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General
What is the difference between the terms "Surge Arrester" and
"Transient Voltage Surge Suppressor (TVSS)"?
These terms are often used interchangeably and rather loosely. However,
the two terms have different meanings as determined by the National
Electrical Code® (NEC®) (www.nfpa.org), their UL listing,
and applicable IEEE/ANSI standards.
A secondary arrester refers to a surge protective device (SPD) that is
Listed for use before the primary disconnect to a facility (the service entrance panel), while a transient voltage surge suppressor (TVSS) is only Listed to be used after this primary disconnect. A violation of the NEC occurs if an SPD Listed under UL1449 as a TVSS device is connected ahead of the primary disconnect to a facility. On the other hand,
secondary surge arresters can be connected both upstream and downstream of
the main-disconnect.
TOP
What do the various product markings on a surge protection device (SPD)
really mean, and which are important?
Underwriters Laboratories (UL)
specifies certain minimum requirements. Some parameters which are
important and should be considered when selecting an SPD include:
- Nominal voltage – should
match the utility system voltage where the device is to be installed
- MCOV – The Maximum Continuous
Operating Voltage, this is the maximum voltage the device can
withstand before conduction (clamping) begins. It is higher than the
nominal system voltage.
- Single pulse surge current rating –
The maximum 8/20 surge current pulse the SPD is capable of surviving
on a single-impulse basis without suffering either performance
degradation or more than 10 percent deviation of clamping voltage at a
specified surge current. Listed by mode, since number and type of
components in any SPD may vary by mode.
- SVR – Suppressed Voltage
Rating. Under UL
1449, a TVSS device is assigned an SVR. This is the
clamping voltage of the device rounded up to one of 330, 400, 500, 600V,
etc. under specific conditions of lead length and injected surge
current. The SVR provides a very approximate measure of an SPD’s
clamping performance. It is important to stress that this test is
conducted at an extremely low current level of 500A 8/20 and as such
can not clearly predict the performance of an SPD under more realistic
conditions to which it is rated. It is also important to realize that
an SPD designed to be installed on a 110V system, which has an SVR of
400V, is not “better” than an SPD designed to be installed on 240V
which has an SVR of 600V. Also bear in mind that an SPD with the
lowest SVR can be rendered less than useless if incorrectly installed
with excessive lead lengths.
- Enclosure rating – ensures
that the NEMA rating of the enclosure matches the environmental
conditions at the location where the device is to be installed.
TOP
What are surges, transients, and
temporary over-voltages, and what are their typical characteristics?
These are all forms of overvoltages and
confused by the often loose and interchangeable use of terminology. The
significance of the terms usually relates to their understood duration.
For example, a transient is generally considered of very short duration
(<10 m) and relatively low energy content. Such electrical activity is
often characterized by voltage switching spikes, which in themselves
contain relatively little energy content, but are sufficient in voltage to
cause junction breakdown in the substrates of semiconductors and failure.
Surges (>10 m & <1 ms), on the other hand, have a greater energy
component and it is generally this which causes the damage and charring of
electronic components and appliances. SPDs are designed to protect
equipment against surges and transients. Temporary over voltages (TOVs)
are created by faults on the utility power distribution system and can
cause extensive damage since their time domain is much longer (ms to seconds
to hours). Note that while UL 1449
(Second Edition) ensures that the SPD
will not create a fire or safety hazard under these conditions, SPDs are
not designed to protect against TOVs.
TOP
What are multi-mode SPDs - why do I need L-L (line-to-line) and N-G
(neutral-to-ground) protection?
Multi-mode surge protection devices (SPDs) are devices which comprise a number of SPD components
within the one package. These “modes” of protection can be connected L-N,
L-L, L-G and N-G across the three phases.
TOP
Is the Joule rating of an SPD important?
While conceptually an surge protection device (SPD) with a larger energy rating will be better,
comparing SPD energy (Joule) ratings can be misleading. More reputable
manufactures no longer provide energy ratings. The energy rating is the
sum of surge current, surge duration, and SPD clamping
voltage.
