16 | SAMLEX AMERICA INC.
SAMLEX AMERICA INC. | 17
• The DC side Positive and Negative outputs of these power supplies are isolated from
the chassis. As explained earlier, the noise currents are filtered to the chassis of the
unit and the chassis is connected to the Earth Ground through the Earth Ground Pin
of the AC power outlet receptacle. Avoid connecting (referencing) the DC Negative
output terminal of the power supply to the Earth Ground.
• Connect a ¼” wavelength of wire on the Negative terminal of the power supply.
Connect one end of the wire to the Negative terminal and leave the other end free.
The wavelength corresponds to the wavelength of the interfering frequency. (May
not be practical for long wave lengths).
[Formula: Wave length (Meters) = 300 / frequency in MHz]
6.6 COMBINED FILTERED NOISE CURRENTS FROM MULTIPLE
SMPS ON A BRANCH CIRCUIT MAY TRIP GROUND FAULT CIRUIT
INTERRUPTER (GFCI)
During malfunction or an accident, the metal chassis of a device may get energized to
unsafe voltage due to internal high voltage section coming in contact with the chassis.
If a person standing on Earth touches this energized chassis, a leakage current
proportional to the person’s skin resistance will flow through the person’s body to
Earth Ground. The leakage current through the body is higher when the skin contact
resistance is lower i.e. if the skin is wet or wounded. This leakage current does not
return to the power source but is dissipated in Earth Ground. A leakage current of
> 4 to 6mA could produce lethal electrical shock. Ground Fault Circuit Interrupter (GFCI)
is used for safety against electrical shock due to leakage. GFCI measures the difference
between the current sent to the load and returned from the load and will trip and
disconnect the power circuit if the difference is > 4 to 6mA. GFCIs are normally installed
in AC Branch Circuits feeding power outlets in wet areas like marine craft, RVs, spas,
hot-tubs, kitchens, washrooms, etc.
As explained earlier, RF noise filtration circuits in SMPS generate intentional Earth
Leakage Current. SMPS are used extensively as DC power sources in modern day
electrical / electronic devices e.g. Audio / Video / Computing devices, power supplies,
battery chargers etc. A single GFCI outlet / GFCI breaker may be serving multiple SMPS
loads and therefore, will be sensing the sum of all the Earth Leakage Currents and, if the
sum is > 4 to 6mA after connecting this unit, the GFCI will trip. In such a case, disconnect
other SMPS based device(s) being served by this GFCI one by one till the net leakage
current is reduced to < 4mA and the GFCI does not trip. Other solution is to power this
unit from a GFCI outlet / GFCI breaker that does not have any SMPS load or power from
an outlet that is not protected by GFCI.
SECTION 6
| Limiting Electro-Magnetic
Interference (EMI)
Summary of Contents for Samlexpower SEC-1223-SM
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