Building Electrical Engineering:
Strong Electricity:
High voltage, high current, so it is customary to call it strong electricity.
The parts involving strong electricity mainly include:
Power substation and distribution: Generally, small and medium-sized enterprises and high-rise buildings mostly use 10KV power supply.
动力：380伏；照明：220伏。
Safety and protection: lightning protection, grounding, equipotential connection, leakage protection, electromagnetic compatibility.
Weak:
Relative to strong electricity, the part based on electronic technology.
The parts involving weak current mainly include:
Communications & Internet, Radio & Audio, Cable TV.
Fire monitoring: fire detectors, fire alarm controllers, fire linkage, etc.;
Security and prevention: access control system, visitor intercom, anti-theft prevention system, electronic patrol system, parking lot management, CCTV surveillance.
Microelectronics (Building Intelligence):
Building intelligence is based on microcomputer, network and communication, automatic control, and integrated circuits, and these four technologies are based on highly integrated and miniaturized intelligent chips, and the processing of weak digital machine analog information is the main content.
The sections dealing with microelectronics include:
office automation system OAS;
Automatic operation management system BAS: fire monitoring system FAS, security automatic system SAS, broadcast automatic system PAS, parking lot supervision system TAS, etc
Communication automation system CAS, integrated wiring PDS.
2. Building power supply and distribution system and electrical equipment
Power supply:
The main power supply types are:
1. Power grid power supply lines: generally small and medium-sized enterprises and high-rise buildings mostly use 10KV power supply.
2. Diesel generator
3. Battery
4.EPS power supply
5.UPS power supply
Power Distribution System:
Non-fire power system: power, lighting;
Fire-fighting power system: main equipment and facilities: distribution cabinets, distribution lines.
Electrical Equipment:
Power equipment: projection equipment;
Lighting equipment: lamps and lanterns;
Fire-fighting electrical equipment: water pumps, etc.
3. Electrical and electrician foundation
Rated voltage, rated current, rated power.
When the electrical equipment is designed and manufactured, the voltage value to ensure safety according to the regulations shall be recorded as UN.
A current value specified in the design and manufacture of electrical equipment to ensure safety for long-term use, denoted as IN, is usually marked on the nameplate of electrical equipment.
The rated current, rated voltage UN, and rated power PN are called the rated value of electrical equipment, and the relationship between the three is:
PN= IN* A
For example: the rated current is 10A, and the rated voltage is 220V, then the rated power can be calculated: PN=10*220=2200W.
Thermal Effect of Current:
The phenomenon of heating up the conductor caused by the passage of electric current through the conductor is also the phenomenon of converting electrical energy into heat energy.
According to the Joule-Lenz law, the heat generated by the current through the conductor Q = I2Rt, it can be seen that the heat generated is proportional to the square of the current.
The thermal effect of electric current is the most basic factor in the occurrence of electrical fire accidents.
Over-current protection, low-voltage protection:
In the electrical system, when there is an overcurrent, the circuit can be automatically cut off to prevent damage to the line or equipment. Includes short-circuit protection and overload protection.
Commonly used overcurrent protection: fuse, low-voltage circuit breaker, thermal relay, thermal breaker, overcurrent protection in high-voltage system is composed of current relay and circuit breaker.
Low-voltage protection is also known as loss-of-voltage protection or under-voltage protection. When the power supply voltage disappears or falls below a certain value, it is a protective measure that can automatically disconnect the circuit, and its function is to avoid the loss caused by the sudden start of the equipment when the voltage is restored, and at the same time to avoid damage due to operation at low voltage.
Electrical Fire Prevention and Control:
The direct causes of electrical fires include short circuit, overload, poor contact, leakage, lightning, static electricity, etc. The deep-seated cause is also caused by unsafe human behavior and natural factors. For example, people's thoughts are paralyzed, they do not comply with relevant fire regulations, violate operating procedures, mismanage and the invasion of natural factors such as wind, rain and thunder.
Why can electrical wiring and equipment cause fire accidents?
A potential source of ignition?
Heating phenomenon: the current flows through the conductor, due to the action of the conductor resistance, Joule heating will be generated, and this part of the energy loss becomes copper loss.
