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The selection of air switches (circuit breakers) for household circuits can be considered from the following aspects:
**I. Select the rated current according to the household electricity load**
1. Calculate the total load current
- Firstly, determine the total power sum of all electrical appliances in the household. Common electrical appliances in general households include air conditioners, electric water heaters, refrigerators, televisions, washing machines, etc.
- According to the power calculation formula P = UI (power = voltage ร current), the voltage in household circuits is usually 220V. Thus, the current I = P/U can be derived. For example, for an air conditioner with a power of 2000W, its current is approximately 2000 รท 220 โ 9.1A.
- Add up the current values of all electrical appliances in the home to obtain the total household load current.
2. Determine the rated current of the circuit breaker
- To ensure that the circuit breaker does not trip frequently during normal operation and can cut off the circuit in time when an overload occurs, the rated current of the circuit breaker should be slightly larger than the total household load current.
- Generally speaking, the rated current of the circuit breaker can be selected to be 1.2 to 1.5 times the total load current. For example, if the total household load current is calculated to be 30A, then a circuit breaker with a rated current of 40A or 45A can be selected.
**II. Select the number of poles according to the type of household circuit**
1. Single-pole circuit breaker
- Applicable to control the lighting circuit or a single electrical appliance in the household circuit. The single-pole circuit breaker only controls the on and off of one live wire, and the neutral wire does not pass through the circuit breaker.
- Advantages: small size and low price. Disadvantage: no protection for the neutral wire.
2. Double-pole circuit breaker
- Used to control socket circuits in household circuits, etc. The double-pole circuit breaker controls the on and off of both the live wire and the neutral wire at the same time, and can cut off both wires simultaneously in case of a fault, improving safety.
- Advantage: high safety. Disadvantage: the price is slightly higher than that of a single-pole circuit breaker.
3. Three-pole circuit breaker
- Mainly used for the control of three-phase power supply, and is rarely used in general households.
**III. Consider short-circuit protection performance**
1. Short-circuit current breaking capacity
- The short-circuit current breaking capacity of a circuit breaker refers to the maximum current value that the circuit breaker can safely cut off when a short-circuit fault occurs. The larger this value, the better the short-circuit protection performance of the circuit breaker.
- Generally, a circuit breaker with a short-circuit current breaking capacity of 4.5kA to 6kA can be selected for general households. If the power grid quality in the area where the household is located is poor or there are large electrical appliances, a circuit breaker with a higher breaking capacity may need to be selected.
2. Instantaneous tripping characteristic
- When a short-circuit fault occurs, the circuit breaker should trip quickly to protect the circuit and electrical appliances. The instantaneous tripping characteristic is an index to measure the response speed of the circuit breaker's short-circuit protection.
- Generally speaking, the instantaneous tripping current of a circuit breaker should be between 5 and 10 times the rated current. For example, for a circuit breaker with a rated current of 40A, its instantaneous tripping current should be between 200A and 400A.
**IV. Brand and quality**
1. Select a well-known brand
- Circuit breakers of well-known brands are usually more reliable in quality, more stable in performance, and better in after-sales service. Some common well-known brands include Schneider, ABB, Siemens, Chint, Delixi, etc.
- You can understand the reputation and quality of the brand by checking the product certification marks, user reviews, professional evaluations, etc.
2. Check product quality
- When purchasing a circuit breaker, check whether the appearance of the product is intact, and whether there are defects such as scratches and deformations. At the same time, check whether the product's markings are clear, including parameters such as brand, model, rated current, and short-circuit current breaking capacity.
- You can also judge the quality of the circuit breaker by feel. A circuit breaker with good quality has a smooth operation feel and tight contact between contacts.
In conclusion, the selection of circuit breakers for household circuits requires comprehensive consideration of factors such as household electricity load, circuit type, short-circuit protection performance, brand and quality, to ensure the safety and reliability of household electricity consumption.
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The P1, P2 and S1, S2 on the transformer have the following important functions:
**I. Current transformer**
For current transformers:
1. **Determine the current direction**:
- P1 and P2 are two terminals of the primary winding and are used to determine the inflow and outflow directions of the primary current. The primary current flows in from P1 and out from P2.
- S1 and S2 are two terminals of the secondary winding. The secondary current flows out from S1 and returns to S2 through an external circuit.
2. **Correctly connect measurement and protection equipment**:
- When connecting measurement equipment such as ammeters and energy meters and relay protection devices, the terminals of the current transformer must be correctly connected. If the connection is wrong, it may lead to inaccurate measurement results or misoperation of the protection device.
- For example, reversing the connection of S1 and S2 will cause the ammeter to deflect in the opposite direction and the energy meter to reverse; in relay protection, incorrect connection may lead to failure to act or misoperation of protection.
