Page 206 - Electrical Safety of Low Voltage Systems

Earth Electrodes, Protective Conductors 189

In addition, to assure protection against indirect contact, the pro-
tective device must open the circuit within a safe time. This is ac-
complished if the minimum r.m.s. value I min of the prospective fault
current is greater than, or equal to, the instantaneous (or magnetic)
trip setting I i of the protective device. The fault current must be in the
instantaneous region of the time–current characteristic of the overcur-
rent device (see chart in Fig. 6.6). In formulas:

I min ≥ I i (11.8)
The left-hand side of Eq. (11.7) is not of immediate calculation,
therefore IEC standards, by conventionally assuming that the fault is
cleared within 5 s, allow the following simplification:

t f 2 2 2 2

∼
i dt = I t ≤ k S (11.9)
0
where I is the r.m.s. value of the prospective fault current circulat-
ing through the PE for a fault of negligible impedance, and t is the
operating time of the protective device in correspondence with that
current. In reality, during the fault the current is not a constant value
as it varies with time, but the error caused by this simplification is
generally acceptable. It is important to note that at the inception of the
fault, thecurrent is asymmetricalduetothe developmentofatransient
d.c. component. Equation (11.9), therefore, cannot be applied when
the fault duration is extremely short (e.g., when current limiter pro-
tective devices are employed), as the d.c. transient might not be over
andthecurrentwavemightnotbeyetsymmetrical,thereby,accentuat-
ing the difference between the left- and right-hand sides of Eq. (11.7). 4
If a protective conductor is common to more than one circuit, it
must be sized for the largest Joule integral.
Calculated values of k from Eq. (11.6) for insulated protective con-
ductors not incorporated or bunched with other cables are shown in
Table 11.6.

k
Conductor
Insulation 0 ( C) M ( C) Copper Aluminum Steel
◦
◦
PVC 30 160 143 95 52
Rubber 30 200 159 105 58
Thermosetting 30 250 176 116 64
Silicone rubber 30 350 201 133 73

TABLE 11.6 Values of k for Insulated Protective Conductors Not Incorporated
in Cables or Bunched with Other Cables