Page 207 - Electrical Safety of Low Voltage Systems

190 Chapter Eleven

Copper Aluminum Steel
Conditions Max Max Max
of Bare Temperature Temperature Temperature
PE ( C)/k Value ( C)/k Value ( C)/k Value
◦
◦
◦
Visible and in 500/228 300/125 500/82
restricted areas
In normal conditions 200/159 200/105 200/58
areas
In fire risk areas 150/138 150/91 150/50
TABLE 11.7 Values of Maximum Temperatures of Bare Protective Conductors

In the case of a bare PE not bunched with other cables, the max-
imum temperature the conductor can achieve is not dictated by in-
sulation’s thermal capabilities, but by the surrounding environment.
The bare PE in fault conditions, in fact, may become a temperature hot
object and trigger fires or explosive atmospheres. For safety reasons,
then, IEC 60364–5-54 lists the maximum temperatures of bare PEs
exposed to touch, as a function of the surrounding conditions, assum-
ing these temperatures are not a risk of damage to any neighboring
material. Table 11.7 shows the maximum temperatures for different
materials and conditions.

11.3.1.1 Iterative Method of Calculation
The following Eq. (11.10), derived by Eq. (11.9), clearly shows that
the cross-sectional area S of protective conductors depends on the
prospective phase-to-ground current I flowing through it:

√
I t
2
S ≥ (11.10)
k
However, the value of I can be calculated only by knowing the
impedance of the PE, which varies according to its cross-sectional
area S. Thus, I and S in Eq. (11.10) are not independent quantities from
each other, especially in TN systems, where ground-fault currents will
return to the source through protective conductors.
To solve this quandary, an iterative method to size S, as depicted
in Fig. 11.3, should be employed.
One can start by selecting the minimum standard cross-sectional
2
area (i.e., 1.5 mm ) and calculate maximum and minimum phase-to-
ground fault currents accordingly. The maximum ground-fault cur-
rent originates for a fault at the beginning of the circuit (e.g., contact
between phase and protective conductors), as there is virtually no