1. How to quickly remove an electric shock victim from the power source?
Answer: There are three situations: 1. When there is a low-voltage electric shock, there are three methods: turn off the power, cut off the power cord, and use insulating objects to disconnect from the power supply; 2. When there is a high-voltage electric shock, there are two methods The first method: turn off the power, short circuit method; 3. Method to escape from the step voltage: turn off the power, wear insulated shoes, jump to the side of the person who got the electric shock with one foot on the ground, close to the head or feet of the person who got the electric shock, put the After dragging him to lie on the equipotential ground (that is, his body lies in a position perpendicular to the electric shock radius), he can rest or be rescued on the spot.
2. What are the requirements for setting up temporary lines?
Answer: 1. When erecting temporary lines, it must be approved by the technical safety department, and the installation requirements, length restrictions, and usage period must be clearly specified. 2. Use qualified equipment and materials. The wires should be four-core or three-core rubber cords. The length should not exceed 10m, and the distance from the ground should not be less than 2.5m. 3. The length of temporary overhead lines shall not exceed 500m, the height from the ground shall not be less than 4-5m, and the distance from buildings, trees or other conductors shall generally not be less than 2m. 4. Temporary lines must be led out through the gate box, and no random connections or connections are allowed. 5. There must be reliable grounding (zero connection) protection, and the connection must be firm and reliable. 6. The temporary line should implement a three-phase five-wire power supply method. The working neutral line and the protective neutral line should be laid separately. The protective neutral line should be yellow and green.
3. What are the organizational measures to ensure safety when working on electrical equipment?
Answer: The organizational measures to ensure safety when working on electrical equipment include: work ticket system, work permit system, work supervision system, work interruption, transfer and termination system.
4. What are the main accidents that often occur in electric welding operations?
Answer: Mainly include: 1. Electric shock accident caused by electric current; 2. Damage to eyes and skin caused by electric arc; 3. Burns to human body caused by molten splashing metal foam; 4. Fire accident.
5. What are the electrical safety requirements for low-voltage live work?
Answer: 1. Guardians should be focused, not allowed to come into contact with the operator, and not leave their posts without authorization to participate in matters unrelated to the guardianship work. 2. Only one person can perform live work, and no two people can work on the same site at the same time. 3. When removing wires while the power is on, remove the phase wire first and then the neutral wire; when wiring, connect the neutral wire first and then the phase wire. When connecting wires during live work, it is not allowed to operate with load. 4. It is strictly forbidden to remove the secondary side wire of the current transformer while it is powered on. 5. The disconnection should be wrapped with insulating tape. 6. When working under multi-layer overhead lines with electricity, the head should not exceed the working line layer. If the upper layer is working with electricity, the lower layers must be powered off and safety technical measures must be taken.
6. What is the method of extrathoracic heart compression?
Answer: The patient lies supine on the hard ground, loosen the collar and undress, and place the palms on the chest. Don’t be reckless. The middle finger should be placed in the groove. Press down hard with the heel of the palm. The pressure should be light and appropriate. Press down an inch or more. Half an inch, once a second is most appropriate.
7. Describe the operation method of mouth-to-mouth artificial respiration?
Answer: The person who gets an electric shock should lie on his back. You can put something under his shoulders to tilt his head back as far as possible with his nostrils facing the sky. Rescue him on the left or right side of his head. Pinch the nostrils with one hand and use the other hand to rescue him. Open your mouth with your hands and blow air. When blowing, make sure your chest expands, and then quickly move your head away to let the person who gets the electric shock exhale on their own. Blow for 2 seconds and exhaust for 3 seconds, a cycle of about every 5 seconds. Children can only blow in small sips.
8. Describe the methods of fighting electrical fires.
Answer: 1. Cut off the power and put out the fire. At this time, it should be noted that the location for cutting off the power supply should be appropriately selected. The live and neutral wires should be cut at different locations. When opening the power switch, insulating gloves should be worn and an insulating rod should be used. If the burning situation affects nearby operating equipment, When there is a serious threat, the relevant circuit breakers and isolation switches should be opened quickly. 2. When extinguishing a live fire, be careful to use non-conductive fire extinguishing agents. Be careful not to let people's bodies or fire-fighting equipment come into contact with electrical parts to prevent electric shock. During fire extinguishing, if electrical equipment fails, you should wear insulating gloves and insulating clothing. shoe. 3. Pay attention to electrical equipment that contains oil and electricity. If the fire spreads, you can use carbon dioxide, 1211, dry powder and other fire extinguishers to extinguish the fire. If the fuel tank ruptures and sprays fuel and burns, in addition to cutting off the power supply, you should try to drain the oil into the oil seepage pit.