In
comparing two products, the lower rated device would be better if this was
as a result of a lower clamping voltage, while the large energy device
would be preferable if this was as a result of a larger surge current
being used. There is no clear standard for SPD energy measurement, and
manufacturers have been known to use long tail pulses to provide larger
results.
Additionally confusing this issue is the possibility that the
rating is just the energy absorbed, diverted, or the sum of both. NEMA
LS 1
by specific omission does not recommend the comparison of SPD’s energy
ratings. Comparison of single shot surge ratings and let-through voltages
is considered sufficient.
TOP
What do waveforms such as 8/20 and 10/1000 mean and how are
they interrelated?
Lightning-induced surge currents are characterized as having very rapid
rising “front edges” and long decaying “tails”. To a first
approximation, the first number in each example of the above surge
waveforms signifies the time taken for the surge to reach 90% of its peak
value, and the second number, the time taken for this surge to decay from
its peak to its half way value. These times are measured in microseconds,
although convention does not require that this unit appear after the wave
shape. The ratio between these different waveforms is a complicated
function based on the integration of the energy content.
TOP
What about repetitive low-level transients?
Concerns have been expressed that electronic components can
deteriorate and fail prematurely under repeated low-level transient
over-voltage activity. While this can be the case, there is no
substantiated body of evidence to support this theory.
TOP
How can I protect equipment that operates on dc?
Protection of equipment connected to direct current (dc) sources or power supplies
generally involves installing protection at the alternating current (ac)
input to the power
supply. In some cases, protection might also be required on the dc side of
such power supplies, particularly if long cable lengths are involved. Most
SPDs will indicate if they are suitable for dc use and the maximum
operating voltage to which they are designed for use on.
TOP
What is NEMA LS 1?
NEMA LS 1 is a voluntary guide established by various manufacturers of
surge protection devices under the auspices of the National Electrical
Manufacturers Association, which primarily designates how surge protective
devices should be specified. This includes labeling and testing.
Purchasers of SPDs are encouraged to ensure their supplier adheres to this
guide. An example of the value of LS 1 is its ability to define key parameters in simple to understand terms. These include
functional description, protection modes, clamping
voltages, safety agency approvals, etc.
TOP
Speed of response - is this important in itself?
Yes and No. The ability of a surge protective device (SPD) or surge component to respond to a
voltage which exceeds its “turn-on” threshold, will govern the
residual clamping
voltage which the downstream equipment will be required
to withstand. If the device is too slow, the clamping voltage will be high
and the equipment may not be adequately protected. This said, too much is
often made of manufacturers of “speed-of-response”. What is more
important is the “clamping or residual voltage” performance of the SPD.
It is also worth noting that nanosecond transients can not travel far on
power wiring, thereby limiting their occurrence in practice.
TOP
What is distributed protection?
This is the process of coordinating protection between the primary service
entrance to a large facility and the internal branch distribution panels.
Generally a surge protective device (SPD) with high surge handling capacity is installed at the
service entrance while SPDs of lower surge ratings will be installed on
the branch panels or dedicated supplies feeding sensitive equipment. This
approach can be taken further to include point-of-use SPDs on long lines
where they terminate to sensitive or critical equipment. A further example
of such a distributed protection philosophy might include hardwired SPDs
at the main and sub-panels and additional plug-in protectors on select
equipment.
TOP
Where is the best place to fit protection?
Ideally, protection should be installed at the main service entrance as
close to the N-G (neutral-to-ground) bond as possible. This will ensure that surge energies
are routed to earth by the most direct path. In larger facilities where
distances between this primary protection and the equipment being
protected are long, it is also good practice to provide point-of-use
protection as close to the terminals of the equipment as possible.
TOP
What is UL 1449?
UL 1449 is the test procedure required to List (or Recognize) a Transient
Voltage Surge Protection product to Underwriters Laboratories, Inc.
specifications. It is
important to note that this is a safety, and not a performance,
specification. UL labels are required on every UL Listed or Recognized
TVSS,
indicating Suppressed Voltage Ratings per UL test criteria.
