5. The conducted RF noise from these power supplies is limited to the maximum
allowable levels by internal filtration. The filtered RF noise currents are by-
passed to the chassis of the power supply. The chassis is, in turn connected to
the earth ground pin of the AC input power cord (for Class 1 units). Thus, the
filtered noise currents are intentionally leaked to the earth ground. This is
termed as the “Earth Leakage Current”. For safety against electric shock, this
earth leakage current is also required to be limited. It will be seen that these
two requirements are conflicting.
NOTE:
In some cases, to prevent electric shock hazard due to abnormal
leakage current (like in marinas, spas, hot tubs, wet spaces etc.), the
AC outlet circuits / receptacles in these areas are served through a
GFCI ( Ground Fault Circuit Interrupter ).
This GFCI is normally set to
trip when it senses an earth leakage current > 5 mA. A single GFCI may
be serving multiple AC outlet circuits / receptacles and therefore, will be
sensing the sum of all the leakage currents of the devices connected to
these. As the switching power supplies have intentional leakage current
as explained above, it may trip a GFCI feeding multiple AC outlet circuits /
receptacles. In such cases, disconnect devices connected to the other AC
outlet circuits / receptacles served by this GFCI.
6. Following additional guidelines may be followed to reduce the effects of RF
noise:
a. Use additional appropriate AC radio frequency interference (RFI) power line
filter immediately before the AC input of the power supply. For example,
250VAC, 20A filter “F1700DD20” from Curtis Industries (www.curtisind.com) or
similar. Filtered, ferrite coated cord set (www.emceupen.com ) is another
choice. These cord sets, with integral line interference filters, reduce common
and differential mode interferences over a wide frequency range. Because
they are shielded, they are also effective against radiated interferences. In
addition to the built-in filter networks, the cable conductors are coated with an
RF absorbing ferrite compound. This provides additional attenuation at high
frequencies that is lacking in most regular LC filters. The RF absorption of the
ferrite-coated cable avoids resonance’s at high frequencies, reducing the
conducted and radiated RF noises even further
b. Use additional appropriate DC radio frequency interference (RFI) power line
filter immediately after the dc output of the power supply. For example, 80VDC,
100A filter from “DC” series by Corcom (www.corcom.com) or 80VDC, 100A
filter from “FD” series by Curtis Industries (www.curtisind.com) or similar.
14.
