Emergency operation mode of electrical network

Consider the situation where one of the mains phases (phase C) withdrew to ground via a relatively low resistance R_g, such as wire breakage and falling to the ground. The network diagrams (see Fig. 8.3), this fact will be reflected in parallel with Y_C conductivity Y _g = 1/ R _g (Y _gis not shown). Consequently, between phase C and ground conductivity be Y_С ⁺ Y _g. Accordingly, the expression (8.8) takes the form:

          (8.21)

Three-phase four-wire network with earthed neutral:

Condition (8.9) is usually performed in the network, so the expression (8.21) can be simplified:

                        (8.22)

Considering the relation (8.5), and R ₀<< R_h we obtain:

                           (8.23)

Three-phase three-wire network with isolated neutral:

For a thisnetwork Y_N = Y ₀= 0. In addition, usually | Y_A|, | Y_B|, | Y_C |<< Y_зм. Then the expression (19) takes the form:

                             (8.24)

or, given the expressions for a and relations (8.5)

I_h= U _ph /(R_h + R _g ) = U _l/ (R_h + R _g ).                         (8.25)

In emergency mode, often have a situation in which R_g substantially more R₀ and simultaneously R_g << R_h. Under this condition, a comparative analysis of the formulas (21) and (23) shows that a person is under tension U_c close to a linear network with isolated neutral, which is more dangerous than the network with earthed neutral, where U_c approaching the phase voltage. For both types of networks is characterized by a decrease in risk of electric shock by increasing the R_h.