TOP
What changes did Edition 2 of UL 1449 bring?
Edition 2 of this standard requires surge protective devices (SPDs) to meet a number of new and more
onerous test regimens. In particular, a section entitled abnormal
over-voltage was introduced, requiring the safety and failure modes of SPDs
to be more extensively evaluated under elevated operating voltages.
TOP
What is the concern about sustained overvoltages?
Sustained abnormal overvoltages of the utility distribution system can
exceed the surge protective device's (SPD’s) Maximum Continuous Operating Voltage or MCOV. If this
happens, the SPD’s non-linear clamping elements will begin to clamp on
the crest of the utility voltage (120 times a second on 60Hz systems).
This will rapidly cause the SPD to accumulate heat, which further lowers
the onset of conduction point of the clamping elements. If this condition
exists for an extended time, a thermal runaway will begin and catastrophic
failure will result with the resultant potential of a fire hazard being
created unless the design had made provision to prevent the thermal
runaway. As a generalization, sustained over-voltages from distribution system problems are a major cause of
SPD failure.
TOP
Are underground cables protected from lightning?
Underground cables offer greater isolation to the effects of lightning
when compared to aerial cables, however they are still subject to induced
electromagnetic coupling of energy from nearby ground flashes. As such,
surge protection should be installed on facilities supplied by both,
overhead and underground, power feeders.
TOP
Grounding & Surge Protection
What is Ground Potential Rise (GPR)?
When a large amount of energy is rapidly deposited into the ground by a
cloud-to-ground lightning strike or by an electrical fault on a utility
power system, the ground potential at this injection point rises to a
higher level with respect to the more distant ground.
This has the effect
of creating a voltage potential gradient in the earth, which can cause
dangerous touch and step potentials to personnel exist. By creating an
equi-potential ground plane beneath a facility by electrically bonding all
separate “grounds” into a “system” or by burying ground mats and
meshes, this danger to personnel and equipment can be reduced. It is also
important to note that GPR is not only dangerous to personnel, it can also
cause damage to equipment – see below.
TOP
I have heard I should not use separate grounding systems?
Separate “grounds” or “ground references” can result in
damage to equipment during lightning activity. A cloud-to-ground discharge
can deposit extensive charge very quickly into the local ground mass of
the earth causing the ground at the injection point to rise up in voltage
with respect to more remote grounds. The resultant potential gradient
established in the ground means that separate grounds could rise to
different potentials resulting in a loop current and possible damage to
equipment referenced to these two different points. This phenomenon can
present itself in a more subtle way when equipment is connected to
multiple services.
An example of this can be a personal computer with modem where
connections are made to utility power and telecom line. If these two
services are not referenced together to create a common, equi-potential,
ground plane, damage can result. In fact, this is one of the more common
causes of equipment damage. A well-designed multi-port protector will
ensure such equalization between services at the equipment.
TOP
What is single point grounding?
It is important to ensure that ground potential differences are not
derived across equipment within a facility during ground potential rises.
One way to ensure this is to adopt a single point approach to grounding of
the equipment and services in the facility. This usually entails
referencing all equipment in the facility to a single ground bar (or a
number of ground bars that are solidly electrically bonded together), and
ensuring that this internal bonded system is connected to the external
ground system. “Single point grounding” refers to the single
connection between the internal facility ground system and the external
ground network. The external ground network can utilize multiple grounding
elements such as ground rods and/or counterpoises.
TOP
How do I measure ground resistance?
There are a number of techniques for measuring ground resistance, the more
popular being the “fall of potential method”. Measurements require a
ground resistance testing instrument and qualified personnel. With larger
facilities, it is important to take ground resistance readings by placing
the injection and reference electrodes in the “far field” –
essentially some few hundred feet from the inspection ground point.
This
will ensure that false or misleading results are not obtained by having
electrodes too close to buried parts of the overall ground system.