9. What are the functions of transformers
Answer: The functions are: 1. To insulate instruments and relays from the main circuit to improve work safety and reliability; 2. To Expand the use scope of instruments and relays; 3. Unify the specifications of instrument relays.
10. How should the cross section of the conductor be selected?
Answer: The conductor cross-section is mainly selected based on three conditions: the continuous allowable current (safe current carrying capacity) of the conductor, the allowable voltage loss of the line, and the mechanical strength of the conductor. The thermal stability of the cable line should also be verified.
11. What are the organizational and technical measures to ensure safety when working on electrical equipment?
Answer: Organizational measures: 1. Work ticket system 2. Work permit system 3. Work supervision system 4. Work interruption, transfer and termination system.
Technical measures: 1. Power outage 2. Electricity test 3. Install grounding wire 4. Hang signboards and install barriers.
12. What issues should be paid attention to when starting a three-phase AC asynchronous motor?
Answer: After the power is turned on, check whether the internal sounds of the bearings and the motor are normal, whether there is any abnormal smell, whether there is smoke and sparks, whether there is violent vibration, whether there is overheating, etc., observe Is the startup process smooth and the rotation brisk?
In order to prevent the insulation of the motor stator winding from being damaged, the number of consecutive starts of the motor is limited. Generally, no-load starts should not exceed 3-5 times. The motor needs to be started again soon after it has been shut down for a long time. More than 2-3 times.
13. How many voltage-reduced starting methods are there for three-phase AC asynchronous motors?
Answer: 1. Series resistor or reactor step-down start 2. Star-delta voltage start 3. Autotransformer step-down start 4. Yanbian delta step-down start.
14. How to perform the correct switching operation?
Answer: 1. The switching operation must be ordered by the electrical person in charge or according to the work requirements; 2. The operation must be performed by two people; 3. During the operation, the operator should keep a safe distance from the live body guard. , wear work clothes and use insulated tools correctly and mark them; 4. Pay attention to the sequence of switching operations to avoid misoperation.
15. What are the uses of relay protection devices?
Answer: When the power supply line or electrical equipment fails or is in abnormal working condition, the relay protection device can act and trip the switch or detect a signal to ensure the safety of electrical equipment in the power system, prevent accidents, and reduce the risk of accidents. accident scope to improve the safety and reliability of power supply and consumption system operations.
1. Fill in the blanks:
1. In a neutral grounding system, in addition to the working ground, one or more places on the neutral line are reconnected to the earth through a grounding device, which is called duplication. Ground.
2. When the load current of electrical equipment exceeds the rated current, it is called overload.
3. The safety voltage ratings are 42V, 36V, 24V, 12V, and 6V.
4. There are two types of harm caused by electric current to the human body: electric shock and electric injury.
5. The grounding of the neutral point on the low-voltage side of the transformer is called working grounding.
6. Grounding method, leakage method, adding antistatic agent method, electrostatic neutralization method and process control method are commonly used for electrostatic protection.
7. The minimum safe distance between the human body and charged objects is: 0.7 meters for 10KV and below, and 1.0 meters for 35KV.
8. Ground resistance testers generally have three terminal buttons: E, P, and C.
9. Protective grounding is used for power grids with ungrounded neutral points; protective grounding is used for power grids with directly grounded neutral points.
10. Common faults in lighting circuits include fault, open circuit and leakage.
11. Phase-shifting capacitors are used to improve the power factor of general R&D power grids.
12. The load switch is suitable for places where the load current is zero or small.
13. Our country stipulates that 42V, 36V, 24V, 12V and 6V are safe voltages.
14. At any node in the circuit, the sum of the currents flowing into the node is equal to the sum of the currents flowing out of the node.
15. The three elements of orthodox alternating current refer to the maximum value, angular frequency and initial phase angle.
16. The heat generated by an energized conductor is proportional to the size of the energizing current, the conductivity of the conductor itself, and time.
