Page 248 - Electrical Safety of Low Voltage Systems

Testing the Electrical Safety
231
Hence,
V ph
R N + R G = R G = R − 1 (14.5)
∼
V R
R G is automatically provided by the tester because all the quantities
at the right-hand side of Eq. (14.5) are known.
It is important to note that during the automatic operations of the
tester, all the enclosures will become energized at the voltage R G I G .
The duration of such energization must not exceed the maximum
permissible times given in Table 6.1.
As examined in Chap. 6, in TT systems RCDs are the most effective
way to protect against indirect contact, as long as the following equa-
tion, already studied in Chap. 6, which ties together residual operating
currents and earth resistance R G , is fulfilled:
50 V
R G ≤ (14.6)
I dn
Equation (14.6) calls for the maximum value of 50 V as the touch
voltage.
The correct operation of the RCD, within the times as per the
chart in Fig. 2.6, as well as the fulfillment of Eq. (14.6) can, and must,
be instrumentally verified.
WithreferencetoFig.14.9,afterclosingtheswitch,theresistanceof
the potentiometer R, which connects the phase conductor to ground,
is decreased until the RCD operates. The ammeter A will measure the
actual ground current I d in correspondence with the tripping of the
RCD,therebyallowingtheverificationthattheoperatingthreshold I dn
of the device (e.g., 30 mA) is not exceeded. The voltmeter V measures
the phase voltage V ph (switch in open position) and then, after closing
the switch, the voltage drops V R across the known resistance R in
correspondence with the tripping. By assuming R N negligible with
respect to R G , we can write
∼
V ph − V R = V G = R G I d (14.7)
Hence, by imposing the safety condition of Eq. (14.6), we will
obtain
50
V ph − I d ≤ V R (14.8)
I dn
Thus, the system is protected against indirect contact if the reading
of the voltmeter exceeds the right-hand side of the previous inequality.
The tester will also automatically measure the tripping time in cor-
respondence with I dn , which must not exceed the permissible clearing
times of 300 ms, as given in Fig. 2.6.