Clamp-on type instruments are not preferred in such situations due to the
possibility of large errors in results.
TOP
What ground resistance must I achieve?
This is probably one of the most often asked questions of grounding
experts. Again there is no one answer. As a rule of thumb, an effective
ground for lightning and surge protection purposes should be somewhere
around 10 ohms. Obviously this can be difficult to reach in poor soil
conditions and a cost benefit relationship comes into play. It is also
important to stress that no definitive applies to grounding values.
As an
example, it is pointless insisting that a contractor achieve a ground
resistance of precisely 10 ohms or less, when the testing method can be
subject to as much as 2 ohms variation depending on how the test rods are
laid. It is also worth keeping in mind that, the soil water content can
vary as much as 50%, depending on the season of the year. There are
“ground enhancing materials” which can be used to improve (decrease)
the local ground resistivity.
More important than the absolute value of the ground resistance, is to
ensure that all the equipment in the facility is referenced to an equi-potential
ground plane through adequate bonding. By ensuring this, all separate
pieces of equipment will raise to the same potential during a surge
condition. This statement can be illustrated by considering the Space
Shuttle, it is not “grounded” however all the equipment onboard will
be referenced to an internal equi-potential ground plane.
TOP
Some people say impedance and some resistance when talking about
grounding?
The lightning surge event is characterized by having very fast changes in
current and voltage, sometimes called the dv/dt and di/dt. In essence it
is a high frequency event and as such the ground system is better
considered as an ac impedance rather than dc resistance. The subject is
complicated and requires knowledge of transmission line theory and special
techniques to measure the effective impedance of the grounding system
under impulse condition. Enough said!
TOP
Installation
For more information on
installing SPDs, visit our industrial,
commercial,
or residential
installation sections.
Are primary SPDs alone sufficient for equipment protection?
There is no one correct answer to this question. For a small facility, a
single SPD installed at the service entrance panel can be sufficient,
while for a bigger facility it is usually necessary to adopt a distributed
protection philosophy where primary protection is installed at the service
entrance panel, and secondary protection at branch panels.
It can even be
necessary to include additional point-of-use SPDs if this equipment is
located some distance (100 ft or more) from the supplying panel. In
addition to providing protection to the power panels, the installation of
additional “multi-service SPDs” is particularly recommended. Such
devices provide their greatest benefit by ensuring multiple services
entering the facility (such as telephone, cable and power) are all tied to
the same ground reference. For more information on where to apply
SPDs, select the type of environment you're interested in: industrial,
commercial,
or residential.
TOP
Why do I need an SPD when I already have an isolation transformer?
Isolation transformers provide very good common mode rejection but do not
provide good differential (normal) mode rejection. In other words, a surge
superimposed equally on both the line (L) and neutral (N) conductors will see rejection by
the isolation transformer, while a surge appearing differentially between
the L and N conductors will pass through the transformer.
TOP
What is a practical surge rating for service entrance protection?
This is a difficult question and depends on many aspects including - site
exposure, regional isokeraunic levels and utility supply. A statistical
study of lightning strike probability reveals that the average lightning
discharge is between 30 and 40kA, while only 10% of lightning discharges
exceed 100kA. Given that a strike to a transmission feeder is likely to
share the total current received into a number of distribution paths, the
reality of the surge current entering a facility can be very much less
than that of the lightning strike which precipitate it.
The ANSI/IEEE
C62.41
standard seeks to characterize the electrical environment at different
locations throughout a facility. It defines the service entrance location
as between a B and C environment, meaning that surge currents up to 10kA
8/20 can be experienced in such locations. This said, SPDs located in such
environments are often rated above such levels to provide a suitable
operating life expectancy, 100kA/phase being typical.
TOP
Do SPDs devices need to be coordinated when several are applied in one
system?
Yes, since ANSI/IEEE C62.41 defines the service entrance as the most severe exposure, category C, a larger (kA per mode) SPD is recommended. Deeper in the facility where the exposure is lessened, categories B & A, smaller (kA per mode) SPD(s) is recommended.