17. Among lighting circuits, indoor wiring can be divided into two types: exposed wiring and concealed wiring.
18. There are three working modes of electric motor: continuous, short-time and intermittent.
19. By connecting appropriate capacitors in parallel at both ends of the inductive load, the factor of the circuit can be improved.
20. In the three-phase symmetrical load Y-shaped connection, Ur line = Ur phase
21. The rated voltage of a three-phase AC asynchronous motor is 380V, 220V. The connection method is Y/△, then the rated voltage of its winding is 380V. When the line voltage of the three-phase symmetrical power supply is 220V, the motor must be connected to △.
22. Low-voltage electrical appliances include protective electrical appliances and control electrical appliances.
23. For any loop, along any direction, the algebraic sum of the electromotive force of each power source is equal to the algebraic sum of the voltage drops of each resistor.
24. The grounding device consists of a grounding body and a grounding wire.
25. Protective appliances function as short circuit, overload, and voltage loss protection respectively.
26. The protective zero connection is suitable for systems in which the neutral point of a three-phase four-wire 380 or 220V transformer is directly grounded.
27. General lighting should use a rated voltage below 250V.
28. Before using the electroscope, it should be tested on the live parts to check the quality of the electrical appliance.
29. The method of changing the steering direction of a three-phase AC asynchronous motor is to change the phase sequence of the power supply.
30. The main purpose of the high-voltage isolation switch is to isolate high-voltage power supply.
31. It is strictly forbidden to open the secondary side of the current transformer during operation.
32. Technical measures to ensure safety include power outages, power inspections, installation of grounding wires, hanging signs and installation of barriers.
33. Single-phase equipment can adopt safety measures such as protective zero or grounding, case voltage, electrical isolation, double insulation and leakage protection.
34. A complete set of lightning protection devices includes air terminals, down conductors and grounding devices.
35. Protective grounding, protective zero connection, and reinforced insulation are safety measures to prevent indirect electric shock.
36. Safety signs are an important piling measure to ensure safe production and prevent accidents. Red means prohibition, firefighting and stop, yellow means pay attention to safety, green means safety, and blue means enforcement.
37. In low-voltage work, the distance between the human body and picked up tools and charged objects should be no less than 0.1 meters.
38. Among safety signs, blue signs indicate regulations that must be followed.
39. Thermal relays are generally used for overload protection.
40. When the three-phase load is connected in star shape, the relationship between line voltage and phase voltage is U line = U phase.
41. 50HZ power frequency alternating current, the average discharge current of adult men is 16MA.
42. The intensity of current passing through the human body depends on the electric shock voltage and human body resistance.
43. When selecting the conductor cross-section, the allowable carrying capacity of the conductor for long-term continuous load, the voltage drop and allowable voltage loss on the line, and the mechanical strength of the conductor should generally be considered.
44. There are three lighting methods: general lighting, local lighting, and mixed lighting.
45. Lightning protection lines are mainly used to protect transmission lines.
46. The maximum allowable operating temperature of commonly used insulating materials under normal operating conditions is 90 degrees Celsius.
47. The cross-section of the conductor is mainly selected based on three conditions: the continuous allowable current of the conductor, the allowable voltage loss of the line, and the mechanical strength of the conductor.
48. There are three types of electric shock: single-phase electric shock, two-phase electric shock and step voltage electric shock.
49. The timing is mainly composed of two basic parts: the stator and the rotor.
50. The heat resistance grades of insulation materials include seven grades: Y (A) E, (B) F, H, and C.
51. Transformers work using the mutual inductance principle of electromagnetic induction.
52. The direct causes of electrical fires and explosions include current heat, sparks, and arcs.
53. Transformers work using the mutual inductance principle of electromagnetic induction.
54. In overhead lines, the phase sequence of the line arrangement should meet the following requirements: high-voltage power lines, facing the load from the left, the conductor arrangement phase sequence is A, B, C; low-voltage power lines, facing the load from the left The load is from the left, and the phase sequence of the conductors is (A) 0, (B) C.
55. Current transformers are used to convert large currents in the power system into standard small currents for supplying instruments and relays in series for measurement and protection.
56. Most of the faults that occur in electrical equipment are phase-to-phase short circuit faults and equipment overloads.
2. Multiple choice questions:
1. Various poles in overhead lines are divided according to their functions: straight poles, tension poles, corner poles, branch poles, and terminals Rod.
2. The requirements for relay protection devices include: protection selectivity and sensitivity. The speed and reliability of protection.
