GE Digital Energy
Power Quality
Integral SPD’s:
A Safe Solution with
Better Performance
1
Introduction
The increasing number of commercially available
Surge Protective Devices (SPD’s) has provided electrical
system designers with a wide range of options to choose
from. SPD’s are currently defined by the 2008 National
Electrical Code as Type 1 (Surge Arrester) or Type 2 (TVSS).
For the purpose of this paper, we will focus on SPD types
commonly referred to as Transient Voltage Surge Suppressors (TVSS) that are intended for use at locations
on the load side of the primary overcurrent disconnect
or main breaker of the electrical distribution system.
Of the many model types, ratings and suppression
technologies available, there are essentially two
distinctive installation methods that are frequently
specified for commercial and industrial applications.
These are SPD’s that are intended for either external
mounting or integral installation.
Externally mounted (also commonly referred to as
“Wall-Mount” SPD’s) are available from almost all major
manufacturers of SPD’s. These devices are typically
housed in a dedicated enclosure and are intended to be
connected to the power distribution system via electrical
conductors. These SPD’s are terminated at a dedicated
breaker, or in some instances directly to the phase
bus of the electrical panel. Externally mounted SPD’s
are designed to be installed by qualified electrical
contractors at the job site.
Integrally installed SPD’s are offered by many electrical
distribution equipment manufacturers. Integral SPD’s
are normally mounted within the electrical gear and
shipped to the job site as a complete package. Integral
SPD’s are factory installed and pre-wired to the electrical
panel bus, so in many cases there is no need for further
installation.
In recent years, a number of marketing publications
have been released by manufacturers and proponents
of externally mounted SPD’s about the potential hazards
associated with placing these devices inside of electrical distribution equipment. The focus of these publications are to create concerns about the possibility
of ancillary equipment damage that might occur in
the event of SPD failure. (It should be noted that SPD
failure is most often the result of product misapplication
or sustained abnormal phase voltage potentials). Many
of these papers attempt to discourage the use of integrally
installed SPD’s while citing IEEE or other industry recognized
standards as the authoritative basis for their position.
These documents, are carefully conceived, but typically fall
well short of legitimacy due to the omission or misinterpretation of key information from the referenced
2
standards. In many instances, the case against integral
SPD application is being made based on historical data
rather than taking into account the present status of
the industry and the regulations that are currently in
place to prevent the potentially destructive failure of
SPD’s for all applications. The focus of this paper shall
be to take a closer look at these standards, how they
relate to SPD’s, and what has been done to remedy
the potentially damaging effects of a failing SPD.
Both externally mounted and integrally installed SPD’s offer
a variety of options and features. There are advantages
and disadvantages for both design types. While externally mounted SPD’s offer a good solution for users that
would like to add surge protection to an existing electrical system, their installation and performance can
be affected dramatically by environmental variables
such as limited panel access, limited wall space, or the
level of experience that an installer has with such devices.
A knowledge of the characteristics of high-frequency
transient currents and the wiring techniques that must
be used to successfully convey these currents is necessary for the proper installation of any SPD. In contrast,
integral SPD’s are not restricted by panel access or
variations in installation. Since the SPD can be installed
very close to the conductor being protected in an integral
mounting arrangement, the connecting lead length is
often much shorter than when connecting an externally
mounted SPD to the same conductor. This reduction in
lead length contributes to improved SPD voltage clamping
performance over that of the externally mounted installation and results in better surge protection (see “The
Influence of Cable Connections on TVSS Performance”
for more details) 1. Equipment manufacturers who have
earned good reputations can usually be relied on to
employ persons who are trained and competent in the
proper wiring procedures for installing SPDs within
their equipment, making the quality of the installation
less of a factor with integrally installed SPDs. Some integral
devices are offered without a dedicated disconnect
feature which would require the panel to be powered
down in the rare instance that the SPD needs to be
serviced or replaced. However, de-energizing the
equipment may be preferred, even with SPD’s equipped
with disconnects, when the SPD assembly is located
adjacent to bare live parts.
2
Standards
IEEE 1100-2005 is an industry-recognized standard
that addresses recommended practices for the powering
and grounding of electronic equipment. While this
standard is not exclusive to SPD’s, there are relevant
sections with considerations for the application of such
devices within the electrical distribution system. Section
8.4.2.5 states that SPD’s “may be installed externally
or internally to the switchboard or panelboard.” And that
“panelboards are available that contain integrally mounted
SPD’s that minimize the length of the SPD conductors,
thus optimizing the effectiveness of the device.”
