Page 82 - Electrical Safety of Low Voltage Systems

The Earth 65

Now consider the metal fence an EXCP, as its ground resistance is
less than 1000 , and the light pole an ECP. Upon failure of the pole’s
basic insulation, persons might be in simultaneous contact with the
pole and the fence, and, therefore, exposed to the total earth potential.
This hazardous condition can generally be avoided by means of the
EBC.
This bonding connection, though, transfers the touch voltage
along the entire length of the fence, which could be miles long. Per-
sons in simultaneous contact with it and another EXCP (e.g., a fire
hydrant) are once again exposed to the whole earth potential. Should
we bond to the fence the fire hydrant too and, thereby, transferring
the fault potential even farther?
There is no general answer to this question: the electrical engineer
must decide on an individual case basis whether to bond each and
every EXCPs in order to minimize the risk of electric shock.
Another case is presented in Fig. 4.25. The fence is connected to
the grounding system, while the fire hydrant is not. In the case where
the grounding system becomes energized, the fence does too, and the
total earth potential establishes between it and the hydrant, which acts
as an EXCP. This hazard can be avoided by means of an EBC between
the two elements.

4.7 Voltage or Current?
The human body is sensitive to, and endangered by, current, not volt-
age. In addition, it has been proved that the human body’s impedance
Z B , as offered by the same person, is not a constant value, but de-
pends on the voltage of the energized object to which he/she is
16
exposed. There is a nonlinear relationship between voltage and body
impedance: the greater the potential difference applied to the body,
the lower its resistance and the greater the hazard.
Consequently, two different touch potentials may, in fact, “pro-
voke” two different body resistances, but cause the circulation of
the same current. This makes the touch voltage a not very effective

FIGURE 4.25 Equipotentialization between fence and fire hydrant.