The basic definition of low-voltage electrical appliances, the principle of selection of low-voltage electrical appliances

In China's economic construction and people's lives, the application of electric energy is more and more extensive. In order to use electrical energy safely and reliably, various electrical equipment that regulates, distributes, controls, and protects must be installed in the circuit. These electrical devices are collectively referred to as electrical appliances. From the perspective of production or use, electrical appliances can be divided into two categories: high-voltage electrical appliances and low-voltage electrical appliances. With the development of science and technology and production, the types of electrical appliances are increasing, the dosage is increasing, and the use is also extremely extensive. Most of the load on the power system is supplied by low-voltage electrical appliances. Most of the various production machinery and equipment of power users are powered by low voltage. In a large low-voltage power distribution system and a low-voltage power system, a large number of control and protection appliances are required, and these appliances are generally referred to as low-voltage electrical appliances.

A low-voltage electrical appliance is a component or device that can manually or automatically turn the circuit on or off according to external signals and requirements to switch, control, protect, detect, change and adjust the circuit or non-electrical object. According to the working voltage of the control electric appliance, it can be divided into two categories: high-voltage control electric appliance and low-voltage control electric appliance, with the boundary of AC 1200V and DC 1500V. In general, low-voltage electrical appliances can be divided into two major categories of distribution electrical appliances and control electrical appliances, and are the basic components of complete electrical equipment. In the industrial, agricultural, transportation, defense, and people's electricity sectors, most use low-voltage power supply, so the quality of electrical components will directly affect the reliability of low-voltage power supply systems.

Low-voltage electrical equipment refers to electrical equipment that operates in circuits with AC voltage less than 1200V and DC voltage less than 1500V, functions as on/off, protection, control or regulation, and uses electrical energy to control, protect and regulate non-electrical and non-electrical devices. Electrical Equipment. Most of the load on the power system is supplied through low-voltage electrical appliances. Most of the various production machinery and equipment of power users are powered by low voltage. In the huge low-voltage power distribution system and low-voltage power system, a large number of low-voltage electrical appliances for control and protection are required.

The purpose of the low-voltage electrical appliance in the circuit is to automatically or manually turn on or off the circuit according to externally applied signals or requirements, thereby continuously or intermittently changing the parameters or states of the circuit to achieve switching of the circuit or non-electrical object, Control, protect, detect, transform and adjust. Low-voltage electrical appliances are characterized by a wide variety, large dosage and wide application.

Low-voltage electrical appliances are the basic components of power distribution systems and machine tool electrical control systems. The pros and cons of these systems are directly related to the performance of the low voltage appliances used. Although some electrical components may be replaced by electronic circuits with the development of electronic technology, automatic control technology and computer technology, as low-voltage electrical appliances themselves are also expanding in new areas (in the performance of low-voltage electrical appliances, the production of new low-voltage electrical appliances, Realizing the integration of machine, electricity and instrument, expanding the application range of the device, etc., it is impossible to completely replace it.

The main low-voltage electrical components used in electric power drag and transmission systems, according to incomplete statistics, China produces more than 120 series. Nearly 600 varieties, tens of thousands of specifications. These switchgears have different uses and different conditions of use, and thus different methods of selection, but the general requirements should follow the following two basic principles:

Safety principle

Safe and reliable use is the basic requirement for any switchgear, ensuring the reliable operation of the circuit and the electrical equipment, which is an important guarantee for the normal production and life.

Economic principle

Economic considerations can be divided into the economic value of the switchgear itself and the value generated by the use of switching appliances. The former requires reasonable selection and application; the latter considers that it must be reliable in operation, without causing production downtime or damage to equipment due to failure, endangering the economic loss caused by personal safety.

Common low-voltage electrical appliance selection

First, the general principles of the low-voltage electrical selection manual:

Second, the selection of circuit breakers

1. Selection of circuit breakers for distribution.

2. Selection of automatic switch for motor protection

Third, the selection of the knife switch.

Fourth, fuse selection

1, the choice of fuse melt

(1) Select according to the normal working current.

(2) Verify the sensitivity of the short-circuit current

2, various types of equipment fuse selection.

(1) Single motor circuit melt selection

(2) Selection of distribution line melt.

(3) Selection of lighting line melt

(4) Selection of high and low pressure melts for transformers

(5) Selection of the melt of the electrostatic capacitor bank.

3. The choice of fast fuses.

(1) Small-capacity rectifier

(2) Large capacity converter

Fifth, the selection of thermal relay

1, long-term work or intermittent long-term work motor thermal relay selection (1) according to motor start time selection

(2) Select according to the rated current of the motor

(3) Select according to the requirements of phase failure protection

Low-voltage electrical equipment selection manual

First, the general principles of the low-voltage electrical selection manual:

1. The rated voltage of the low-voltage electrical appliance should not be less than the working voltage of the circuit, ie Ue≥Ug.

2. The rated current of the low-voltage electrical appliance should not be less than the calculated working current of the loop, ie Ie≥Ig.

