Basic knowledge of electrical safety and technical measures to prevent electric shock.
Insulation, safe spacing, leakage protection, safe voltage, fences and fences are all protective measures to prevent direct electric shock. Protective grounding and protective zero connection are the most basic measures in indirect electric shock protection measures. The so-called indirect electric shock protection measures refer to the technical measures to prevent all parts of the human body from contacting the metal parts of electrical appliances that are not charged under normal circumstances but charged under fault conditions.
When professional electricians work on all or part of the electrical equipment with power failure, they must first complete the work of power failure, electricity inspection, grounding wire installation, hanging signs, shielding strips, etc., before they can start working.
(1) insulation
1. The role of insulation
Insulation is to isolate charged bodies with insulating materials to realize electrical isolation between charged bodies and between charged bodies and other objects, so that the equipment can work safely and normally for a long time, and at the same time, it can prevent the human body from contacting the charged parts and avoid electric shock accidents, so insulation plays a very important role in electrical safety. Good insulation is a necessary condition for the normal operation of equipment and lines, and also an important measure to prevent electric shock accidents.
Insulation has a strong ability to isolate electricity and is widely used in many electrical appliances, electrical equipment, devices and electrical engineering, such as bakelite, plastics, rubber, mica and mineral oil.
2. Insulation damage
Thermal insulation materials will be damaged after a period of use. In addition to being damaged by breakdown under strong electric field, natural aging, electrochemical breakdown, mechanical damage, humidity, corrosion and thermal aging will also reduce its insulation performance or lead to insulation damage.
When the voltage borne by the insulator exceeds a certain value, the current discharges through the insulator, which is called electrical breakdown.
After the breakdown voltage disappears, the insulation performance of gas insulation can be restored; After many times of liquid insulation breakdown, the insulation performance will be seriously reduced; However, after solid insulation breakdown, the insulation performance can no longer be restored.
When there is voltage for a long time, due to the natural aging, electrochemical effect and thermal effect of insulating materials, its insulation performance gradually declines, and sometimes the voltage is not very high, which will also cause electrical breakdown. Therefore, insulation should be tested regularly to ensure the safety and reliability of electrical insulation.
3. Insulation safety equipment
In some cases, operators holding power tools must wear insulating gloves, insulating shoes (boots) or stand on insulating mats (tables) to work. These insulating safety appliances are used to isolate people from the ground or the metal shell of the tool, including the connected metal conductors. This is a simple and feasible safety measure at present.
In order to prevent mechanical injury, gloves are not allowed when using hand drills. Insulation safety appliances shall be subjected to pressure test and appearance inspection regularly according to relevant regulations. It is forbidden to use unqualified safety appliances, and the insulation appliances shall be kept and inspected by special personnel.
Commonly used insulation safety includes insulating gloves, insulating boots, insulating shoes, insulating pads and insulating tables. Insulation safety equipment can be divided into basic safety equipment and auxiliary safety equipment. The insulation strength of basic safety appliances can withstand the working voltage of electrical equipment for a long time, and can directly contact the live parts of electrical equipment when in use. The insulation strength of auxiliary safety appliances is not enough to withstand the working voltage of electrical equipment, which can only strengthen the safety function of basic safety appliances and must be used in conjunction with basic safety appliances. When working on low-voltage live equipment, insulating gloves, insulating shoes (boots) and insulating pads can be used as basic safety appliances, and can only be used as auxiliary safety appliances under high-voltage conditions.
Basic knowledge of electrical safety and technical measures to prevent electric shock II
(2) Screen protection
Screen protection refers to a safety technical measure to isolate the charged body from the outside world by using fences, fences, shields, shields or isolation plates to prevent the human body from contacting or approaching the charged body. Some shielding devices can not only prevent electric shock, but also prevent arc injury, arc short circuit or facilitate maintenance. If the live parts of distribution lines and electrical equipment are inconvenient to be insulated or the insulation strength is insufficient, shielding protection measures can be adopted.
Generally speaking, the movable parts of switchgear cannot be insulated, but they need to be shielded. Among them, the protective switch cabinet itself has screen protection devices, such as the rubber of rubber knife switch and the iron shell of iron shell switch; Opening the slate knife switch requires an additional screen saver. Additional screen protection devices are also needed for crane sliding contact lines and other exposed wires. For high-voltage equipment, it is often difficult to completely insulate it, and serious electric shock accidents will occur when people get close to a certain extent. Therefore, whether the high-voltage equipment is insulated or not, shielding protection or other measures to prevent access should be taken.