TOP
How does installation affect the performance of SPDs? (What is the
effect of lead length, what size conductor is required, what is the real
risk of cross coupling between adjacent conductors, etc.)?
The installation of SPDs is often poorly understood. A good SPD,
incorrectly installed, can prove of little benefit in real-life surge
conditions. The very high rate-of-change of current, typical of a surge
transient, will develop significant volt drops on the leads connecting the
SPD to the panel or equipment being protected. This can mean higher than
desired voltages reaching the equipment during such a surge condition.
Measures to counteract this effect include locating the SPD so as to keep
interconnecting lead lengths as short as possible, twisting these leads
together. Using a heavier gauge AWG cable helps to some extent but this is
only a second order effect. It is also important to keep protected and
unprotected circuits and leads separate to avoid cross coupling of
transient energy.
TOP
What are the different power systems in use in the US and what are the
protection needs for each?
The US power distribution system is a TN-C-S system. This implies that the
Neutral and Ground conductors are bonded at the service entrance of each,
and every, facility or separately derived sub-system. This means that the
neutral-to-ground (N-G) protection mode within a multi-mode SPD installed at the service
entrance panel is basically redundant. Further from this N-G bond point,
such as in branch distribution panels, the need for this additional mode
of protection is more warranted. In addition to the N-G protection mode,
some SPDs can include line-to-neutral (L-N) and line-to-line (L-L) protection. On a three phase WYE system,
the need for L-L protection is questionable as balanced L-N protection
also provides a measure of protection on the L-L conductors.
Changes to the 2002 edition of the National
Electrical Code® (NEC®) (www.nfpa.org) have precluded the use of SPDs on
ungrounded delta power distribution systems. Behind this rather broad
statement is the intention that SPDs should not be connected L-G as by so
doing these modes of protection are creating pseudo grounds to the
floating system. Modes of protection connected L-L are however acceptable.
The high-leg delta system is a grounded system and as such allows for
protection modes to be connected L-L and L-N or L-G.
TOP
What are NEMA and IP enclosure (or environmental) ratings?
National Electrical Manufacturers Association (NEMA) (www.nema.org) Standards Publication 250-1997, "Enclosures for Electrical
Equipment (1000 Volts Maximum)" provides a comprehensive definition
of NEMA Enclosure Types
(www.nema.org) for interested parties. Those of interest to
installers of surge protection products in non-hazardous locations
include:
- Type 1 - Enclosures constructed
for indoor use to provide a degree of protection to personnel against
incidental contact with the enclosed equipment and to provide a degree
of protection against falling dirt.
- Type 2 - Enclosures constructed
for indoor use to provide a degree of protection to personnel against
incidental contact with the enclosed equipment, to provide a degree of
protection against falling dirt, and to provide a degree of protection
against dripping and light splashing of liquids.
- Type 3 - Enclosures constructed
for either indoor or outdoor use to provide a degree of protection to
personnel against incidental contact with the enclosed equipment. To
provide a degree of protection against falling dirt, rain, sleet,
snow, and windblown dust; and that will be undamaged by the external
formation of ice on the enclosure.
- Type 3R - Enclosures
constructed for either indoor or outdoor use to provide a degree of
protection to personnel against incidental contact with the enclosed
equipment. To provide a degree of protection against falling dirt,
rain, sleet, and snow; and that will be undamaged by the external
formation of ice on the enclosure.
- Type 4 - Enclosures constructed
for either indoor or outdoor use to provide a degree of protection to
personnel against incidental contact with the enclosed equipment. To
provide a degree of protection against falling dirt, rain, sleet,
snow, windblown dust, splashing water, and hose-directed water; and
that will be undamaged by the external formation of ice on the
enclosure.
- Type 4X - Enclosures
constructed for either indoor or outdoor use to provide a degree of
protection to personnel against incidental contact with the enclosed
equipment. To provide a degree of protection against falling dirt,
rain, sleet, snow, windblown dust, splashing water, hose-directed
water, and corrosion; and that will be undamaged by the external
formation of ice on the enclosure.