1. Fire prevention and control of electrical circuits
Definition and type of short circuit: commonly known as touching the line or connecting electricity, refers to the phenomenon of short connection between the phase wire and the phase wire, the phase wire and the neutral line in the electrical circuit.
Definition and types of short circuits:
The three-phase impedance of the three-phase short-circuit circuit is equal, so the three-phase short-circuit current and voltage are still symmetrical, but the current is higher than the normal value and the voltage is lower than the rated value. Three-phase short circuit has the lowest probability of occurring, only about 5%, but it is the most harmful form of short circuit.
Heating during short circuit of conductors:
The short-circuit current is dozens of times larger than the normal current during normal operation, and sometimes it can reach hundreds of times.
Causes of short circuit:
Insulation damage of electrical wiring;
Operators violate operating procedures or make mistakes, and poor maintenance and management.
Measures to prevent short circuits in electrical circuits:
Design, installation, commissioning, use and maintenance of electrical circuits in strict accordance with the provisions of the "Electrical Design Regulations".
To prevent the aging of electrical line insulation, in addition to considering the influence of environmental (ambient temperature, humidity and chemical corrosion) conditions, the insulation of the line should be checked regularly.
Install circuit breakers or fuses according to regulations, and install valve-type lightning arresters for line protection.
Strengthen the safety management of electrical lines to prevent man-made operation accidents and unauthorized unauthorized connection of lines.
Overload: Overload, also known as overload, refers to the phenomenon of exceeding the allowable load of electrical lines and equipment.
Cause of overload: The safe current carrying capacity is determined by the test method, and the temperature rise inside the conductor when the current flows through the conductor does not exceed the limit allowable temperature. The main reason for the overload of the line is that the cross-section of the wire is too small, and the actual load far exceeds the safe ampacity of the wire.
Main measures to prevent overload:
Reasonable selection of wire cross-section, the use of the unit should be responsible for the installation of the line by the power management personnel, and strict system, not allowed to pull the wire and increase the load at will. Regularly measure and check the overload of the line, and install appropriate fuses and overload protection devices, so that when the overload in the line is serious, the power supply can be cut off in time. With the increase of line load, the wire section suitable for the corresponding capacity should be replaced in time, or the electricity load should be reasonably adjusted according to the production procedures and requirements, so as to avoid the peak of electricity consumption or stagger the time to achieve the purpose of preventing overload. In order to avoid overload on the line, an automatic switch can also be added on the line for load protection.
Leakage: The electrical circuit is insulated from the ground under normal operation. The current flows through the conductor surrounded by the insulator and does not cause harm. However, when the outer insulation layer of the line is damaged due to physical and chemical actions such as friction, extrusion, cutting, thermal aging, humidity, pollution, corrosion, etc., and loses or partially loses the insulation performance, if it encounters rain or humid air, etc., at the insulation breakage, it will form a conductive path between moisture, impurities, etc. and the earth - bypass conductor. In this way, a part of the current in the conductor bypass will enter the ground through the conductive pathway, and then form a loop with the power supply, a phenomenon called leakage. This insulation break is called the leakage point, and the current entering the large area is called the grounding point.
In the actual leakage fault, there is only one leakage point, but there are many grounding points. Leakage of electrical lines can cause electric shock to personnel due to local electrification of objects, and in serious cases, leakage sparks or high-temperature surfaces ignite gas gas, thus causing leakage fires.
Measures to prevent leakage:
When designing and installing electrical circuits, the insulation strength of the conductor should not be lower than the rated voltage of the line, and the insulator supporting the conductor should also be selected according to the line and power supply voltage.
In particularly humid or acid-alkali corrosive gas places, it is strictly forbidden to insulate the wires openly, and polyvinyl chloride casing or (water gas) steel pipe wiring should be used.
When installing the circuit, the wire joint should be wrapped firmly, and at the same time, it should be prevented from scratching the wire insulation layer by the knife and pliers.
At ordinary times, inspection and maintenance should be strengthened, and repairs and replacements should be made in time if the wire insulation is damaged.
Poor contact (excessive release resistance):
At the connection between the power line and the electrical equipment or wire, the power line should be electrically connected with the switch, the protection device and the larger electrical equipment.