Additionally, IEEE 1100-2005 also cites IEEE preliminary
draft standard PC62.72 by stating, “there is concern
that failure of the SPD can cause collateral damage to
the switchboard or panelboards.” However, readers of
IEEE 1100 might not be aware that this statement was
not a direct quotation from PC62.72. Instead it was an
interpretive comment made on section 14.1 of PC62.72
that describes the failure mechanisms of Metal Oxide
Varistor (MOV) components that are commonly found
within most SPD assemblies.
PC62.72 has since been formally released as IEEE
C62.72-2007, and is the IEEE guide standard for the
application of Surge Protective Devices. The approved
standard does not, nor has ever contained any specific
language which would prohibit or warn against the
use of integral SPD assemblies.
Instead, IEEE C62.72-2007, Section 14.1 cautions that
MOV’s might expel hot metal fragments, conductive
ionized gases, or conductive smoke/soot upon reaching
a destructive thermal runaway condition. Of greater
importance, C62.72 also advises that manufacturers will
anticipate possible MOV failure and lessen or eliminate
these potentially damaging effects using a variety of
methods. These methods can include, but are not limited
to, containment of the failing components by a fortified
encasement, the addition of current limiting fusing, employing thermal cutoffs, using non-flammable or flame
quenching material/media, or any combination thereof.
3
MOV Design Considerations
Because of the many ways to prevent or contain the
potentially damaging effects of MOV’s, it is unlikely that
any two SPD manufacturers will employ identical design
approaches. Thus, it is important to realize that the
severity of failure is dependant upon the SPD design and
destructive failure prevention techniques and not the
location of the SPD installation. An effectively designed
SPD should not fail in a manner that compromises the
integrity of surrounding electrical equipment, regardless
of integral or external design types. All major manu-
facturers within the surge protection industry should be
aware of the potential for catastrophic failure of MOV’s
and should design for each application accordingly.
MOV’s will eventually reach an end-of-life condition
should the power system voltage elevate beyond the
Maximum Continuous Operating Voltage (MCOV) rating
of the SPD. MOV degradation can also lead to a permanent
failure condition by gradually reducing the voltage
clamping characteristic of the MOV until the clamping
level eventually coincides with the normal system voltage.
While degradation due to excessive surge energy remains
a possibility within lower surge energy rated designs,
it is not considered a normal occurrence in the majority
of today's high-energy SPD products.
Regardless of the cause of MOV failure, the end result
will be the same. When an extended overvoltage is
present, or if the MOV degrades below the nominal
operating voltage of the electrical power system, the
MOV will attempt to “clamp” the system phase voltage.
The MOV begins to cycle with the power frequency,
initially “clipping” the voltage peaks of AC sinewave.
The MOV attempts to dissipate the residual current,
resulting in a rapid heating of the MOV body. This initial
cycling/clipping condition of an MOV, attempting to
absorb the overvoltage energy, begins the end-of-life
stage of the MOV. This process is known as “thermal
runaway”. When the MOV can no longer dissipate this
energy, it will typically fail at a random location on the
MOV body. This location is sometimes referred to as
the “punch-through” site where the low impedance
breakdown and subsequent shorting of the failing
MOV develops. Once the MOV reaches a low resistive
state, it can rapidly become a fragmentation and/or fire
hazard if not immediately removed from the electrical
system fault current path. The level of available system
fault current will then drive the subsequent destructive
energy release from the MOV.
As stated in IEEE C62.72, section 14.1, SPD manufacturers
employ various techniques in an attempt to limit the
damage caused by a failing MOV. For example, designs
that include a combination of current limiting fuses and
thermal cutoffs rely on the current limiting fuses to interrupt high fault currents and the thermal cutoffs to
interrupt low fault currents by opening as a result of
radiant heat emitted from the body of the failing MOV.