3. The breaking current of the equipment should not be less than the short-circuit current, ie Izh≥Ich

4. The thermal stability guarantee value should not be less than the calculated value. (thermal relay);

5. Select low-voltage electrical equipment according to the starting condition of the circuit. For example, the fuse and the automatic air switch need to be selected according to the starting condition. (Need to consider the starting current)

Second, the selection of circuit breakers

Protection: overload, short circuit, under voltage

General selection:

1. The rated voltage of the circuit breaker ≥ the rated voltage of the line;

2. The rated current of the circuit breaker ≥ the line calculates the load current;

3. The rated current of the circuit breaker release ≥ the line calculates the load current;

4, the circuit breaker limit on-off capacity ≥ the maximum short-circuit current in the line;

5. The automatic switching instant (or short delay) tripping setting current of the single-circuit short-circuit current at the end of the line is not less than 1.25 times;

6. The rated voltage of the circuit breaker undervoltage release is equal to the rated voltage of the line.

1. Selection of circuit breakers for distribution:

1. The long-delay action current is set to 0.8~1 times of the allowable current carrying capacity of the wire;

2. The return time of the motor of the 3 times long delay action current setting value is not less than the starting time of the motor with the maximum starting current in the line;

3. The short-delay action current setting value is not less than 1.1 (Ijx+1.35kIedm). Ijx calculates the load current for the line; k is the motor starting current multiple, and Iedm is the maximum rated current of one motor;

4. Short delay time according to the thermal stability check of the protected object;

5. When there is no short delay, the instantaneous current setting value is not less than 1.1 (Ijx+1.35k1kIedm). K1 is the impact coefficient of the motor starting current, which is 1.7~2. If there is a short delay, the short-circuit current is calculated at the input end of the lower-stage switch whose instantaneous current setting is not less than 1.1.

value.

2. Selection of automatic switch for motor protection:

1. Long delay current setting value = motor rated current;

2. The return time of 6 times long delay current setting value ≥ motor starting time;

3. The instantaneous setting current of the squirrel cage is 8~15 times of the rated current of the trip unit; the instantaneous setting current of the winding shape is 3~6 times of the rated current of the trip unit.

Selection of automatic switch for lighting:

1. The long delay current setting value is not greater than the line calculation load current;

2. The instantaneous current setting value = 6 times the line calculates the load current.

Third, the selection of the knife switch

Protection: It is mainly used as an isolating switch. It does not cut off the fault current and can only withstand the electric power and thermal effects caused by the fault current.

Selection:

1, according to the rated voltage:

The rated voltage of the knife switch is ≥ the working voltage of the knife switch.

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Low-voltage electrical equipment selection manual

2, according to the rated current:

The rated current of the knife switch is ≥ the working current of the knife switch. If there is a motor in the circuit, the operating current should be from the motor.

Dynamic current calculation.

3. According to the thermal stability and dynamic stability check:

Imax ≥ ich. Imax: Maximum allowable current. Ich: Three-phase short-circuit inrush current.

Fourth, fuse selection

Protection: Short circuit, if overload protection, the reliability is not high.

1, the choice of fuse melt

(1) Select according to normal working current

Melt rated current ≥ line calculation current

(2) Verify the sensitivity of the short-circuit current

Idmin/Ier≥Kr Idmin: protected line

Minimum short-circuit current Kr: fuse action coefficient, generally 4

2, various types of equipment fuse selection

(1) Single motor circuit melt selection

Ier≥Iqd/α :Ier: melt rated current Iqd: motor starting current; α: calculation coefficient, fuse characteristics depending on starting condition

(2) Selection of distribution line melt

Iqd1 + Ig (n -1) Ier e;

Iqd1: Starting current of the motor with the highest starting current in the line

Ig(n-1): line operating current other than the motor with the highest starting current

α: Calculated coefficient.

(3) Selection of lighting line melt

Ier≥Ig/αm ; Ig: line calculation current; αm: calculation coefficient

(4) Selection of high and low pressure melts for transformers

1 Transformer with a capacity of 160KVA, its high-pressure melt is selected according to 2~3 times rated current; the transformer with capacity above 160KVA, high-pressure melt is selected according to 1.5~2 times rated current.

2 Transformer low pressure melt is selected according to its __________ rated current or 20% of overload current.

(5) Selection of electrostatic capacitor bank melt

Ier ≥ (1.5 ~ 2) Iec

Iec: rated current of the capacitor bank

3, the choice of fast fuse

Fast-acting fuses are mainly used for internal rectifier protection of silicon rectifier components, thyristor components and complete sets of devices.

(1) Small-capacity rectifier

Ier=1.57IF Ier: Rated fuse rms current value. IF: thyristor forward average current

(2) Large-capacity converter device

Me 1 Ar

Ki Ak

m: number of parallel paths

Ki: dynamic current sharing coefficient, generally 0.5 to 0.6

Ar: Fuse maximum blow

Ak: Silicon component surge

Fifth, the selection of thermal relay

Protection: overload

1. Selection of long-term work or intermittent long-term work motor thermal relay

(1) Select according to motor starting time

Tf=(0.5~0.7)td

Tf: the return time of the thermal relay at 6Ie

Td: the operating time of the thermal relay at 6Ie

(2) Select according to the rated current of the motor

Iz=(0.95~1.05)Ied

Iz: Thermal relay setting current

Ied: rated motor current

(3) Select according to the requirements of phase failure protection

For the star-connected motor, a three-pole thermal relay can be used; for a delta-connected motor, a thermal relay with a phase-breaking operation protection device should be used.

Precautions

Classification and use environment of control objects (such as electric motors or other electrical equipment); confirm relevant technical data, such as rated voltage, rated power, starting current multiple, load nature, operating frequency, working system, etc. of the control object; Normal working conditions, such as ambient air humidity, relative humidity, altitude, allowable installation azimuth and impact vibration, harmful gases, conductive dust, rain and snow infiltration; understand the main technical performance (or technical conditions) of electrical appliances, such as Use, classification, rated voltage, rated control power, connection capacity, breaking capacity, allowable operating frequency, working system and service life.