Voltage touch

For example, referring to Example 22.9 and Table 22.6, the safe touch voltage, E, In the power plant area is estimated at 267 V. For this voltage, the leakage current, /, through the two feet when in parallel which is a more severe case. 

The touch voltage diminishes up to I m depth of the grid and then rises rapidly. The ideal depth for economic considerations may be taken as 0.5 m when the touch and slep voltages are reasonably low. 

In ihe above equaiion.s for maximum step and touch voltages the best results will be obtained when the following parameters are achieved ... 

After the final designs are complete it is recommended that the actual touch (actual) and step voltage (actual) are rechecked for both power plant and switchyard areas separately, to ensure that they are within the tolerable limits as determined above. After the ground stations have been finally installed the actual step and touch voltages must be measured to verify the designs. 

Average weight of a huinati body (kg). . Safe touch voltage. 

The IPB system must therefore be designed so that it allows only a moderate field in the space. The field should be incapable of heating structures in the vicinity beyond a reasonable limit, or cause step and touch voltages. These voltages are undesirable and may become dangerous to a human coming into contact with them. 

At this stage the 50 kg step and touch voltages can be calculated from Cs hs, K, since Ps and ts are constants. The step voltage step5o is -... 

Table H.lc. Data for earth resistance, touch voltage, ground potential rise and corner mesh voltage for different grid and rod designs...

Case A. The resistivity of the lower soil was chosen to be a moderate value of 100 ohm-m. Low values of resistance to earth at the pole, R p, were easily obtained. The main criterion is that the corner mesh voltage L must be less than the 50 kg touch voltage iitouchso- Only one case A.6 satisfies this criteria, 343 volts is less than 361 volts. This case requires a relatively large site area of 256 m for a pole and its associated equipment. 

Electric shock is normally caused either by touching a conductive part that is normally live, or by touching an exposed conductive part made live by a fault. The touch voltage curve in Fig. 6.8 shows that a person in contact with 230 V must be released from this danger in 40 ms if harmful effects are to be avoided. Similarly, a person in contact with 400V must be released in 15 ms to avoid being harmed. 

D 1 Injury to living being due to step and touch voltages... 

The phase voltage at the substation transformer will be a little higher than 230 V to allow for the inevitable voltage drop in the distribution cables. In urban areas, the line/neutral and the line/earth loop impedances will be comparable and will probably be only a small fraction of an ohm, whereas the victim s hand-to-hand impedance will be in the order of 2000 ohms. Under these circumstances the effects of the circuit impedances can be ignored. The victim s touch voltage will be about 230 V and, for a total body impedance of 2000 ohms, the shock current would be 230/2000 = 0.11 A. This is high enough to cause ventricular fibrillation in many people should the current flow for about 0.5 s. 

The nature of the fault described means that the touch voltage is in the order of 230 V. However, many indirect contact shock accidents occur at less than mains voltage. This can be quite fortunate for the injured person because the shock current will be lower, thereby reducing the adverse effects and improving their chances of being able to let go of the conductors and survive the incident. 

The increased risks arising from the harsh environment are recognised by reductions in the required disconnection times for TN systems, set out in Table 604A. For example, for 230 V systems, the normal 0.4 s disconnection time is reduced to 0.2 s. It may not be possible to achieve these disconnection times because of earth loop impedance restrictions, in which case RCD protection will normally need to be specified. Note also that the 50 V touch voltage value used in Section 413 is reduced to 25 V. 

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