3.5. Causes of electric injuries

Major causes of electric traumatism are:

• personality low training, improper testing of knowledge and giving qualification groups on safety rules;

• breaking operation and safety rules of power facility;

• improper organization of work ;

• breaking regulation referred to work in protected areas, electric cables and transmission lines;

• insulation damage;

• grounding wire damage;

• using protective equipment not appropriate to work conditions;

• conducting mounting or repairing works under voltage;

• using wires and cables not admitted for certain work condition and voltage;

• low quality of mounting and repairing work;

• wrong risk assessment of pace voltage when feet of person are at the points with different electric potentials;

• repairing neutral wire of aerial power line when power is on;

• power supply of several users from joint switch equipped with fuse designed to shut down the most powerful user;

• neglecting necessity of shutting power down in time off work;

• not using personal protective equipment or using not tested one;

• skipping periodical checking of insulation resistance (power line, engine winding, communication line, relay) and grounding resistance;

• using power facility with insulation resistance lower the standard; using not licensed power facility;

• low instructing workers handling manual power facilities;

• missing of monitoring work by supervisors or employees;

• absence of labeling, signal words, blocking, signaling on restricted areas or repairing works.

Those causes can be classified by the next factors:

• touching conductor of facility under voltage caused by breaking safety rules, engineering defects, mounting defects;

• touching part of facility which turned out to be under voltage because of insulation damage, short circuit;

• accidental power supplying of facility operated by workers;

• missing of reliable protective equipment.

3.6. Assessing risk associated with operating power facity

Assessing risk associated with operation of power facility is focused in estimation of current in all possible variants of its flowing through the human body, which turned out to be under voltage after touching conductor or some part of power facility not purposed to transfer electricity but being under voltage because of insulation damage, or under pace voltage.

Power lines are of two types: AС and DC. Alternative current power lines can be one-phase or polyphase. The most wide spread polyphase power lines are three-phase AC power lines. Depending on mood of neutral operation power lines are ones with insulated neutral and ones with grounded neutral. Insulated neutral means that power line neutral wire is insulated from grounding or is connected to it through facility with very high resistance (voltage transformer, compensation coil). Neutral is called grounded when it is connected to grounding directly or through facility with small resistance (current transformer).

3.6.1. Danger in one-phase power line.

One-phase power lines and direct current power lines can be insulated from the ground, have ground pole or center tap.

During one-pole touching of conductor a person gets connected to another conductor through ground resistance (fig. 3.1).

Since one-phase power lines are not extended capacitance of their conductors can be neglected and as for DC lines such capacitance equals to zero. To simplify formula for current calculation ground currents of both conductors are assumed equal, so are their insulation resistances:

. (3.1)

Current passing through human body can be found out of the scheme (fig. 3.1):

, (3.2)

where U – voltage, V; R_h – human body resistance, ohm; r – conductors’ insulation resistance, ohm.

Fig. 3.1 Scheme of person connection to conductor of insulated power line

Touching not grounded conductor of power line with grounded pole (fig. 3.2) causes current passing through human body that is:

, (3.3)

where R_o – grounding resistance, ohm.

Taking into account that R_o<< R_h formula will look as:

. (3.4)

Fig. 3.2 Scheme of person connection to not grounded conductor of power line with grounded pole

Connection to normal conductor when another one is damaged and has short circuit with the ground is shown in the fig. 3.3. That connection causes current:

, (3.5)

where R_sc – resistance of short circuit, ohm.

Fig. 3.3 Scheme of person connection to conductor of damaged power line

If person touches a conductor of power line with grounded center tap (fig. 3.4) he turns out to be under voltage making half of total line voltage, so the current will be:

, (3.6)

where R_ct – grounding resistance of center tap, ohm.

Fig. 3.4 Scheme of person connection to conductor of power line with grounded center tap

Two-pole connection to power line (fig. 3.5) gets a person under total line voltage and current will be:

. (3.7)

Analysis of all studied formulas testifies that case of two-pole connection to power line is the most dangerous irrespective to neutral operation mode, because in this case current passing through person depends only on human body resistance. The least dangerous is connection to conductor of insulated power line in normal operation mode.

Fig. 3.5 Scheme of person connection to two conductors of power line