Page 77 - Electrical Safety of Low Voltage Systems
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60    Chapter Four


                                  FIGURE 4.17 The
                                  earth potential V G
                                  assumed by the
                                  faulty ECP .












                                     R BG is due to the presence of the floor, whose resistance in dry con-
                                  ditions is at least 1 k . In the absence of floor (i.e., outdoor locations),
                                  R BG can be calculated by considering the person’s feet as two circular
                                  plates, of radius r = 0.1 m, in parallel on the soil. Each foot/plate has
                                  a ground resistance approximately equal to
                                                              2
                                                            ∼
                                                       R Foot =  = 4                   (4.22)
                                                              5r
                                  where   is the superficial soil resistivity in ohm meters (  · m). Thus,
                                  the human body resistance-to-ground R BG equals 2 . The tendency of
                                  international standards for low-voltage installations is not to consider
                                  the resistance of footwear, by assuming the person, conservatively,
                                  shoeless. The presence of shoe resistance in series to the body, in fact,
                                  would limit the body current’s circulation, benefiting the person’s
                                  safety.
                                     Let us assume a person standing in an area sufficiently far from the
                                  electrodetobeconsideredatzeropotential.Inthisworst-casescenario,
                                  be the person exposed to indirect contact by touching a faulted ECP.
                                  Assume the ECP leaks the ground current I, so that the person’s hand
                                  is subject to the potential V ST = V G (Fig. 4.18).
                                     The prospective (or source) touch voltage V ST is defined as the po-
                                  tential difference between the faulted ECP (i.e., the dryer of Fig. 4.18)
                                  and the earth occupied by the person, at a distance of1mfromthe
                                  ECP, 12  when the ECP is not being touched by the person.
                                     In reality, the person touching the enclosure is not subject to the
                                  potential difference V ST but to the touch voltage V T , which is ≤V ST .A
                                  voltage divider, in fact, takes place between the body resistance R B
                                  and the person resistance-to-ground R BG (Fig. 4.19).
                                     In low-voltage systems, V T is defined as the voltage differential
                                  which a person, 13  standing at1mfromthe grounded ECP, may be
                                  subject to, between both hands and both feet. 14
                                     Upon the human touch of the faulted ECP, the natural presence of
                                  R BG causes an “elevation” of the electric potential in correspondence
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