Page 33 - Electrical Safety of Low Voltage Systems

16 Chapter Tw o

FIGURE 2.6 Permissible operating time as a function of the ground-fault
current.

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The term residual current I d indicates the vector sum of all al-
ternating currents flowing through a circuit’s wires, single-phase or
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three-phase, including the neutral conductor, and is expressed in
terms of the root mean square (r.m.s.) value. The RCD executes this
sum, which is zero in normal conditions. Should a fault occur, I d be-
comes greater than zero and is equal to the r.m.s. of the ground-fault
current I G . The RCD compares this nonzero value to its rated oper-
ating current I dn and if I d > I dn disconnects the supply to the faulty
circuit. The clearing time will occur within a conventional safe time
as established by applicable standards. RCDs, in fact, do not limit the
magnitude of the ground-fault current, but only the time this cur-
rent circulates to ground. Figure 2.6 shows the permissible operating
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times not to be exceeded by general purpose RCDs as a function of
the residual current I d , usually expressed as a multiple of the rated
operating current I dn .
Besides the residual operating current, the RCD is characterized
by another important parameter: the residual nonoperating current
I dNO , which represents the maximum r.m.s value of the residual cur-
rent that does not cause its operation. Standard value for I dNO is
0.5I dn and therefore the RCD does not operate for I d < 0.5I dn ; it might
operate in the range 0.5I dn < I d ≤ I dn and must surely operate for
I d < I dn .
For a better understanding of the functioning of the residual cur-
rent devices, let us examine Fig. 2.7, which shows a single-phase RCD.
In the absence of ground faults, we have

I Ph = I N ⇒ I Ph − I N = 0 (2.1)