However, coordination of fault interruption at intermediate
fault current levels can present a significant problem
for SPD design engineers. Thermal cutoffs are only good
for limited levels of available current and can only react
if the MOV can radiate enough heat directly at the cutoff
during the thermal runaway cycle. The higher the
available fault current, the more rapidly the failing MOV
3
will be driven into a low resistive state. In most cases,
this happens much too fast for even the closest proximity
thermal cutoffs to react. And once the MOV has shorted,
the initial energy release from the MOV will be concentrated
at the location on the MOV surface where the short has
originated. After this sequence occurs, the thermal cutoffs become ineffective. And if the fault current potential
is not significant enough to open the primary current
limiting fuse, the SPD could remain in a low resistive state
with an unstable MOV that continues to emit intense
energy in the form of flame, molten material, smoke, etc.
fault testing levels would reveal the vulnerability of many
SPD designs that did not have full current limiting coordination across ranges that are much lower than the
tested maximum short circuit interrupt levels.
During the UL test sequence for each specified fault
current, the SPD is purposely driven into a failure condition
by applying twice the nominal phase voltage that the
SPD is designed for. In order to obtain a passing grade,
the SPD cannot fail with any form of catastrophic results
that would include emission of flame, molten metal,
glowing or flaming particles through any openings,
charring, glowing, or flaming of the supporting surface,
tissue paper, or cheesecloth, ignition of the enclosure,
creation of any openings in the enclosure that result
in accessibility of live parts or loss of structural integrity.
If only limited or intermediate system fault conditions
are present, or if the SPD is not designed to effectively
deal with fault currents at less than the maximum short
circuit rating of the SPD, it is easy see how certain SPD
designs could cause damage to the surrounding electrical
equipment once the SPD housing has been breached
by a failing MOV.
4
The mandatory date for UL certification using the intermediate level fault current testing revisions of UL 1449
was February 9, 2007. SPD’s that are manufactured
after this date will not have authorization to apply the
UL mark unless UL witness testing has been performed
on each of the manufacturer’s representative SPD models
and voltage types and are found to be in compliance.
Safety Testing
UL1449 is the industry-recognized safety standard for
Surge Protective Devices. During reviews of the UL 1449
standard in 2005, the UL Standards Technical Panel
agreed that certain “blind spots” still existed within the
abnormal overvoltage/fault current testing program
that is defined within the standard.
As a result, UL 1449 was revised to incorporate additional
requirements for abnormal overvoltage testing to be
applied with available intermediate fault currents of
10, 100, 500 and 1000 amperes. These new testing
levels were made in addition to the maximum short
circuit current and limited current testing levels that
were introduced in the initial release of UL 1449 2nd
edition in 1996. The addition of these new testing levels
are intended to address conditions that SPD’s are likely
to be exposed to under a more extensive range of fault
current potentials that exist within typical power systems.
Many times, SPD’s are installed on secondary power
systems with power transformers or other power
generation sources that cannot produce the maximum
fault current potentials the SPD is rated for. As an example,
an SPD that has been rated for use on power systems
with a short circuit fault current potential of 200kA will
be connected to a power system that is only capable
of delivering a few thousand short circuit amperes.
Prior to the addition of the new testing requirements,
UL did not evaluate the SPD for safe current interruption
at these lower, fault potentials. The new intermediate
These recent and significant testing updates to the UL
1449 standard should help to assure those who specify
and purchase UL 1449 certified SPD’s that the SPD’s will
not become a fire or fragmentation hazard, nor cause
damage to surrounding gear, when the MOV’s or other
suppression components fail.
5
Conclusion
Integral SPD’s provide an excellent choice for new
construction applications by eliminating installation
variables, saving wall space and reducing the performance degrading effects of SPD connecting lead length.
Many proponents and manufactures of wall mount only
type SPD’s will argue that integrally mounted designs
are unsafe, or will cause substantial collateral damage
upon failing while often citing incomplete or misinterpreted passages from various standards and historical
documents as the basis. However, due to very recent
changes in safety testing standards, SPD devices that
are certified by UL to the latest revisions of UL 1449
will provide a safe and reliable solution for applications
where the SPD is factory installed within electrical distribution equipment.
1
The Influence of Cable Connections on TVSS Performance Marshall, E.; Hander;
Valdes, M.; Britton, J.; Jones, T.; Whitehead, J.; McIntyre, B. Industrial and Commercial
Power Systems Technical Conference, 2005 IEEE Volume, Issue, 8-12 May 2005
Page(s): 212-217
GE Digital Energy – Power Quality
701 E 22nd Street, Lombard, IL 60148 USA
800 637 1738 www.gedigitalenergy.com/tvss
www.getvss.com
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