Page 124 - Electrical Safety of Low Voltage Systems

TT Grounding System 107

FIGURE 6.13
Vectorial current
summation sensed
by the RCD.

IfthefaultyUnitBhasno,ormalfunctioning,protectivedevice,the
voltage rise will persist in the system, as the RCDs of other dwelling
units will not be able to sense and clear the ground fault.
Interferences can occur even in the absence of faults. Dwelling
unitsmightimpressacurrenttogroundbelowtheirRCDsresidualset-
tings. Some equipment (e.g., computers, high-frequency luminaries,
UPS, etc.), in fact, incorporate radio frequency filters, with capacitors
connected between the “hot” conductors and the ground. These filters
cause functional leakage current (e.g., in excess of 3.5 mA) during the
regular functioning of the equipment in order to limit electromagnetic
8
effects. The earth currents from all the units flow through protective
conductors and converge into the common R GT . The leakage currents
may not cancel each other if they are supplied by the same phase con-
ductor (worst-case scenario). Therefore, they can induce a permanent
ground potential on all the grounded metal parts.
In addition, the continuous circulation of the ground currents
interferes with the sensitivity of the RCD, by increasing it. If, for
instance, 10 mA constantly flow through a 30-mA-rated RCD, it will

FIGURE 6.14 Electrical interferences upon a fault between dwelling units in a
condominium.