The United States' National Electrical
Code (NEC) recently expanded Arc-Fault
Circuit Interrupter (AFCI) requirements (see "Arc-Detecting Circuit Breakers Will See
Wider Use" at www.electronicdesign.com, ED
Online 15682). This decision led to some passionate mail from Electronic Design readers
who said that AFCIs saved them from disaster
or were totally incompatible with small appliance motors (see the figure).
Ordinary circuit breakers trip on gross faults. Ground-fault circuit interrupters (GFCIs) trip on
an unbalance between line and neutral. But
AFCIs trip on arcs. They're now required for circuits in bedrooms and other critical areas in
new houses, and as of 2008, they will be
required throughout new homes.
AFCIs detect in-line arcing and arcing
between phases and between phase in neutral. Ordinary circuit breakers have relatively slow tripping times that may not catch arcs.
But AFCIs are supposed to respond to digitized current signatures that characterize dangerous arcs while ignoring arcs in appliance motors.
SAVED THE HOMESTEAD
Milan Trcka is an EE at an
engineering design firm that specializes in medical equipment.
While he was adding on to his 1953-vintage house, he had to
relocate the main service panel. The inspector accepted everything without question. The only requirement was to add AFCIs
to the bedroom sockets. So, Trcka decided to upgrade both
socket and light circuits. He was skeptical of the usefulness of
AFCIs to begin with. Immediately after the installation of two
$36 breakers, one started popping.
"I read all the available information on the Web running
into all manner of grousing, but I also saw an article explaining what the breaker is and what it does. Encouraged, I
started looking for the culprit. On and off it took me two
weeks to separate the circuits to discover that an improperly
installed GFCI socket in the bathroom (on the bedroom
socket circuit) compressed the hot wire against the grounded socket box," he said.
"Old Romex insulation fractured, crumbled, and allowed arcing to the box. I do not know how long this condition lasted, but
now it is fixed," he added. "In six months, neither of the two
breakers disconnected. I am a convert and I would encourage
retrofitting AFCI breakers, especially in older installations."
FALSE-TRIP NIGHTMARES
Then, there's the other
side of the issue. "Last October, my wife and I purchased a new
house that had AFCI breakers connected to circuits to the bedrooms per the current NEC," said Joe Blaschka Jr., PE. The AFCI breakers were manufactured by one of the major U.S. manufacturers
of electrical panels and switching equipment.
Blaschka expected them to be of high quality
and designed correctly.
Once the Blaschkas moved in, they tried
their top-of-the-line vacuum cleaner, which
was several years old but in good condition.
But using the vacuum opened the breaker so
fast, the motor didn't even have a chance to
get started. It was virtually instantaneous. Yet
when they plugged the vacuum into another
outlet that was protected by a standard thermal breaker, it worked fine.
Blaschka said that in his house, the outlets in the bedrooms as well as those in the
hallways in the bedroom area are AFCI-protected. So, powering the vacuum from an
outlet that doesn't have an AFCI is generally
inconvenient.
There is plenty of documentation on the
Web describing botched installations and pre2005 AFCIs that were recalled. But Blaschka isn't Joe Homemaker. He's a licensed Professional Engineer. He said he had
detailed conversations with the AFCI manufacturer, who simply
blamed the vacuum maker.
ENGINEERING A SOLUTION
In the end, Blaschka
bought a new vacuum for the bedrooms, and it only creates
false AFCI trips 10% of the time. But that's not a very satisfactory end. Evidently, AFCIs provide safety, but they also have a
threshold problem. Could universal appliance motors be fitted
with low-pass filters? Would they work? Is there an aftermarket
for external LP filters?
CONSUMER EDUCATION
The general public really
hasn't been getting the word about AFCIs. It's still hard to find
much consumer-directed online information about true and
false tripping.
One poster on a consumer-forum thread described how one
AFCI in his new house seems to be tripping at random. "I could
not find a pattern," he wrote. "Sometimes it would be once a
day while I was out at work, and then it would be weeks before
it would trip again. Sometimes, it could be twice in a day."
And all of this essentially happened without any loads on
the circuit. Other posters responded to tell him to replace his
AFCIs with ordinary breakers. Considering Milan Trcka's email about his poor, crumbling insulation, perhaps it's time
for this poster to start opening up his drywall!