Protection against electric shock and electrical fires

1. Electric shock casualties

Definition of electric shock injury: "When the human body touches conductive parts of different potentials at the same time, the potential difference causes current to flow through the human body, which is called electrical contact."

Electric shock (eIectric shock) is the current flowing through the human or animal body, causing it to produce pathophysiological effects.

When the contact current is small, it is harmless to the human body, and it is used to diagnose and cure certain medical electrical equipment. This electrical contact is called micro-electric contact.

When the current passing through the human body is large and the duration is too long, people can be injured or even killed. This electrical contact is called electric shock. The electric shock mentioned here mainly refers to the electrical contact of alternating current with voltage not greater than 1000V and frequency not greater than 100Hz.

In addition, there is still destructive damage caused by high voltage, which refers to the damage caused by thermal, chemical and mechanical effects when a large current above the ampere level flows through the human body when lightning strikes and high-voltage electric shocks occur.

DC current will also flow through the human body. Tests have shown that the danger of DC current to people is much smaller than that of AC current, which is only about 25% of the 50 Hz AC current. This is because when the DC current passes through the organic tissues of the human body, it only causes electrolysis, which weakens the effect of the current due to polarization.

For low-voltage (less than 1000V) 50Hz AC current, the human body has three main effect thresholds:

1) Sensory threshold: 0.5mA

2) Get rid of the threshold: 10mA, mainly refers to when a person uses a hand-held live conductor, if the current flowing through the palm exceeds this value, the palm muscle reaction will be to hold the live conductor unintentionally instead of getting rid of the live conductor; Conductors, the human body will suffer injury or even death under the action of a large current for a long time.

When other parts of the human body come into contact with the live conductor, they can get rid of the live conductor instantly, and there is no danger of death from electric shock. Therefore, "handheld devices (such as hand drills) or mobile devices (such as floor lamps) have a greater risk of death from electric shock than fixed devices." The power must be turned off within the corresponding time, which is also required to use the handheld The reason for the transient RCD installed on the socket circuit of portable and mobile equipment.

3) Threshold value of ventricular fibrillation: 30mA, ventricular fibrillation caused by current passing through the human body is the main cause of electric shock.

According to the IEC60479 standard, "the curve of the relationship between the 15 ~ 100Hz AC current Ib and the energizing time t that causes ventricular fibrillation" is obtained. "From the curve L, as long as Ib is less than 30mA, the human body will not be killed by electric shock due to ventricular fibrillation. In accordance with this international standard, the rated operating current value of a high-sensitivity residual current action protector (hereinafter referred to as RCD) that protects against electric shock is 30mA. "

According to Ohm's law, "The current Ib is generated by the contact voltage applied to the body impedance Zt. The greater the contact voltage, the greater the Ib. It is difficult to calculate Ib when designing electrical devices, and it is more convenient to calculate the contact voltage. Put forward the corresponding expected contact voltage Ut-time curve under dry and humid environment conditions ". The test proves that the body impedance Zt is related to the person's age, sex, weight, and dryness and wetness of the skin, and decreases with the increase of the contact voltage, and the average value is calculated for the convenience of calculating Zt. It follows from this: "Under dry conditions when Ut is not greater than 50V, the human body will not experience ventricular fibrillation when exposed to this voltage;

IEC sets the rated voltage of ultra-low voltage equipment in dry environment to 48V (36V is still used in China). Under humid environment conditions, Ut greater than 25V can cause contact current Ib of more than 30mA that causes ventricular fibrillation. According to IEC, the UL value under humid environment conditions is set to 25V, and the rated voltage of ultra-low voltage equipment is specified Is 24V.

Under the conditions of underwater or particularly humid environment, such as in the bathroom or swimming pool, the rated voltage of the ultra-low voltage equipment is only 12V or 6V due to the soaked skin. In recent years, there have been many accidents of electric shock injuries in fountains or surfing bathtubs. In addition to poor equipotential bonding measures, failure to select extra-low voltage equipment as required is the main reason for the accident.

2. Direct contact and indirect contact electric shock

There are two types of personal electric shock: direct electric shock and indirect electric shock.

