Page 115 - Electrical Safety of Low Voltage Systems

98 Chapter Six

FIGURE 6.3 V ST as a function of R GT for three increasing values of R N (1, 10,
and 100 ).

the resistivity of the soil and the size and shape of the electrode. For
example, a theoretical hemispherical electrode, buried in a fine sandy
soil of resistivity = 300 · m, should have the following radius to
contain the prospective touch voltage to 25 V:

R GT = = 0.1 ⇒ r 0 = 477 m (6.3)
2 r 0

The above value, not feasible for use in practice, shows that in TT
systems the limitation of V ST to harmless values by the sole means
of the house grounding system is not easily achieved due to the ex-
tremely low value of its earth resistance that is required.
Safety must be provided by employing protective devices, over-
current and/or residual, which can promptly disconnect the supply
upon ground faults. Each protective device has a built-in an inverse
time–current operating curve, which allows the disconnection of the
circuit in a time inversely proportional to the magnitude of the fault
current. Safety is assured if the protective device operates within the
permissible time as per the time–voltage safety curve (Fig. 5.19). The
protective device’s inverse time–current curve must always be below
the safety curve (Fig. 6.4).
During a ground fault, the protective device senses the current
I G and initiates the opening action, which will part its contacts. Let
us assume that its fault clearing time is t G . During this period, the
enclosure will be energized at the potential V G = R GT I G . As per the
safety curve in Fig. 6.4, this voltage can be withstood for the permissible