Page 128 - Electrical Safety of Low Voltage Systems

TT Grounding System 111

(e.g., the ground rod). This fortunate result attenuates the touch volt-
ages eventually occurring in the building.

6.10.1 Should We Bond Incoming Pipes Made of Plastic?
In some cases water pipes entering the building are made of plastic.
There is no need to say that it is not possible, nor required, to bond
such pipe. Formally speaking, the plastic pipe is neither an EXCP, as its
resistance to ground is much greater than 1000 , nor an ECP. For this
reason there is no requirement to link it to the equipotential system,
even if it originates outside of the building.
There should be no concern either about the possibility that the
tap water present in the plastic pipe can conduct electricity. It has
been demonstrated via measurements, in fact, that tap water is a poor
conductor of electricity, even in the presence of impurities. 10
If the incoming pipe is made of plastic, but within the building is
made of metal, the main bonding is still necessary (Fig. 6.18).
If, in fact, ECP A, accidentally not bonded, energized the metal
water pipe, the fault potential would be transferred within the build-
ing. Persons simultaneously touching pipe and ECP B (bonded equip-
ment) would be exposed to the earth potential in the absence of the
main equipotential connection. The bonding connection between the
metal pipe and the grounding system, realized downstream of the wa-
ter meter, would virtually cancel this potential difference, by equaliz-
ing the pipe and ECP B.

FIGURE 6.18 Main
equipotential
bonding of the
metal pipe.