Lecture 6, 7
1. Dangers of operation of electroinstallations
2. Classification of methods of safe operation of electroinstallations
3. Protective actions in electroinstallations
4. Electroprotective means in electroinstallations
5. Protective actions in electroinstallations
1. Dangers of operation of electroinstallations
1.1. Dangers of operation of electroinstallations consist in an opportunity of a touch to current-carrying parts energized, in an opportunity of a touch to current-carrying parts of the electric equipment which have appeared energized as a result of damage of isolation, and to danger of inclusion on a step-by-step pressure.
1.2. We shall consider dangers of a touch to current-carrying parts, for this purpose we shall consider a network a pressure 220 V.
Single-phase touch of the person
The size of a current which proceeds through the person, at a single-phase touch is equaled:
Iч = Uф / (Rч + Rр ) Uф / Rч ,
Where Rч - resistance of the person, Rч > 1 до 2 кОм;
Rр - working resistance, Rр 10 Ом.
Biphase touch
The size of a current which proceeds through the person, at a biphase touch is equaled:
Iч = Uл / Rч ,
Where Rч = 1 кОм.
Touch to a phase and zero wire
The size of a current which proceeds through the person, at a touch to a phase and zero wire is equaled:
Iч = Uф / Rч ,
Where Rч = 1 кОм.
Single-phase touch at emergency operation
The size of a current which proceeds through the person, at a touch at emergency operation is equaled:
Iч = 2Uф / (3Rч ).
1.3. Distinguish three kinds of short circuits on the ground:
o Breakage and falling on the ground of wire ПЛ energized;
o Damage of isolation of a cable and short circuit on the ground;
o Damage of isolation of electroinstallation and short circuit on the earthed case.
Danger of short circuit on the ground in electroinstallations caused by property of a current to proceed on the closed circuit and presence of resistance in a ground on which the current proceeds.
Let's consider a case of short circuit on the ground.
The potential of a point And, that is on distance х from a point of short circuit, will be is equaled:
,
Where r- specific resistance of a ground.
The potential on a surface of the ground is distributed under the law of a hyperbole and decreases in process of removal from a point of short circuit. In connection with such distribution of potential enter concept zone on which the current of short circuit on the ground spreads.
It is such zone of a surface of the ground outside which the potentials caused by a current of short circuit on the ground can be conditionally accepted equal to zero.
If a ground homogeneous on a surface circles of equal potentials are formed. The radius of such circle for any possible currents of short circuit on the ground is not exceeded with 20 For a line 10 кВ it approximately 6-8 is equaled
In connection with such distribution of potentials are entered concept of a pressure прикосновенья and pressure of a step.
Under a pressure of a touch the person who, being in a zone растекания a current of short circuit on the ground, touches the earthed case of electroinstallation gets. The pressure of a touch is equal to a potential difference of the case and a point where there is a person.
.
Thus the pressure of touch Uпр depends:
From Iз - direct dependence;
From х - direct dependence.
Outside a zone растекания Uпр = к.
The pressure of touch Uпр can be written down as function of a pressure on the case:
Uпр = Uк1 2 ,
1-factor of a pressure of a touch which considers the form of a potential curve which value to a thicket is less than unit, and outside a zone растекания1=1;
2-factor of a pressure of a touch which considers recession of a pressure on a support of a basic surface of legs which value is less than unit.
The current which proceeds through the person caused by a pressure of a touch, is defined under the formula:
Iч = IзRз1/ Rч.
Under a step-by-step pressure the person who moves in a zone растекания a current of short circuit on the ground gets.
And = 0,7 ¸0,8 г - width of a step.
.
Thus the pressure of step Uш depends:
From Iз - direct dependence;
From r- direct dependence;
From and - direct dependence;
From х - inverse relationship.
Outside a zone where the current spreads Uш = 0.
On substations for reduction of step-by-step pressure Uш it is carried out gravel near the electric equipment and on paths. Gravel has the big specific resistance and is not humidified.
Step-by-step pressure Uш can be written down as function of a pressure on the case:
Uш = Uк1 2 ,
1-factor of a step-by-step pressure which considers the form of a potential curve;
2-factor of a step-by-step pressure which considers recession of a pressure on a support of a basic surface of legs.
The current which proceeds through the person caused by a step-by-step pressure, is defined under the formula:
Iч = IзRз1/ Rч,
Where Rч от 2 кОм до 3 кОм.
For measurement of a pressure of a step on a surface of the ground or a floor on distance of a step 2 metal plates with the sizes 250х125 mm are placed, simulating the form of a surface of legs. Plates are loaded with a cargo of 25 kg everyone, the scheme gathers. Are then used actions of care, from this territory all gets superfluous and short circuit on the case is carried out.