‰  NOW 35 WPM  ‰  TEXT IS FROM NOVEMBER 2012 QST  PAGE 65‰

GROUND FAULT CIRCUIT INTERRUPTER GFCI FAILURE I AM WRITING WITH REGARD TO
THE SEPTEMBER 2012 QST ARTICLE POWER CARTS FOR SCOUTS AND FIELD OPERATIONS.
THE MOST LIKELY CAUSE OF THE FAILURE OF THE GFCI TO RESET WHEN OPERATED
FROM AN INVERTER IS THE VOLTAGE PRESENT ON THE 120 V AC OUTPUT NEUTRAL
CONNECTION.  GFCIS ARE DESIGNED TO OPERATE IN A COMMERCIAL POWER SYSTEM
THAT, IN ACCORDANCE WITH THE NATIONAL ELECTRIC CODE, HAS A GROUNDED
NEUTRAL.  FIGURE 1 SHOWS A TYPICAL POWER SOURCE FOR A HOUSE.  A TRANSFORMER
LOCATED ON A POLE OR PAD IN THE NEIGHBORHOOD HAS A 240 V AC CENTER TAPED
SECONDARY WINDING, WHICH PROVIDES 120 V AC TO THE HOT LEADS ON EACH SIDE OF
THE CENTER TAP.  THE CENTER TAP IS CONNECTED TO THE NEUTRAL LEAD, WHICH IS
GROUNDED AT THE MAIN POWER PANEL INSIDE YOUR HOUSE.  THE ONLY VOLTAGE
PRESENT BETWEEN THE NEUTRAL AND GROUND IS THE RESULT OF VOLTAGE DROP IN THE
NEUTRAL WIRE, WHICH IS AT MOST A VOLT OR TWO.  THE TYPICAL 12 V DC TO 120 V
AC STATIC INVERTER USED IN AN UNINTERRUPTIBLE POWER SUPPLY UPS OR
STANDALONE INVERTER DOES NOT HAVE A TRANSFORMER FOR THE 120 V AC OUTPUT
BECAUSE THIS WOULD ADD GREATLY TO THE SIZE, WEIGHT AND COST OF THE UNIT.  A
ROUGH SCHEMATIC OF THE POWER PORTION OF AN INVERTER IS SHOWN IN FIGURE 2.
THE CONTROL, MONITORING AND INDICATION CIRCUITS AND RFI FILTERS HAVE BEEN
OMITTED.  THE INVERTER OPERATES IN TWO STAGES.  THE FIRST STAGE BOOSTS THE
‰  END OF 35 WPM TEXT  ‰  QST DE W1AW  ƒ