3. The insulation resistance of portable electrical equipment should not be less than: 0.5 megohms.
4. In low-voltage work, the distance between the human body and the tools carried and the electrified body shall not be less than: 0.7 meters.
5. During low-voltage maintenance work, the distance between the human body and its carrying tools and live objects should not be less than: 0.1 meters.
6. For inductive loads, if the frequency of the power supply is increased while keeping the power supply unchanged, the total current in the circuit will: decrease.
7. In order to improve the accuracy of measurement, the internal resistance requirement of the voltmeter is: the bigger, the better.
8. If a delta-connected squirrel-cage asynchronous motor is mistakenly connected to a star connection, its copper loss and temperature rise will increase when running at rated load torque.
9. High-voltage electrical appliances that cannot be connected and disconnected with load under normal circumstances include: isolation switch.
10. The insulation test period of insulated shoes is: 0.5-1 year.
11. Our country stipulates that power lines below 1000V are called low-voltage power lines, and those above are called high-voltage lines.
12. The most common abnormal condition of electrical equipment is: overload. If it continues for a long time, the current-carrying parts and insulating materials of the electrical equipment will overheat.
13. The rated voltage of a certain three-phase asynchronous machine is 380V, and its AC withstand voltage test voltage is: 500V.
14. Thermal relay has overload protection function.
15. If a delta-connected squirrel-cage asynchronous motor is mistakenly connected to a star connection, its copper loss and temperature rise will increase when running at rated load torque.
16. The safety color used to mark prohibition, stop and fire protection is: red.
17. When working on electrical equipment with complete or partial power outage, the technical measures to ensure safety include: power outage, power inspection, installation of grounding wires, hanging signs and installation of barriers.
18. The three elements of sinusoidal alternating current are: maximum value, frequency, and initial phase angle.
19. When installing a single-throw knife switch, the static contact should be placed on it to connect to the power supply.
20. For resistive loads, the rated current of the fuse melt should be 1.1 times the load current.
21. The distance between concealed sockets and the ground should not be less than 30CM.
22. In order to improve the accuracy of measurement, the internal resistance of the ammeter should be as small as possible.
23. In overhead lines, when abnormal climate changes or natural disasters in areas along the line will seriously affect the safety of the line power supply, special inspections need to be carried out immediately.
24. The following insulating materials include: rubber.
25. The installation height of surface-mounted sockets should generally be 1.2M.
26. Work at heights above 2 meters with the possibility of falling vertically is considered high-altitude work.
27. For inductive loads, if the power supply voltage is kept unchanged and the frequency of the power supply is increased, the total current in the circuit will: decrease.
28. In order to prevent the transformer oil from deteriorating too quickly, the upper oil temperature should not frequently exceed: 85 degrees Celsius.
29. Y--△ reduced voltage starting is suitable for motors with stator windings in delta connection (△).
30. To measure the resistance below 1 ohm, you should use: DC double-arm bridge.
3. True or False Questions
1. Y--△ reduced voltage starting of three-phase squirrel-cage asynchronous motor can only be applied in no-load or light-load starting situations. Yes
2. In a three-phase four-wire power supply system, the neutral wire can be thinner than the phase wire. Yes
3. The secondary side of the current transformer can be open-circuited. Wrong
4. For indoor wiring, the maximum load current of each branch of general lighting branches should not exceed 15A. Yes
5. The frequency of electromagnetic waves that are harmful to the human body is between 30-300MHZ. Yes
6. The voltage of row lights is generally 36V or lower. Yes
7. The insulation grade refers to the heat resistance grade of the insulating material. Wrong
8. When the reference point in the circuit changes, the voltage between two points will also change accordingly. Wrong
9. The biggest danger caused by static electricity is explosion and fire. For
10, Y/△ step-down movement, it is suitable for motors with various wiring methods. Wrong
12. It is strictly forbidden to open the voltage transformer when it is working. Right
13. In the same power supply system, the power absorbed by the three-phase load connected in Y shape and △ is equal. Wrong
14. Electric shock occurs when electric current passes through local organs of the human body, causing damage to local organs of the human body. Wrong
15. After removing the electric shock victim from the power source, he should be sent to the hospital immediately for rescue. Wrong
16. In order to ensure the reliability of conduction, switches and fuses are not allowed to be installed on the protective neutral line. Yes
17. In AC circuits, active power is useful power. Wrong
18. When maintaining incandescent lamps, please note that the rated voltage of the bulb should be consistent with the power supply voltage. Right
19. In order to facilitate the distinction between the three phase wires of three-phase alternating current, red represents the first phase, yellow represents the second phase, and green represents the third phase. wrong.
