Page 264 - Electrical Safety of Low Voltage Systems

Applications of Electrical Safety 247

FIGURE 15.4 Hazardous situation caused by the accidental loss of the PE.

The leakage current, in fact, will circulate through the person’s
body if she/he comes into contact with the enclosure. Thus, for safety
reasons, the bonding of high leakage current equipment must be as-
sured by enhancing the reliability of the PE. This can be obtained by
doubling its cross-sectional area, with respect to minimum permissi-
ble values, by using more than one PE conductor in parallel, and/or
monitoring its electrical continuity.
If the leakage current exceeds 3.5 A, the aforementioned IEC stan-
dard requires a warning label to be affixed adjacent to the equipment
power connection, indicating the necessity of connecting the protec-
tive conductor before switching on the supply.
4
To prevent electrical noise from interfering with sensitive elec-
tronic equipment, manufacturers may require a dedicated grounding
system. An erroneous interpretation of this condition may lead the
designer to install in the same building one separate ground for the
high-frequency electronic apparatus and one for the 50/60-Hz equip-
ment (Fig. 15.5).
The above arrangement is extremely unsafe and must be avoided,
because it does not assure equal potential between equipment under
fault conditions. In the case of ground faults on either ECP, persons
in simultaneous contact with both pieces of equipment are subject to
the whole earth potential. Each ECP becomes, in fact, an EXCP to the
other one.
In addition, separate earthing points, possibly energized at differ-
ent potentials under fault conditions, can cause circulation of ground
currents, and be the source of the electrical noise one wants to elimi-
nate.
Another issue pertaining to continuous high leakage currents may
be the inevitable nuisance trippings of protective residual current
devices. In this situation, the general protection provided by RCDs