Page 132 - Electrical Safety of Low Voltage Systems

TT Grounding System 115

This important characteristic compensates the unavoidable energiza-
tion of all the healthy ECPs and EXCPs, thereby benefiting the persons’
safety.

FAQs

Q. What is the difference between underlined (i.e., V ST ) and nonunderlined
(i.e., V ST ) symbols?
A. The underlined symbols represent the phasor quantities, that is, vectors
or complex numbers, characterized by magnitude and angular displacement.

The symbol V ST , as well as the symbol enclosed between bars V ST , indicate

the magnitude of the phasor (see Appendix A).
Q. Equipotential bonding connections cancel potential differences between
EXCPs and ECPs within reach. What about the touch voltage between faulty
ECPs and the floor?
A. The main equipotential bonding links together all the ECPs and EXCPs
to the grounding system, including structural metallic parts of building and
reinforcing bars embedded in concrete. Ergo, if the floor is conductive, and
in contact with these components, the main equipotential bonding also elim-
inates ECP-to-floor potential differences. If the floor is made of insulating
materials, no ECP-to-floor potential differences arise.
Q. Do we need to supplementarily bond metal window frames in areas at
increased risk?
A. Metal frames of windows are generally not EXCPs, that is, they are not
likely to introduce remote or dangerous potentials. In addition, they are not
the part most likely to be touched in a window. Therefore, there is no need
for their supplementary bonding. On the other hand, their connection to the
equipotential system may result in their undue energization, in the case of a
fault occurring somewhere else in the system.
Q. To improve the performance of grounding systems in TT systems, should
we connect it to the utility neutral wire that is earthed?
A. The TT grounding system is used when the electrical utility cannot guar-
anteea safemeans ofearthing fortheir users.The neutralwire, in fact, although
earthed, might assume dangerous potentials with respect to ground, because
of high values of the utility’s ground R N . The connection between the user
grounding system and the neutral wire would transfer the neutral potential
over the user’s enclosures. For these reasons, this bond is not permitted.

Endnotes

1. The underlined quantities indicate complex numbers (or phasors) as represen-
tative of sine waves in the circuit. Symbols not underlined designate magni-
tudes of complex numbers (see App. A).