Direct contact refers to an electric shock caused by direct contact between a person or animal and a live part, such as an electric shock caused by a person who did not cut off the power supply when repairing an electrical circuit, when a person touched a damaged socket, or a lamp head.

Indirect contact (indirect contact) refers to the exposed conductive parts of electrical equipment with fault voltage due to damaged insulation, when people or animals touch the exposed conductive parts and suffer electric shock. For example, when a person moves due to insulation damage, the phase wire core hits the electric shock when it hits the floor lamp of the metal pillar.

It should be noted that different definitions of "live part" and "exposed conductive part".

The "live part" refers to the phase line and neutral line that carry current when the electrical equipment is working normally.

The exposed conductive part is the conductive part that may be touched in daily use of electrical equipment. It does not carry voltage under normal circumstances, but may carry voltage when a ground fault occurs due to damage to the basic insulation, such as the metal casing of electrical appliances, steel pipes for laying wiring, metal ladder frames, tray boxes, etc.

The voltage of exposed conductive parts is often called "leakage". In the actual electric shock accident, because the electrical equipment is structurally free of direct contact with live parts, direct electric shock accidents are rare; on the contrary, because people may contact exposed conductive parts at any time, the possibility of indirect contact with electric shock accident It's much bigger. In addition, the stepping voltage electric shock accident caused by electric leakage is an indirect contact electric shock accident.

3. Electrical fire

A fire must have three conditions: ignition source, combustibles, and oxygen. If the electrical installation is not properly designed and installed, a fire source is often formed in the building due to electricity. Electric ignition sources usually appear in the form of abnormally high temperatures and arcs (electric sparks), and their occurrence is complicated and diverse. Generally, they can be summarized as three types of causes: short circuit, poor connection, and improper installation of electrical devices. Three elements of combustion: fire source, combustibles and combustion aids, the three are indispensable.
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Short circuit fire is the main form of electrical fire. Short circuit, also known as earth fault, refers to "caused by the conduction of the loop live conductor to ground (including the ground, the exposed conductive part connected to the ground and the external conductive part of the device), or the insulation to the ground becomes less than the specified value failure.

3.1 There are two main reasons for short circuit:

a mechanical damage, the core of the wire exposed to different potential conductors and short-circuited;

b. The insulation level of electrical circuits is reduced due to overheating, water immersion, mildew, sunlight radiation, etc. Under external electrical triggers, such as the impact of lightning transient overvoltage or grid transient overvoltage, the withstand voltage is too low The insulation is broken and short-circuited. The heat source that causes the line to "overheat" may be an external heat source such as a heating pipe that is too close to the line, a high-temperature stove, or an internal heat source that has an excessively high temperature rise due to electrical circuit overload (overload).

3.2 Short-circuit ignition There are two types of metallic short-circuit ignition and arc short-circuit ignition:

a Metal short-circuit fire: When two conductors of different potentials are in contact during short-circuit, the large short-circuit current "produces high temperature through the contact resistance and melts the metal at the contact point" It is also possible to melt and weld the two contacts firmly. At this time, its impedance is very small, so the short-circuit current can reach several hundred times to several thousand times the rated ampacity of the electrical circuit! At this time, the short-circuit protective device on the circuit should act quickly, but "if the short-circuit protective device fails to operate (for example, the fuse is mistakenly replaced by copper or iron wire, the circuit breaker fails to operate), the short-circuit state will continue. At 355oC, the hydrogen chloride decomposed by PVC insulation will burn due to violent oxidation. At this time, the core along the full length of the line will burn red, and the PVC insulation will naturally form a "fire dragon", which poses a great risk of fire.

b. Arc short circuit fire: if the two electrodes are contacted and then opened to establish an arc, only 20V is required to maintain this 10mm long arc. That is to say, as long as it is contacted first and then separated again, it is likely to produce an arc with a high local temperature and become a fire source. According to the different parts of the arc, it can be divided into the arc between the live conductor, the arc between the live conductor and the ground, and the creepage of the insulating surface.