Page 191 - Electrical Safety of Low Voltage Systems

174 Chapter Te n

FIGURE 10.4 Failure of the insulation between the secondary winding and the
grounded metal screen.

the higher voltage faulty circuit takes to clear the fault. Thus, in this
arrangement the protection against electric shock does not solely de-
pend on the SELV system, but also on the protective provisions of the
low-voltage system.
In general, ECPs in SELV systems must not be connected to earth,
4
to ECPs of other earthed systems, or to EXCPs. The absence of the
earthing connection will prevent the transfer of dangerous potentials
originating in other locations to the SELV equipment. In addition,
plugs and receptacles must have no protective conductor contacts,
as well as should not be able to enter/admit any receptacles/plugs
assigned to other non-SELV electrical systems.

10.2.2 Protection Against Direct Contact
The direct contact with one live terminal in SELV systems is harmless,
because the supply, having no reference to earth due to the electri-
cal separation, causes ground currents of very small magnitude to
circulate. Even the simultaneous direct contact with both source’s ter-
minals is not dangerous, as the resulting touch voltage is considered
harmless (i.e., V T ≤ 50 V a.c.). In addition, in dry conditions if the
nominal voltage of the SELV does not exceed 25 V a.c., or 60 V d.c.,
the basic insulation of live parts is not deemed necessary for protec-
tion against direct contact. However, in wet conditions the extra-low
voltage may be dangerous in the case of fault to ground of one of the
output terminals (Fig. 10.5). In this case, the direct contact may cause
electrocution.