20. The voltmeter should be used in parallel with the load. Right
21. In a single-phase line with one phase and one zero, if fuses are installed on both the phase and neutral lines of the branch line, zero protection cannot be used. Right
22. In a purely inductive AC circuit, the current phase leads the voltage phase by 90 degrees. Wrong
23. The leads of mobile electrical appliances must be three-core solid rubber-covered wires or plastic-protected soft wires, and must not have joints. Yes
24. Thermal relays are mainly used for short circuit and overload. Wrong
25. The protection neutral line uses a yellow and green two-color wire, and the working neutral line uses a black wire. Yes
26. When using a single-pole lighting switch, the wiring switch must control the phase line.
27. For equipment powered by a phase-phase four-wire neutral point grounding system, protective grounding or protective zero connection can be used. Wrong
28. Too high or too low voltage will increase the operating current of the motor. Yes
29. When using ammeter and voltmeter, please note that the ammeter is connected in series in the circuit and the voltmeter is connected in series in the circuit. Wrong
30. Protective grounding is suitable for power grids with ungrounded neutral points, and protective grounding is suitable for power grids with directly grounded neutral points. The above two cannot be used in the same power grid at the same time. Right
IV. Explanation of terms
1. Power factor: refers to the ratio of active power to apparent power.
2. Protective grounding: refers to tightly connecting the metal parts that may exhibit dangerous ground voltage under fault conditions with the earth.
3. Fatal current: a current that causes ventricular fibrillation and kills people.
4. Overload protection: When the line or load exceeds the allowable range, a protection that can delay cutting off the power supply.
5. Single-phase electric shock: When the human body directly contacts the one-phase conductor of the live equipment or line, the electric shock occurs when the current passes through the human body.
6. Switching operation: The operation (cut off or put in) of the switch (isolating switch and circuit breaker) and its auxiliary operations due to the need for maintenance or operation.
7. Accident lighting: After the work lighting goes out due to an electrical accident, lighting is set up to continue working or evacuate personnel.
8. Alternating current: Current, voltage, and electromotive force whose magnitude and direction change with time are usually collectively referred to as alternating current.
9. Screen protection: that is, using barriers, shields, covers, boxes, etc. to isolate live objects from the outside world.
10. Electric shock: Electric shock current flows through the internal organs of the human body, causing damage to the internal organs of the human body.
11. Electrical injury: It is the harm caused to the human body by the thermal energy or chemical energy converted by electric current, including burns, electric branding and skin metallization.
12. Overload protection: refers to a protection method that can delay the power supply when the load of the line or equipment exceeds the allowable range.
13. Short circuit: In the power system, a conductor with a small resistance is short-circuited between phases or between phases and ground.
1. How to quickly remove an electric shock victim from the power source?
Answer: There are three situations: 1. When there is a low-voltage electric shock, there are three methods: turn off the power, cut off the power cord, and use insulating objects to disconnect from the power supply; 2. When there is a high-voltage electric shock, there are two methods The first method: turn off the power, short circuit method; 3. Method to escape from the step voltage: turn off the power, wear insulated shoes, jump to the side of the person who got the electric shock with one foot on the ground, close to the head or feet of the person who got the electric shock, put the After dragging him to lie on the equipotential ground (that is, his body lies in a position perpendicular to the electric shock radius), he can rest or be rescued on the spot.
2. What are the requirements for setting up temporary lines?
Answer: 1. When erecting temporary lines, it must be approved by the technical safety department, and the installation requirements, length restrictions, and usage period must be clearly specified. 2. Use qualified equipment and materials. The conductor should be a four-core or three-core rubber cord. Its length should not exceed 10m, and the distance from the ground should not be less than 2.5m. 3. The length of temporary overhead lines shall not exceed 500m, the height from the ground shall not be less than 4-5m, and the distance from buildings, trees or other conductors shall generally not be less than 2m. 4. Temporary lines must be led out through the gate box, and no random connections or connections are allowed. 5. There must be reliable grounding (zero connection) protection, and the connection must be firm and reliable. 6. The temporary line should implement a three-phase five-wire power supply method. The working neutral line and the protective neutral line should be laid separately. The protective neutral line should be yellow and green.
