Page 119 - Electrical Safety of Low Voltage Systems

102 Chapter Six

of cables and equipment. On the other hand, they must allow the
circulation of the normal continuous currents required by the loads.
Therefore, in correspondence with large loads, the CB’s rating will also
be large. This implies that Eq. (6.4) is more difficult to fulfill for large
loads than for small ones, as I a for large loads is, in fact, a multiple of
a larger nominal current.
Thus, one might think that small electrical loads are safer than
large loads only because they are protected by smaller CBs, and the
fulfilling of Eq. (6.4) is, therefore, facilitated. This is, of course, not
true, as the risk of indirect contact does not change with the power
of the load. The complication, if not the impossibility, of fulfilling
Eq.(6.4)canberesolvedbyusingresidualcurrentdevices(RCDs;already
introduced in Chap. 2).

6.4 Protection Against Indirect Contact by Using
Residual Current Devices
The presence of RCDs in TT systems does not exclude, of course, the
overcurrent devices, which must still be employed against overloads
and short circuits.
When RCDs are used, the following safety condition, which ties
together residual operating current, permissible touch voltage, and
the electrode’s earth resistance R GT , must be fulfilled:
50
R GT ≤ (6.5)
I dn
where I dn istheresidualoperatingcurrentoftheRCD,whosestandard
values are (in mA) 10, 30, 100, 300, 500, and 1000. Equation (6.5) fixes
at 50 V the maximum permissible touch voltage upon circulation of
the earth current I dn through R GT . If the ground-fault current exceeds
I dn , the prospective touch voltage will be greater than 50 V, but it is
assumed that the RCD will open the circuit in a shorter time, follow-
ing the time–voltage safety curve, thereby, “compensating” for a larger
touch potential. On the other hand, if the earth current is less than I dn ,
the RCD may not trip at all, but the prospective voltage appearing
over the enclosure would be less than 50 V, which is assumed a safe
value.
It appears clear that Eq. (6.5) allows higher values of R GT than Eq.
(6.4) and therefore is easier to fulfill. Permissible maximum values of
R GT , calculated per Eq. (6.5), are shown in Fig. 6.7 as a function of the
residual operating currents of the RCD.
In the presence of a RCD rated 30 mA, the ground resistance must
not exceed 1667 , which is a fairly easy condition to meet. RCDs,
therefore, are the most effective way to protect against indirect contact
in TT systems.