3. Key questions
Explain influence of an electric current on an organism of the person.
Name threshold values of electric current.
What can protect the person at a bipolar touch to current-carrying wires?
What can protect the person at a unipolar touch to current-carrying wires in a three-phase network with an isolated midpoint conductor?
What can protect the person at a unipolar touch to current-carrying wires in a three-phase network with an earthed midpoint conductor?
4. The method of solving problems about danger of electric networks
4.1. The bipolar touch to current carrying wires
The bipolar touch to two various wires (a phase in a three-phase network) is the most dangerous, as the current proceeds through a body of the person on the most dangerous way " a hand - a hand " and nothing protects the person (fig. 1).
The current through a body of the person is equal
The value of current exceeds threshold value. Such touch is fatally dangerous to the person (the current is equal 380/1000 - 380 mА).
4.2. The unipolar touch to doublewire network, isolated from the earth
Concerning a wire 1 in a two-wire network, the person affects switched on in parallel resistance of isolation r1 this wire and is consecutive with resistance of isolation г2 of the wire 2.
The value of current which can flow through his body as it is clear from fig. 26 is determined basically by resistance of isolation г2.
At equality of resistance of isolation гиз=г1=г2 the formula for force of the current proceeding through the person, looks like
The value of current is determined by resistance of isolation of a network. The person is a floor protection of resistance of isolation.
The value of resistance of isolation of wires of a network concerning the ground should be not less than 0,5 МОм.
In this situation of the person protects also resistance of footwear which can be depending on its status from 0 up to 100 and more whom, and resistance of a floor (800 Ohm an earthen dry floor and up to 10 МОм a dry wooden floor).
4.3. The unipolar touch in a three-phase network with an isolated midpoint wire
In a network with an isolated neutral usually for simplicity of the analysis phase voltage U1, U2 believe, that, U3 resistance of isolation of wires concerning the ground г1, г2 are equal and symmetric, and, г3 are identical and equal гиз as shown in figure 3.
Owing to full symmetry the potential of an average point concerning the ground is equal to zero (fig. 3á, 3в). Identical voltage of phases are enclosed to resistance of isolation.
The touch of the person to one of phases (fig. 3б) creates a circuit of a current through a body of the person and isolation of other wires.
The current through a body of the person will be equal
As resistance of isolation big enough the current through a body of the person will be small.
In a three-phase network with an isolated neutral the person is under protection of resistance of isolation of a network.