Page 141 - Electrical Safety of Low Voltage Systems

124 Chapter Seven

However, protective devices, RCDs included, must fulfill the fol-
lowing condition applied at the farthest point of the circuit being pro-
tected, where the fault current is at its minimum due to the build-up
of the wires’ impedances:

V
ph
I (7.4)
G =
≥ I a
Z Loop
or equivalently, by solving for |Z Loop |,

V

ph
Z Loop ≤ (7.5)
I a
where |I | is the minimum phase-to-protective conductor fault cur-
G
rent and Z Loop is the series of the impedances of the components that
form the fault-loop, specifically the source, the line conductor up to
the farthest point of the fault from the source, and the protective con-
ductor up to the farthest point of the fault. V ph is the system nomi-
nal voltage to ground. I a is the current causing the automatic opera-
tion of the overcurrent protective device within the time specified in
Table 7.1.
IfRCDsareemployed, I a representstheresidualoperatingcurrent,
which provides the disconnection of supply within the time specified
in Table 7.1.
For example, an overcurrent device operating at 230 V, with con-
tinuous rating I n = 16 A, may trip in 0.4 s in correspondence with
I a = 4I n ; therefore, by applying Eq. (7.5), Z Loop ≤ 3.6 . This inequal-
ity is fairly easy to fulfill.
As said, Eq. (7.4), applicable to any point of the circuit being pro-
tected, should, indeed, be verified for ground faults at its end because
that is the point where the smallest amount of fault current is gen-
erated. If the overcurrent device can sense the lowest current in the
circuit and trip within the safe times, the protection against indirect

Maximum Disconnection
Voltage Range (V) Times t a (s)
50 < V ph ≤ 120 0.8
120 < V ph ≤ 230 0.4
230 < V ph ≤ 400 0.2
V ph > 400 0.1

TABLE 7.1 Maximum Disconnection Times as a Function of
the Nominal A.C. Voltage of the TN System