The unipolar touch in a three-phase network with an earthed midpoint wire

At a unipolar touch of the person to a phase wire in a network with an earthed neutral small resistance of grounding of a neutral (no more than 4-10 Ohm) shunts resistance of isolation of wires of a network and a current of short circuit will pass (fig. 4) through bodies of the person consistently connected resistance and resistance of grounding of a network.

If to neglect small resistance of grounding (in comparison with resistance of a body of the person) the size of a current through a body of the person can be calculated according to expression

 

At the unipolar touch to a wire with current in a three-phase network with an earthed midpoint conductor the person gets under a phase voltage, isolation of a network does not rescue him from defeat.

5. EXAMPLES OF PROBLEMS

Problem 1. To calculate a current through a body of the person at an involuntary contact of current-carrying wire in the room with two-wire electric system (the second wire is earthed):

a) The person concerns simultaneously earthed water-pipes (resistances of a contact are equal to zero);

b) The person stands on a cement floor in wet footwear;

c) The person stands on a dry wooden floor in dry footwear.

To determine a degree of lesion.

To make conclusions.

Problem 2. To calculate a current through a body of the person at an involuntary contact wires with current in a three-phase network with an earthed neutral:

a) The person during a rain stands on the ground;

b) The person stands on a cement floor in wet footwear;

c) The person stands on a dry wooden floor in dry footwear.

To determine a degree of lesion.

To make conclusions.

Problem 3. To calculate a current through a body of the person at an involuntary contact wires with current in a three-phase network with an isolated neutral:

a) The person concerns simultaneously earthed water-pipes (resistances of a contact are equal to zero);

b) The person stands on a cement floor in wet footwear;

c) The person stands on a dry wooden floor in dry footwear.

To determine a degree of lesion.

To make conclusions.

Problem 4. To calculate a current through a body of the person at an involuntary simultaneous contact of two wires with current of a three-phase network:

a) Hands are not protected;

b) Hands are protected by a dry fabric (resistance of isolation 2-10 kΩ);

c) Hands are in (r_из = 100...500 kΩ).

To determine a degree of lesion.

To make conclusions.

The note: Both the teacher, and students can suggestand solve other similar tasks.

 

 

6. THE LABORATORY TASK AND THE CONTENTS OF THE REPORT

1. To study and summarize dangerous and affecting factors of electric current, action of current on an organism of the person.

2. To study methods of the analysis of electrosecurity.

3. To familiarize with ways of first aid to injured from action of electric current.

4. To execute practical calculations by definition the value of a current proceeding through the person in various cases of an involuntary contact wires with current.

In the report decisions of not less than four problems should be resulted, each task is illustrated by the scheme.

To make conclusions on influence of electric current on an organism of the person in conditions of a specific task.

5. To make the common conclusions.