More on low-current interruption and the UL and IEC fuse standards
circuit designers naturally equate the labeled rating of a circuit
protection device to closely approximate the load current that will
cause that device to open. However, in circuit protection devices that
are designed to interrupt DC voltage, high AC voltage or to offer
extreme levels of short-circuit current limitation, this is often not
the case. Because of the extremely challenging nature of interrupting
high-energy faults, circuit protection designers often have to sacrifice
low overload protection. In most applications that need this type of
circuit protection, such as UPS systems or variable frequency drives
(VFD), this is a very acceptable trade off, as the UPS or VFD systems
use microprocessor or solid-state based controls to detect and interrupt
these low-overload currents. Using this type of low-overload
protection across tens, hundreds or thousands of junctions in a solar PV
system is very cost prohibitive, so designers use individual OCPDs.
is a perfectly acceptable design option as long as the OCPDs are
designed and certified as full-range fuses. A full-range fuse would be
defined as any fuse designed to interrupt currents between 110 percent
of labeled rating for UL and 113 percent for IEC and the labeled maximum
interrupting rating. For UL this would include all Listed fuses and
some Recognized fuses. When using a Recognized fuse, care must be taken
to insure that it is also a full-range fuse. For IEC this would
include all fuses with a characteristic designation that begins with “g”
(e.g. gPV and gR). Fuses with a characteristic designation that begins
with “a” (e.g. aR and aM), are not acceptable for DC PV circuit
protection and should not be used.
Other PV system circuit protection issues
addition to the critical coordination of string protection devices with
panels and the requirement for full-range protection, the UL and IEC
standards also address other unique electrical characteristics of solar
PV power systems, such as, difficult environmental condition and high
levels of current cycling.
Solar energy systems frequently
operate in harsh outdoor environments under temperature conditions that
can cause thermal shock. Temperature cycle testing, like those mandated
in U L 2579 and IEC 60269-6, helps ensure there is no significant
temperature drift (aging characteristic or other performance shift)
associated with a fuse’s operation. The UL and IEC requirements for
temperature cycle testing further restrict the fuses that can be used in
solar energy systems.
Solar energy systems use high voltage to
transmit power efficiently, which requires designs that are
substantially different from those powered by 120 or 240 volts. When
designing circuit protection and other elements of a solar energy
system, keep in mind the requirements for robust, long-life performance.
Although a five-year life might be acceptable for a DC power supply in
consumer electronics equipment, it would be completely unacceptable in
solar energy systems that are often expected to perform for 25-years.
Remember that electronics will be located outdoors, exposed to high and
low ambient temperatures, and experience ESD surges caused by nearby
lightning strikes. Therefore, it’s crucial to select components that are
robust, from board traces to bus bars to mechanical elements.
Enclosures should be both sturdy and waterproof. Surge suppression
devices should be installed in appropriate circuit locations.
are always under pressure to come up with cost-effective designs, but
taking shortcuts in PV system development can lead to problems. For
example, using a circuit breaker in a PV system combiner box to merge
disconnect and OCPD functions may sound appealing but using properly
approved devices would likely result in a less-than-optimum cost
structure and using inexpensive devices would likely result in safety
and reliability problems. In most cases, the cost of fuses, fuse
holders, and separate disconnects, when needed, will have a lower
initial cost, and be less costly to maintain. Many circuit breaker
manufacturers require annual exercising and re-calibration of their
products to ensure proper operation and to avoid invalidating the
manufacturer’s warranty. Circuit breakers must be removed from service,
allowed to cool, and re-calibrated according to the manufacturer’s
instructions. This annual maintenance requirement adds considerable
expense, difficulty and safety hazards.
This article focuses on circuit protection needed on the DC side of a PV system. However, as Figure 1
shows, many other locations require other circuit protection devices.
Protection of other components in the system from transient overvoltage,
ESD and AC over-currents also must be addressed by the system
designer. Thankfully, application of devices, such as MOVs, TVS Diodes
and AC fusing that protect from these threats is typical to other
systems that have been properly designed and protected for decades.
There are generally no differences in PV power systems for these
applications and safe diligent design work would be recommended.
About the Author
Gilman is Global Sales Engineering Director, POWR-GARD Products, at
Littelfuse, Inc. in Chicago, IL. He can be reached by email at
, or by telephone at 773-628-0714. He graduated
with honors with a Bachelor of Science in Mechanical Engineering from
Bradley University. In June of 2007 Dan Graduated from Northwestern
University’s Kellogg School of Management with his Masters in Business
Administration He is currently responsible for sales, marketing,
applications support, new product development and manufacturing support
for Littelfuse’s photovoltaic product offering. In this role Dan and
his team are actively engaged with OEMs and System Designers across
Europe, Asia and North America.