3. What are the organizational measures to ensure safety when working on electrical equipment?
Answer: The organizational measures to ensure safety when working on electrical equipment include: work ticket system, work permit system, work supervision system, work interruption, transfer and termination system.
4. What are the main accidents that often occur in electric welding operations?
Answer: Mainly include: 1. Electric shock accidents caused by electric current; 2. Damage to eyes and skin caused by electric arc; 3. Burns caused by molten splashing metal foam; 4. Fire accidents.
5. What are the electrical safety requirements for low-voltage live work?
Answer: 1. Guardians should be focused, not allowed to come into contact with the operator, and not leave their posts without authorization to participate in matters unrelated to the guardianship work. 2. Only one person can perform live work, and no two people can work on the same site at the same time. 3. When removing wires while the power is on, remove the phase wire first and then the neutral wire; when wiring, connect the neutral wire first and then the phase wire. When connecting wires during live work, it is not allowed to operate with load. 4. It is strictly forbidden to remove the secondary side wire of the current transformer while it is powered on. 5. The disconnection should be wrapped with insulating tape. 6. When working under multi-layer overhead lines with electricity, the head should not exceed the working line layer. If the upper layer is working with electricity, the lower layers must be powered off and safety technical measures must be taken.
6. What is the method of extrathoracic heart compression?
Answer: The patient lies supine on the hard ground, loosen the collar and undress, and place the palms on the chest. Don’t be reckless. The middle finger should be placed in the groove. Press down hard with the heel of the palm. The pressure should be light and appropriate. Press down an inch or more. Half an inch, once a second is most appropriate.
7. Describe the operation method of mouth-to-mouth artificial respiration?
Answer: The person who is electrocuted should lie on his back. You can put something under your shoulders to tilt his head back as far as possible with his nostrils facing the sky. Rescue the person who is electrocuted on the left or right side of his head. Pinch the nostrils with one hand and use the other hand to rescue him. Open your mouth with your hands and blow air. When blowing, make sure your chest expands, and then quickly move your head away to let the person who gets the electric shock exhale on their own. Blow for 2 seconds and exhaust for 3 seconds, a cycle of about every 5 seconds. Children can only blow in small sips.
8. Describe the methods of fighting electrical fires.
Answer: 1. Cut off the power and put out the fire. At this time, it should be noted that the location for cutting off the power supply should be appropriately selected. The live and neutral wires should be cut at different locations. When opening the power switch, insulating gloves should be worn and an insulating rod should be used. If the burning situation affects nearby operating equipment, When there is a serious threat, the relevant circuit breakers and isolation switches should be opened quickly. 2. When extinguishing a live fire, be sure to use non-conductive fire extinguishing agents. Be careful not to allow people's bodies or fire-fighting equipment to come into contact with live parts to prevent electric shock. During fire extinguishing, if electrical equipment fails, you should wear insulating gloves and insulation. shoe. 3. Pay attention to electrical equipment that contains oil and electricity. If the fire spreads, you can use carbon dioxide, 1211, dry powder and other fire extinguishers to extinguish the fire. If the fuel tank ruptures and sprays fuel and burns, in addition to cutting off the power supply, you should try to drain the oil into the oil seepage pit.
9. What are the functions of transformers
Answer: The functions are: 1. To insulate instruments and relays from the main circuit to improve work safety and reliability; 2. To Expand the scope of use of instruments and relays; 3. Unify the specifications of instrument relays.
10. How should the cross section of the conductor be selected?
Answer: The conductor cross-section is mainly selected based on three conditions: the continuous allowable current (safe current carrying capacity) of the conductor, the allowable voltage loss of the line, and the mechanical strength of the conductor. The thermal stability of the cable line should also be verified.
11. What are the organizational and technical measures to ensure safety when working on electrical equipment?
Answer: Organizational measures: 1. Work ticket system 2. Work permit system 3. Work supervision system 4. Work interruption, transfer and termination system.
Technical measures: 1. Power outage 2. Electricity test 3. Install grounding wire 4. Hang signboards and install barriers.
