplete short circuit or overload of technological equipment;
- complete disappearance or too much decrease in voltage.
As for short circuit protection, it must be implemented for all electrical receivers. The main requirement is that disconnecting the device from the network when a short circuit occurs should be the minimum possible. When choosing protection devices, it is also important to know that full protection against overload current should be provided, with the exception of several of the following cases:
- when the overload of electrical receivers for technological reasons is simply impossible or unlikely;
- if the electric motor power is less than 1 kW.
In addition, the electrical protection device may not have an overload protection function if it is installed to monitor an electric motor that is operated in short-term or intermittently-short-term operation. An exception is the installation of any electrical appliances in rooms with increased fire hazard. In such rooms, overload protection should be installed on all devices without exception.
Undervoltage protection must be installed in the following cases:
- for electric motors that do not allow inclusion in the network at full voltage;
- for electric motors in which self-start is not allowed for a number of technological reasons, or it is dangerous for employees;
- for any other electric motors whose power off is necessary in order to reduce the total power of all connected electrical receivers in this network to an acceptable value.
Varieties of currents and selection of a protective device
The most dangerous is a short circuit current. The main danger is that it is much larger than the normal inrush current, and its value can vary greatly depending on the part of the circuit where it occurs. Thus, when checking the protection device, which protects the circuit from a short circuit, it should disconnect the circuit as quickly as possible if such a problem occurs. In this case, it should in no case be triggered when a normal value of the starting current of any electric device appears in the circuit.
As for the overload current, everything is pretty clear here. Such a current is considered any value of the characteristic that exceeds the rated current of the electric motor. But here it is very important to understand that not every time an overload current occurs, the protective device must disconnect the circuit contacts. This is also important because short-term overloading of both the electric motor and the electric network in some cases is permissible. It is worth adding here that the shorter the load, the higher values it can reach. Based on this, it becomes clear what is the main advantage of some devices. The degree of protection of devices with a "dependent characteristic" in this case is maximum, since their response time will decrease with an increase in the load factor at this moment. Thus, such devices are ideal for protection against overload current.
To summarize, we can say the following. To protect against short circuit, an inertialess device must be selected, which will be configured to operate a current that is much higher than the starting value. To protect against overload, on the contrary, the switching protection device must have inertia, as well as a dependent characteristic. It must be selected in such a way that it does not work while the normal start-up of the electrical device takes place.
The disadvantages of different types of protective devices
Fuses, which were previously widely used as protection devices for switchgear, have the following disadvantages:
- a rather limited opportunity for use as protection against overload current, since the detuning from inrush currents is quite complicated;
- the electric motor will continue to work in two phases, even if the third disconnect the fuse, which is why the motor often fails;
- in certain cases, the cut-off ultimate power is insufficient;
- There is no way to quickly restore power after a shutdown.
As for the air types of automatic machines, they are more perfect than fuses, but they are not without drawbacks. The main problem with the use of electrical protection devices is that they are not selective in terms of action. This is especially noticeable if an unregulated cutoff current occurs at the installation machine.
There are installation machines in which overload protection is carried out using thermal releases. Their sensitivity and delay are worse than that of thermal relays, but at the same time they act on all three phases at once. As for universal automatic machines for protection, here it is even worse. This is justified by the fact that only electromagnetic releases are available.
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I = ÷(√3*U *cos *n), :
I n is the rated current of the motor, which will have a dimension in A;
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cos c is the coefficient of active power;
n is the coefficient of performance.
Knowing these data, it is possible to easily calculate the rated current of the motor, and then easily select the appropriate protection device for the purpose.
Varieties of damage to protective equipment
The main difference between the means of protecting electric circuits from other devices is that they not only fix the defect, but also disconnect the circuit if the values of the characteristics go beyond certain limits. The most dangerous problem, which often disables protective equipment, is a dull short circuit. During the occurrence of such a short circuit, the current indicators reach the highest values.
When a circuit break occurs when such a problem occurs, an electric arc often arises, which in a short period of time is quite capable of destroying the insulation and melting the metal parts of the apparatus.
If too much overload current occurs, it can lead to overheating of the conductive parts. In addition, there are mechanical forces that significantly increase the wear of individual elements in the equipment, which sometimes can even lead to damage to the device.
There are high-speed switches that are prone to problems such as grazing of the movable arm and movable contact on the walls of the arcing chamber, as well as the closure of the demagnetizing coil bus to the housing. Quite often there is too much wear on the contact surfaces, pistons and drive cylinders.
Repair of high-speed devices
Repair of any type of high-speed protection apparatus must be performed in the same sequence. A high-speed switch, or BV, is purged with clean compressed air under a pressure of not more than 300 kPa (3 kgf / cm 2 ). After that, the device is wiped with napkins. Next, it is necessary to remove such elements as an arcing chamber, a locking device, a pneumatic drive, an armature with a movable contact, an inductive shunt, and others.
The device is directly repaired at a special repair stand. The interrupter chamber is disassembled, its walls are cleaned in a special shot blasting unit, after which they are wiped and inspected. Chips may be allowed in the upper part of this chamber if their dimensions do not exceed 50x50 mm. The wall thickness at the break points should be from 4 to 8 mm. It is necessary to measure the resistance between the horns of the arcing chamber. For some samples, the indicator should be at least 5 megohms, and for some at least 10 megohms.
A damaged partition must be cut down along its entire length. All similar places of log cabins should be carefully cleaned. After that, the surfaces to be glued are lubricated with an adhesive solution based on epoxy resin. If broken fan sheets were found, then they are replaced. If there are curved, then they must be leveled and returned to operation. There is also an extinguishing coil, which should be cleaned of soot and reflow, if any.