Due to the absorption current between the two (because the contact area between the two is reduced), the metal surface film (including dust film, oxide film, inorganic film, organic film, etc.) and electrochemical corrosion and other phenomena make the local resistance of the contact part too large. Of course, poor contact is also one of the important reasons. If the line current is very large, when the contact resistance is too large, it will generate great heat, which can discolor or even melt the metal, destroy the insulation layer, and cause a fire in nearby combustibles.
The main reasons for poor line contact:
Poor installation quality causes the connection between the wire and the wire or the connection point between the wire and the electrical equipment is not firm.
Oxidation occurs at the junction due to thermal action.
Long-term vibration contact loosening. Copper and aluminum conductors are mixed.
The wire connection is stained with impurities, oil, mud, etc.
Precautionary measures:
The connection between the conductor and the conductor and the electrical equipment must be strong and reliable.
Frequently inspect the running lines, and deal with the loose joints or heating in time.
The connections between wires with a large cross-sectional area should be welded or crimped.
When copper and aluminum wires are connected, copper-aluminum transition joints should be used. It is also possible to pad tin foil between copper and aluminum wires or tin on the nose of copper wires. In order to check the temperature rise of the contact head, it is also possible to apply color-changing paint or candles to the parts with high contact resistance to monitor the heat generation of the contact points.
Conduct regular electrical safety inspections, find problems, and repair and replace them in time.
Arc and spark:
Arcing and sparking are electrical discharges caused by the breakdown of gases. For example, transformers or electrical equipment are switched off the grid when they are put on or off the grid. When the two electrical contacts are separated, as long as the power supply voltage exceeds 12~20 volts and the interrupted current exceeds 0.25~1 amperes, a mass of extremely high temperature, emitting strong light, and conductive gas similar to a cylindrical shape will be generated between the contacts, which is called an arc.
Due to the extremely high temperature of the arc and electric spark, the temperature of the arc column can reach more than six or seven thousand degrees, which can completely melt and vaporize the metal conductor, and sputter molten beads and sparks around, which has a large ignition and detonation ability, so it is a terrible ignition source.
The main causes of arcing and sparks:
High-voltage breakdown, short circuit of the wire, damage to the outer insulation layer of the insulated wire, and arcing phenomenon of the interrupting induction circuit.
A high current blows the fuse melt.
Caused by loose wire connections, etc.
Measures to prevent arcing and sparks
A sufficient safety distance should be maintained between the bare conductors or between the wires and the grounding body, and the insulating support of the wires should be kept intact to prevent the wires from being laid too loosely and the vertical arc from being too large.
There is no damage to the insulation layer of the insulated wire.
The fuse or switch should be installed on the basis of non-combustible materials and protected by non-combustible materials.
Installation and repair of electrical equipment without electricity.
Install lightning protection devices and line overvoltage protection devices for protection.
Fire Management of Electrical Equipment:
There are many commonly used electrical equipment, which can be summarized into five types: power equipment, electric lighting equipment, electric heating equipment, control equipment, and electrical equipment in explosion hazard places. These electrical equipment have thermal effects in the working process, and sometimes they will produce electric sparks, arcs, etc., if they are not installed and used properly, they may cause fires, so we must pay attention to fire safety.
Fire protection measures for lighting fixtures:
1. Different types of lamps and lanterns should be selected according to the use place and environmental requirements of the lamps.
2. Incandescent lamps and iodine-tungsten lamps should be kept at an appropriate distance from combustibles.
3. For the wires used near tungsten halogen lamps, anti-baking and ignition measures should be taken.
4. Attention should be paid to ventilation and heat dissipation during the installation of the ballast, and the voltage and capacity of the ballast and the lamp must be the same and matched.
Fire Hazard and Precautions for Electric Heating Equipment:
1. Fire hazard of electric heating equipment
(1) The temperature control system is out of order or damaged.
(2) The increase in the ambient temperature of wires and cables is easy to cause the aging of the insulation layer and the occurrence of breakdown and short circuit accidents.
(3) The contact point between the wire and the heating wire of the electric heating equipment is burned off, which has a certain fire hazard.
(4) Improper use and poor management are also one of the important causes of electric heater fires.
2. Preventive control measures for electric heating equipment
(1) Small electric heating equipment should be set up separately in the room of non-combustible materials, and good ventilation, exhaust and explosion-proof pressure relief measures should be taken.