3. **Ensure safe operation**:
- The secondary side of the current transformer must be reliably grounded during operation. Usually, the S2 terminal is grounded. This can prevent high voltage from being generated when the secondary side is open-circuited, endangering personal and equipment safety.
**II. Voltage transformer**
For voltage transformers:
1. **Determine voltage polarity**:
- P1 and P2 are two terminals of the primary winding, corresponding to the high-potential and low-potential ends of the primary voltage.
- S1 and S2 are two terminals of the secondary winding and also have definite polarity. Correct connection can ensure that the measured secondary voltage has the correct phase relationship with the primary voltage.
2. **Provide accurate voltage signals for measurement and protection**:
- The voltage transformer transforms high voltage into low voltage for use by measurement instruments such as voltmeters and power meters as well as relay protection and automatic devices.
- Correctly connecting the terminals can ensure the accuracy of measurement and the reliable operation of the protection device. For example, in three-phase three-wire and three-phase four-wire systems, the connection methods of voltage transformers are different, and the terminals must be correctly connected according to regulations to obtain correct measurement results.
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Wiring method of one-in-use and one-standby circuit is as follows:
**I. Main circuit wiring**
1. Power inlet: Connect the three-phase power supply to the inlet terminal of the main circuit breaker or fuse. Ensure a firm connection and that the wire specification meets the load requirements.
2. Connection of main equipment: Connect from the outlet terminal of the main circuit breaker or fuse to the main contacts of the contactor of the main equipment (such as motor, pump, etc.). The upper end of the contactor is connected to the power supply, and the lower end is connected to the equipment.
3. Connection of standby equipment: Similarly, connect from the outlet terminal of the main circuit breaker or fuse to the main contacts of the contactor of the standby equipment. The contactor of the standby equipment is connected in parallel with the contactor of the main equipment.
**II. Control circuit wiring**
1. Control power supply: Take one phase of power supply and the neutral wire as the power supply for the control circuit. The voltage can be reduced to a voltage suitable for the control circuit through a transformer, such as 220V or 110V.
2. Control of main equipment:
- Start button: Connect the start button (normally open contact) in series with the coil of the contactor of the main equipment. When the start button is pressed, the contactor coil is energized, the main contacts close, and the main equipment starts.
- Stop button: Connect the stop button (normally closed contact) in series with the start button and the contactor coil. When the stop button is pressed, the power supply to the contactor coil is cut off, the main contacts open, and the main equipment stops.
- Running indicator light: A running indicator light can be connected to the auxiliary contacts of the contactor of the main equipment. When the main equipment is running, the indicator light is on.
3. Control of standby equipment:
- Automatic switching control: The automatic switching function is realized through a relay or controller. When the main equipment fails or the power supply is interrupted, the relay or controller detects the fault signal and automatically switches to the standby equipment. Voltage relays, current relays or other sensors can be used to detect faults.
- Manual switching control: Set a manual switching button. When it is necessary to manually switch to the standby equipment, press the button to energize the contactor coil of the standby equipment and start the standby equipment.
- Fault indicator light: Connect a fault indicator light to the auxiliary contacts of the contactor of the standby equipment. When the main equipment fails, the indicator light is on.
**III. Protection device wiring**
1. Overload protection: Install overload protection devices such as thermal relays or motor protectors in the main circuits of the main equipment and the standby equipment respectively. When the equipment is overloaded, the protection device acts to cut off the power supply to the contactor coil and stop the equipment from running.
2. Short-circuit protection: Install short-circuit protection devices such as fuses or circuit breakers in the main circuit. When a short-circuit fault occurs, the protection device quickly cuts off the power supply to protect the equipment and circuit.
3. Phase loss protection: A phase loss protector can be used to detect the phase loss condition of the power supply. When there is a phase loss in the power supply, the protection device acts to stop the equipment from running and prevent the equipment from being damaged due to phase loss.
During the wiring process, operate strictly according to the electrical schematic diagram to ensure correct and firm wiring. At the same time, pay attention to safety. Cut off the power supply before wiring and use appropriate tools and protective equipment. If you are not familiar with electrical wiring, it is recommended to ask a professional electrician to install and debug.
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Wankong Electrical Equipment Manufacturing Co., Ltd is located in the most famous transformer production base in China - Baoding, is a manufacturing enterprise that can provide the production, sales and installation of power equipment products, the company's main sales products are 1000KV and below in line with TS/ISO/IEC/ANSI/GOS/BS standards for various applications of small high-prototype box transformer, YB European standard transformer, American standard box transformer, photovoltaic power transformer and various types of CCC/ICE/UL/TRCU/EAC-EX/ GOST/saber certified switchgear.