Electrical networks and equipment

Electrical installations and electric networks are divided into two classes by electrical safety conditions: up to 1 kV and over 1 kV.

Supply of electricity to consumers is usually carried on the lines of a high voltage (above 1 kV), which is then reduced using three-phase voltage transformers to less than 1 kV. Local power networks of industrial and domestic purposes are fed from the secondary windings of the transformers.

The secondary windings of three-phase transformers connected in «star» form a common point – neutral. If neutral is connected to earth ground, directly or through a low resistance (Fig. 1), then it is called earthed. The conductor connected to the earthed neutral point, called zero (denoted by the letter N₀ if it is a working, and the PEN, if the working and protective at the same time). Along with this neutral can be isolated from the ground (Fig. 2), i.e. not connected to earth ground, or connected through a high resistance. Thus, the three-phase network to 1 kV, depending on the power source neutral mode, and the availability of the neutral conductor can be a four-earthed neutral (see. Fig. 8.1) or with isolated neutral, three-wire (see. Fig. 8.2).

a              b      c

Fig. 8.1. The three-phase four-wire network with earthed neutral:
a and b – single-phase touch; c– biphasic touch; OH – lighting load;

CH – power load

 

a              b      c

 

Fig. 8.2. Three-phase three-wire network with isolated neutral:

a, b – single-touch; c– biphasic touch

 

In the windings of the transformer, as well as between each phase conductor A, B, C, and ground (or neutral conductor N) acts as the phase voltage U_ph and between the phase conductors – linear voltage U_l. The following ratings are taken in the four-wire network: U _l/ U _ph: 220/127, 380/220, 660/380 V, in the three-wire network – U_l: 36, 42, 127, 220, 380, 660 V.

Distributed along the length of the conductors A, B, C, N insulation resistance and capacity of each of these networks to ground circuits conventionally presented lumped elements R and C with corresponding indices. Inductive component insulation resistance is usually neglected due to its insignificance.

If a person touches to the phase network wire or housing equipment, for example in the breakdown of insulation (variants a and b in Fig. 1 and 2), through the human body to ground current passes, which then through the ground, the insulation resistance of the conductors and earthing neutral (if available) is returned to the power supply (transformer windings). This is called a touch of a single phase. Less frequently, in practice, the two-phase integration of human is realized in the current circuit (option c).

The severity of damage depends on the value of the current flow path through his human body, duration of exposure, the individual characteristics of the person and other factors.

Allowable values I_h of current flowing through the person along the way «hand – the hand», or «hand – foot», and contact voltage U _с (voltage applied directly to the human body), and depending on the duration τ effects are specified in GOST 12.1.038-82. Some of these values in relation to industrial electrical installations with voltage up to 1 kV are shown below.

Table 8.2