(2) The power of the electric heating equipment is relatively large, and the line should be prevented from being overloaded, and it is best to use separate wiring for power supply, select fire-resistant or heat-resistant cable wiring, and install fuse protection.
(3) Small electric heating equipment and electric heating appliances, such as electric ovens, electric irons, electric ferrochrome, etc., can not leave when they are used with electricity, and they should develop a good habit of turning off electricity.
(4) In the process of use, the electric heating equipment should be equipped with an appropriate amount of fire extinguishing equipment, and special personnel should be assigned to manage it, and the corresponding safety operation procedures should be formulated and strictly observed.
Fire protection of electric welding equipment:
1. Fire hazard of electric welding equipment and electric welding process
The arc temperature in electric welding can reach more than 6000°C, which can quickly melt the metal and produce a large number of sparks, so it is easy to ignite the surrounding combustibles. The temperature of the weldment after welding is very high, and there is a certain fire hazard. When the interior decoration of the construction site or the store, the fire danger is greater, the splashed sparks can immediately catch fire and burn when they fall on the combustibles, and they can smolder and ignite when they fall into the sack or cotton bale, so the electric welding process has a greater fire hazard.
2. Safety and fire prevention measures for electric welding
(1) When welding work is carried out in a place where combustibles are present, the fire approval system shall be strictly observed.
(2) In places with fire and explosion hazards, as well as during the business period of public entertainment venues, welding operations are generally not allowed, and the equipment that needs to be overhauled should be disassembled and moved to a safe place for repair.
(3) When welding operations are carried out in important parts of key projects, they should be guarded by special personnel and fire extinguishing equipment should be prepared.
(4) The welding equipment should be kept in good condition.
(5) Arc welding and welding should be carried out in a special building. It is strictly forbidden to use the metal components, pipes, tracks or other metal objects of the plant as conductors.
Air Conditioner Fire Protection:
(1) The power load must be considered when purchasing air conditioning equipment.
(2) The air conditioner used for heating must be equipped with a temperature control device.
(3) The air conditioner must be supported by non-combustible materials, and should not be placed directly on the wooden floor or carpet, but should be supported with non-combustible materials. Do not put a flammable shield on the air conditioner, close to the flammable curtain. Do not put used oil cotton silk, oil rags, etc. on the air conditioner to avoid fire.
(4) The air conditioner should be inspected and maintained frequently. It is necessary to ensure that the electrical components are well insulated to prevent short circuit and fire.
(5) The air conditioner shall be operated under the close supervision of the owner. When the owner leaves, the power should be turned off.
Electrical Fire Management:
A fire safety management system for electricity should be established in densely populated places, and the following contents should be clarified:
Clarify the responsible departments and responsible persons for the safety management of electricity and fire safety;
procurement requirements for electrical equipment;
requirements for the safe use of electrical equipment;
inspection content and requirements of electrical equipment;
Job qualifications and responsibilities of operators of electrical equipment.
The fire safety management of electricity shall meet the following requirements:
Qualified products should be selected for the purchase of electrical and electric heating equipment, and should meet the requirements of relevant safety standards;
Electrical wiring and electrical equipment installation and maintenance should be operated by electricians with professional qualifications;
It is not allowed to connect wires at will, and increase electrical equipment without authorization;
The distance between the area around the electrical equipment and combustible materials should be more than 0.5m;
Electrical circuits and equipment should be regularly inspected and tested, and it is strictly forbidden to overload for a long time;
When shops, catering establishments, and public entertainment venues close for business, the power supply of the business premises shall be cut off.
Check the operation status of distribution boxes, power supply lines, lighting fixtures, posters (advertising) light boxes, projection equipment and electrical equipment in the sales area every day, and fill in the operation records, file for future reference, and deal with abnormalities in time.
Inspect the distribution room or distribution room once a month for frogs, rats, ants and other insect pests, and take measures to eliminate them immediately if found.
The residual current action protector needs to press the button once a month when the power supply is turned on, and the number of tests should be appropriately increased in the wet season of thunderstorms, and the test and operation records should be made.
The low-voltage power distribution device, electrical circuits, socket switches, lighting devices and electrical equipment are tested twice a year, the grounding resistance is tested once a year, and the insulation resistance is tested once a year.