Danger in three-phase power line with insulated neutral

As to the ground all power line conductors have capacitance and resistance what makes insulation that separates conductors from the ground (fig. 3.6). Actually values of capacitance and resistance differ for different conductors. However to simplify analysis we’ll consider them the same: C_a=C_b=C_c=С and r_a=r_b=r_c=r.

Case of person touching one of three phase conductors (one-phase connection) of power line in normal mode results in current which can be calculated by formula:

, (3.8)

where U_ph – phase voltage, V; Z - complex resistance of phase conductor to the ground:

, (3.9)

where w=2pf – angular frequency of the power line, f – current frequency.

Fig. 3.6 Scheme of person connection to one-phase of power line in normal mode

If power lines are short and that’s why phase conductors have low capacitance to the ground (C=0) formula to calculate current passing through the person will look as:

, (3.10)

In case of two-phase connection (fig. 3.7) person turns out to be under linear voltage, so current will be:

,

where U_l – linear voltage, V; it is defined as: .

Fig. 3.7 Scheme of two-phase connection to power line in normal mode

Case of one-phase connection to power line in emergency mode (fig. 3.8), when another phase has short circuit with the ground, causes current:

. (3.12)

If R_sc << R_h then

. (3.13)

Fig. 3.8 Scheme of one-phase connection to power line in emergency mode

Thus, current passing through the person in case of one-phase connection to power line with insulated neutral in normal mode depends on insulation resistance and capacitance to the ground. Case of short circuit of one phase to the ground significantly rises the danger of one-phase connection since person turns out to be under voltage close to linear. The most dangerous is case of two-phase connection.