Basic concepts and definitions

Laboratory work 8

Electrical safety in the different types
of electrical networks

Objective: explore the dependence of the current flowing through the person touching the wire three-phase mains voltage up to 1 kV, from the power supply neutral system (transformer), wire insulation parameters of the network and resistance in the human body circuit.

Allowable values of current flowing through the person

τ, s	0.01-0.08	0.1	0.2	0.3	0.5	0.7	1.0	More than 1.0
U c, V	650	500	250	165	100	70	50	36
Ih, mА	650	500	250	165	100	70	50	6

 

These values I_h, U _c provide an opportunity to allow the assessment of the risk of injury and to develop means of protection (earth and neutral, protective earth, safety shutdown, etc.).

Hazard analysis of electrical networks is to determine the current flowing through the human body:

I_h =U _1,2 /R_h,                                                                             (8.3)

 

where U_1,2 – voltage between points 1 and 2, which touches people directly or through some intermediate elements; R_h – resistance in the electrical circuit of the human body.

When the two-phase touch U_1,2 = U_l  and

                                         I_h =U_l /R_h,                                                                (8.4)

 

In this case, the current is determined by line voltage and the resistance R_h and practically independent of the neutral network mode. A sufficient increasing R_h (for example, when using dielectric gloves) provides an opportunity to avoid injury. Otherwise R_h =1000 Ohms and U_l = 380 I_h reaches 380 mA, that when τ> 0.1 is very dangerous.

When you touch a single-phase, if a person is shod and on any basis:

R_h = R _h+ R _sh +R _b ,                                   (8.5)

 

where R_h, R_{s h} and R_b – the human body resistance, the shoe and the base, respectively.

Description of the laboratory bench

 

Research carried out on the simulator laboratory bench «Electrical safety of three-phase networks».

On the front panel of the bench there is a mnemonic diagram of the network and its elements, the human figure and other objects, and also has the indicator and switching devices, with which you can set the necessary types of three-phase networks and modes of their work.

Resistance R _АЕ, R_BE, R_CE, R_PEN, capacity С_АЕ, С _BE, С _CE, C_PEN   simulate distributed insulation resistance and capacitance conductors real network relative to earth. R ₀ element depicts the resistance neutral earthing transformer windings. S₁₃ switch allows discrete set these values R_h. Emergency mode, caused by the closure of the supply voltage for the land is realized to the bench by connection R _3M resistance to the phase conductor.

On the right side panel there is a digital display of the ammeter which can measure the current I_h toggle handle installed into the «A1» position and the upper limit of the measuring range set placed next to the switch, depending on the current value. Below is a voltmeter display panel designed for measuring the voltage between ground and the phases A, B and C at the corresponding position of the switch handle.

LED’s glow in the phase conductors indicates the presence of voltage and the LED’s glow in the image of man – the danger (for I_h > 10 mA).

Security measures

 

1. Connect the bench power supply only with the permission of the teacher.

2. After the end work the laboratory bench must be turned off by toggle switch S₂.

3. If there is a problem turn off the bench from the electricity power network and inform the technician or teacher.

 

Order of work realization

 

Carry out a comparative study of three-phase networks with voltage up to 1 kV, insulated and earthed neutral from the point of view of the danger of electric shock to persons.

Laboratory work 8

Electrical safety in the different types
of electrical networks

Objective: explore the dependence of the current flowing through the person touching the wire three-phase mains voltage up to 1 kV, from the power supply neutral system (transformer), wire insulation parameters of the network and resistance in the human body circuit.

Basic concepts and definitions

 

Electricity is used in all industries of the national economy and in everyday life. Practice shows that in all areas of use of electrical energy accidents happen where the victim is struck by electric shock. Electric shocks make up a small percentage of the total number of work-related injuries, but it occupy one of the top places on the number of injuries with severe and fatal consequences. The greatest number of electric traumas (60 … 70%) occurs during the work of electro installations up to 1000 V.

The actions of electric current on the human body are diverse. Passing through the body, the electric current causes thermal, electrolytic, and biological action.

The thermal effect of the current burns is shown in some of the individual parts of the body, heating of the blood vessels, nerves, blood, etc. The electrolytic action of the current is shown in the expansion of the blood and other organic body fluids, causes significant violations of their physical and chemical composition. The biological effect of the current manifests itself as anger and excitement of living tissues of the body, as well as a violation of the internal biological processes. The mechanical action results in the current bundle break the tissues as a result of electrodynamic effect, as well as the explosive instantaneous formation of steam from the tissue fluid and blood. This variety of electrical current actions can lead to two types of lesions – electrical injury or electric shock.

Electrical injuries are distinct when local damage to tissues caused by exposure to an electric current or an electric arc where these following electrical injuries occur: electrical burns, electric signs, metallization, ophthalmia, mechanical damage.

Electrical shock is the excitement of living tissues of the body passing through electric shock, accompanied by involuntary spasmodic contractions of the muscles. Depending on the outcome of the current impact on the body electric shocks are divided into the following four degrees: I – twitching muscles without loss of consciousness, II – muscular twitching, unconsciousness, but the preservation of breathing and heartbeat, III – loss of consciousness and cardiac abnormalities or breathing (or both together), IV – clinical death, i.e., the absence of respiration and circulation.

The main causes of electric shock are:

– Violation of rules of technical operation of electrical installations;

– Contact with current carrying parts;

– Touching the metal not current carrying parts, become live due to faulty insulation or grounding devices.

If a person comes under stress, then through his body electric current flows. The effects of the electric current through humans depends on many factors: the current type (AC or DC), alternating current at - on its frequency; the magnitude of the current (or voltage); duration of current flow; from the current path through the human body; physical and mental state of a person.

The most dangerous for the human being is an alternating current with a frequency of 50... 500 Hz. The ability of self-liberation from the power of this frequency, most people only saved when its value is very small (up to 10 mA). The magnitude of current flowing through an affected person voltage depends on the voltage setting circuit and the resistance of all the elements on which the current flows.

It is exposed to the greatest danger of people, when current passes on vitals (heart, lungs) or cells of the central nervous system. However death is possible even at a small tension (12 … 36 V) as a result of contact of current carrying parts with the most vulnerable parts of a body – the palm back, a cheek, a neck, a shin, a brachium.

It is established that at the time of electric shock is very important physical and mental condition of the person. If a person is hungry, tired, drunk or ill, his body's resistance is reduced, i.e., the probability of severe injury increases. At observance of safety rules, i.e., with attentive and careful work, the possibility of electric shock is reduced. The extent of the current human exposure is given in Table. 8.1.

State of the environment (temperature, humidity, dust, acid vapors) affects the resistance of the human body and the insulation resistance, which ultimately determines the nature and consequences of electrical shock. From the standpoint of environmental production facilities may be dry, wet, damp, especially raw, hot, dusty with conductive and nonconductive dust, or with a reactive organic medium. All but dry, the body resistance is reduced.

 

Table 8.1