12. What issues should be paid attention to when starting a three-phase AC asynchronous motor?
Answer: After the power is turned on, check whether the internal sounds of the bearings and the motor are normal, whether there is any abnormal smell, whether there is smoke and sparks, whether there is violent vibration, whether there is overheating, etc., observe Is the startup process smooth and the rotation brisk?
In order to prevent the insulation of the motor stator winding from being damaged, the number of consecutive starts of the motor is limited. Generally, no-load starts should not exceed 3-5 times. The motor needs to be started again soon after it has been shut down for a long time. More than 2-3 times.
13. How many voltage-reduced starting methods are there for three-phase AC asynchronous motors?
Answer: 1. Series resistor or reactor step-down starting 2. Star-delta voltage starting 3. Autotransformer step-down starting 4. Yanbian delta step-down starting.
14. How to perform the correct switching operation?
Answer: 1. The switching operation must be ordered by the electrical person in charge or according to the work requirements; 2. The operation must be performed by two people; 3. During the operation, the operator should keep a safe distance from the live body guard. , wear work clothes and use insulated tools correctly and mark them; 4. Pay attention to the sequence of switching operations to avoid misoperation.
15. What are the uses of relay protection devices?
Answer: When the power supply line or electrical equipment fails or is in abnormal working condition, the relay protection device can act and trip the switch or detect a signal to ensure the safety of electrical equipment in the power system, prevent accidents, and reduce the risk of accidents. accident scope to improve the safety and reliability of power supply and consumption system operations.
Six: Calculation questions
1. The line voltage of the three-phase AC power supply is 380V, and a three-phase electric furnace with a heating wire resistance of 22 ohms per phase is connected: find (1) When the electric furnace is Y-connected, what is the total power Pr? (2) When the electric furnace is connected in △, what is the total power P△?
Solution: (1) In Y connection: I line = I phase U phase/R = 380/ *22 = 10A
Pr = *U line *I line = * 380*10＝6.6KW
(2) During △ connection: I line = I phase = U phase/R＝ *380/22＝30A
P△＝ *U Line*I line= *380*30=19.8KW
2. If the three-phase symmetrical load is star-connected, what is the phase voltage when the line voltage is 220V? When the phase voltage is 380V, what is the line voltage?
1) When the line voltage is 220V: U phase = U line / =220/1.73 = 127V
2) When the phase voltage is 380V: U line = *U phase ＝1.73*380V＝660V
3. A temporary lighting line was erected at the construction site of a factory. When a worker moved the lighting at night, he grabbed the screw cap and was electrocuted. He fell to the ground and was found to be sent to the hospital. Hospital, deceased.
(1) Analysis of the cause of the accident
A: When the electrician connected the lighting wire, he mistakenly connected the intersecting phase wire to the screw port of the lamp holder. The bulb and the lamp holder did not cooperate well, and part of the bulb The screw mouth is exposed, and after power is supplied, the exposed part of the lamp head becomes electrified, causing electric shock.
B. Temporary lines and lighting do not meet safety requirements.
C. The person who received an electric shock did not understand safety knowledge and grabbed the electric lamp holder.
D. If the patient was not discovered in time, the patient would be given the correct first aid on the spot and sent to the hospital, which would delay the rescue time.
(2) Preventive measures to be taken
A. When installing lighting, the single-pole switch must control the phase line. For screw lamp holders, the phase wire must be connected to the core of the lamp holder, and bipolar switch control is required in more dangerous places.
B. Mobile lamps for local lighting in humid environments should use safe voltage (36V). Under normal circumstances, highly sensitive leakage protectors should be installed.
C. Strengthen electricity safety education. People who do not understand electrical safety knowledge should not tamper with electrical equipment.
(3) Summary of accident lessons: Although lighting wiring is not complicated, many people were injured and killed due to electric shock. According to the requirements of regulations, it is strictly prohibited to connect and pull temporary lighting wires indiscriminately to prevent personal electric shock accidents.
4. Try to choose a fuse for a three-phase 380V, 20KW motor?
Solution: Motor rated current Ie=20*2=40A
Motor starting current 40*6=240A
The rated current Ire of the fuse is:
p>
Ire≥Iq/1.8-2.5, take Ire≥240/1.8-2.5＝133-96A
Select Ire＝100A fuse
5. Three-phase AC asynchronous motor The data on the nameplate is 3KW, Y-shaped connection, 380V, 7.36A, N=1430, rpm, f=50HZ
No more, MB is too difficult