| The Need for High Reliability in GFCIs |
When it comes
to saving a human life, any electrical equipment that one uses should be well insulated and
protected from the body relative to earth ground, so that there will be no
danger of electrocution or adverse effects from AC leakage current passing through one's
body to ground.
This is especially necessary if one is working in a damp environment or in
the rain, where the hazard is more prevalent.
All this in existence would be adequate for human protection, but what is often the case
is that insulation cracks due to age or abuse and leakage paths can form that would lead
through
the body to ground. This is a ground fault.
Even if one takes good care of his tools and cables, what might not be visible could
still
be a problem. This is why a device called a GFCI should be placed between the AC source
and
the tool or light. The GFCI will detect small leakage currents, and if they exceed 6
milliamps
(the present threshold level established by the Underwriters Labs) the AC will be
switched
off and the user protected until the problem is found and fixed.
This sounds well and good if the above advice is followed. But there is one more catch
to
make the story complete. The GFCI not only must do the job in detecting a leakage fault
in
an external tool to save a life, but it must not fail itself. Herein lies the discussion
about reliability. How reliable is the GFCI? Will it always be ready?
Reliability may be touted by the GFCI manufacturers, but how can we be sure?
Just because a GFCI is UL/CSA approved only means that is has passed specific tests written
by the safety organizations. The certifications do not address reliability of the devices.
Occasional testing of the GFCI could still allow electrocution in between the times that the test
is tried. So testing is not much help either. Just a little.
The U.S. Navy came to Power Electronic Systems, Inc. to find a way around the problem of questionable reliability in GFCIs. What could be done to save the lives of divers? The existing GFCIs they examined were not as guaranteed for reliability against failure.
Power Electronic Systems examined existing GFCIs and determined that the Navy was right: "reliability" left a lot to be desired. So a new design was made that did not have the same characteristics. This was called PharosTM.
That product has been modified and improved and is now being offered as the Sandy HookTM portable GFCI for home, industrial, marine, fire safety, and construction use.
The Sandy Hook is all-electronic, eliminating the faulty relays used for switching the
main power AC on and off. It is so reliable that a fault anywhere in the device either shuts down
the GFCI so it can't be used to supply power or it still functions properly. This says a
lot,
of course.
How can an all-electronic GFCI still function properly with a failure in it? The answer lies in the patented design and the accepted definition of a single point failure.
A single point failure is: what happens if only one part fails? How is the operation
affected? The switching mechanism of the GFCI is a very likely entity to fail, because it
can short the input and output terminals together. In this circumstance, the power cannot
be disconnected upon ground fault occurrence and the user can still be electrocuted. To
bypass this, the design must contain more than one switch element placed in series, so if
one shorts, the other can still function to open the circuit upon ground fault detection.
Since a single point failure occurs rarely, we are still protected with a high reliability
GFCI in our possession. That is to say, more reliable than most. Now, obviously, this
creates an argument as to how long reliability lasts, since given enough time and
conditions, eventually everything can fail, no matter how long we wait.
The federal government has been studying this for many years for military eqipment.
Their reasons may not always be life saving, but the equipment must work as required for a given
period of time before it is taken out of service. This is especially important in space
activities and missles, because once launched, there is no chance to repair the unit if it fails.
Reliability has been studied and quantified for various electronic and electrical
parts used in equipment, the same parts used in modern-day GFCIs. A mathematical
reliability
analysis can be applied to the GFCI and its lifetime can be estimated; the
"single point failure" definition
comes from this analysis.
To summarize: reliability can be calculated and applied to GFCIs. A high reliability
GFCI
can be produced to give users more assurance that there exists a device with more
protection from the dangers of electricity.
High reliability unfortunately comes with a higher price attached. The better the GFCI, the higher the price tag. But it is a small fee to pay for saving a life, a priceless value.