For power transformation and distribution equipment, all transformers installed on the outdoor ground and power transformation and distribution devices installed in workshops or public places need to be provided with fences or fences as shielding. When working near electrified bodies, movable temporary barriers should be installed between workers and electrified bodies, corridors, people and other places.
The screen protector is not in direct contact with the charged body, so there is no strict requirement on the electrical properties of the materials used. Materials used for screen protection devices should have sufficient mechanical strength and good fire resistance. However, the shielding device made of metal material must be grounded or connected to zero to prevent electric shock caused by accidental electrification.
The types of screen protectors include permanent screen protectors, such as shielding strips of power distribution devices and covers of switches; Temporary screen protectors, such as those used in maintenance work and those of temporary equipment; Fixed screen protection devices, such as bus bars; Mobile screen protector, such as the screen protector of bridge crane.
When using the screen protector, you should also pay attention to the following:
(1) The screen protector shall keep a sufficient safe distance from the charged body.
(2) The shielded live parts shall be clearly marked with specified symbols or painted with specified colors.
Fences, fences and other screen protection devices should have obvious signs, such as hanging according to the object to be protected. Stop, high pressure danger! ? 、? No climbing, high pressure danger! ? If it is marked, it should be locked if necessary. Signs can only be arranged and removed by personnel in charge of safety.
(3) The doors blocking the crowd should be locked as required, or signal devices and interlocking devices should be adopted. The former usually uses lights or meters to indicate that there is electricity; The latter is a special device. When the human body exceeds the shielding device and may approach the charged body, the shielded charged body will automatically cut off the power supply.
Technical measures for preventing electric shock based on electrician's basic safety knowledge III
(3) Leakage protector
Leakage protector is a device that can automatically disconnect the circuit or give an alarm when the leakage (contact) current (mA) value in the circuit reaches or exceeds its specified value under specified conditions.
Leakage means that when the electrical insulation is damaged or the conductive part collides with the shell for other reasons, if the metal shell of the electrical appliance is grounded, then electricity will form a path through the earth, thus forming a current, that is, leakage current, also known as grounding current. When the leakage current exceeds the allowable value, the leakage protector can automatically cut off the power supply or give an alarm to ensure personal safety.
The leakage protector is sensitive in action, and it takes a short time to cut off the power supply. Therefore, as long as it can be reasonably selected, correctly installed and used, it can not only protect personal safety, but also prevent electrical equipment from being damaged and prevent fires.
Equipment and places where leakage protectors must be installed:
(1) Portable electrical equipment and hand-held power tools belonging to the first category;
(2) Electrical equipment installed in harsh environments such as humidity and strong corrosiveness;
(3) Electric construction machinery and equipment on the construction site, such as pile drivers and mixers;
(4) Electrical equipment for temporary electricity use;
(5) The guest rooms of hotels, restaurants and guest houses and the socket loops in buildings such as institutions, schools, enterprises and houses;
(6) Underwater lighting equipment for swimming pools, fountains and baths;
(7) Power supply lines and equipment installed in water;
(8) Electrical and medical equipment in direct contact with human body;
(9) Other places where leakage protectors need to be installed.
The installation and inspection of leakage protector should be carried out by professional electricians. Electricians should be trained and assessed on the knowledge of leakage protectors. The contents include principle, structure, performance, installation and use requirements, inspection and testing methods, safety management, etc.
(4) safe voltage
Limit the voltage that may be applied to the human body to a certain range, so that the current passing through the human body does not exceed the allowable range under this voltage. This voltage is called safe voltage, also called safe ultra-low voltage. However, it should be noted that under no circumstances can safe voltage be understood as an absolutely non-dangerous voltage. Equipment with safe voltage belongs to Class III equipment.
Safe voltage standards determined by China are 42V, 36V, 24V, 12V and 6V. Hand-held electric tools used in particularly dangerous environment should adopt 42V safe voltage; In the environment with electric shock danger, 36V or 24V safe voltage should be used for hand-held lighting and local lighting; Hand-held lighting lamps used in particularly dangerous environments such as metal containers and particularly humid places should adopt 12V safe voltage; When working in underwater operation and other places, 6V safe voltage should be used.