Page 158 - Electrical Safety of Low Voltage Systems

TN Grounding System 141

the same probability to cause ventricular fibrillation. The left-foot-to-
right-foot current path is, therefore, less hazardous and so is the step
voltage, if compared to an equal value of touch voltage.
A European standard 10 takes into account the above considera-
tions, obtaining the permissible values of the step voltage by multi-
plying by the factor 3 the permissible values of the touch voltage, in
correspondence with the same clearing time.

FAQs

Q. What is the difference in the status of the neutral wire in TT and TN
systems?
A. The neutral wire is generally always energized, in some case even at dan-
gerous potentials. In TT systems, the magnitude of the neutral potential de-
pends on faults caused by the utility, therefore, out of customers’ control. For
this reason, the TT neutral wire is conventionally treated as a live conductor,
and always switched off together with the phase conductors.
In TN-S systems, users own and maintain the supply source; therefore,
they can evaluate the risk of the dangerous energization of the neutral wire
and decide how to treat it. In TN-C systems, the neutral is also a protective
conductor (PEN) and, therefore, must not be switched off.
Q. If the fault-loop in TN systems does not comprise the actual earth, why
do we have to ground the source?
A. TN systems require the system ground. Its purpose is to guarantee the
stability of the operating voltage-to-earth under regular and fault condi-
tions. In addition, the absence of the earth connection of the source would
expose persons to the risk of electric shock, in the conditions depicted in
Fig. 7.3.

Endnotes

1. As earlier mentioned, we herein indicate with the underlined symbols the
complex numbers as representative of sinusoids. The same symbols, but with
no underline, or between bars, signify the magnitude of the complex number.
All the network theorems can be applied to sinusoids by means of complex
numbers, as it is illustrated in the final appendices of the book.
2. This sizing is permitted by international standards and codes, as further ex-
amined.
3. In Chap. 12, the effects of high-voltage faults on low-voltage equipment will
be examined.
4. See Chap. 3 for the definition of acceptable risk.
5. Common nonlinear loads may include variable frequency drives and uninter-
ruptible power supplies.