- Type 12 - Enclosures
constructed (without knockouts) for indoor use to provide a degree of
protection to personnel against incidental contact with the enclosed
equipment. To provide a degree of protection against falling dirt;
against circulating dust, lint, fibers, and flyings; and against
dripping and light splashing of liquids.
What are the new NEC® Short Circuit Current Ratings requirements?
The NFPA 70, NEC 2002 edition, Article 285.6, introduced a new requirement that the SPD be tested and labeled with a short circuit current rating equal to, or greater than the available fault current at that point in the system. At this point in time, TVSS devices passing through UL 1449, have inherently been Listed to
short circuit current ratings (SCCR) of 5-25 kA. This new requirement is currently being reviewed by UL 1449 as they consider how best to test to higher short circuit fault currents. Products can be Listed with higher ratings if the manufacturer has specifically requested this. This section will be revised once the new UL requirements are published.
TOP
Surge Protection with Filtering
Should I be concerned about really
fast transients?
Switching pulses and subsequent re-strikes in multi-stroke lightning, can
produce very fast transients, with rise times in the fraction of
microseconds. These can capacitively and inductively couple to equipment
and cause induced over-voltages. To eliminate such fast spikes, it is
usual to incorporate a level of filtering in the SPD device. This can
simply be a capacitor connected in parallel across the SPD’s output, or
it might be a true series LC filter – often called a two port SPD where
there are distinct sets of input and output terminals. SPDs incorporating
series LC filters generally provide superior filtering performance,
however they are more expensive and need to be sized for the continuous
load current. It should be point out that SPDs with so called
“filters” would more accurately be described as wave-shaping devices
as the filter’s prime role is to slow the very fast rate of voltage rise
dv/dt rather than to “filter”.
TOP
What is meant by dB attenuation in an SPD?
Generally this applies to surge protective devices (SPDs), which incorporate
filters. The dB of
attenuation stated is usually the point at which the filter has reduced
the incident transient by 3dB (or the voltage by a factor of 20). A more
effective surge filter will have a higher dB at a lower frequency. For
example, an SPD with 60dB attenuation at 30kHz is more effective than a
filter with 60dB at 100kHz. The attenuation can also apply to in-line,
series connected, type SPDs used in data communications. In such a case
the SPD should have a low attenuation (dB number) at the operating
frequency of the data system.
For SPDs protecting ac power circuits, it is common for SPD manufacturers
to quote the dB result at 100kHz, rather than the frequency at which 3dB
attenuation occurs. Rather than quote a single performance figure, a graph
of frequency response from 10kHz to 1MHz is more useful. Performance above
1MHz is of little value as at these higher frequencies large variations
will occur between installations. While many specifications call for
60-80dB at 100kHz, little practical performance benefit is obtained beyond
30dB.
The subject of SPDs with filters is not complete without reiterating the
point made in the preceding the term “filter” when applied to a power
SPD is confusing. Such “filtering” devices would more appropriately be
described as wave shaping devices and their performance expressed by the
reduction in dv/dt, which they present to the steep rising, edge of the
surge, rather than in dBs. Filters per sec are linear attenuation whilst
SPDs with filtering components exhibit a non-linear action and slope
attenuation.
TOP
What is Sine Wave Tracking technology?
This is a rather fancy term given to a surge protective device (SPD) which includes radio-frequency
interference (RFI)/electromagnetic interference
(EMI) filtering. All SPDs with capacitive RFI/EMI filtering exhibit sine wave tracking abilities.
TOP
Why should filters have air cored inductors?
Series-installed surge protective devices (SPDs), which comprise LC networks with a series ferrite inductor in the
line-side conductor, can experience saturation under the high current
levels during surge activity. Air gap inductors do not suffer from
problems of saturation, however they are more expensive to produce for the
same figure of inductance than ferrite wound inductors.
TOP
What is the difference between shunt protection, parallel filters, and
series filters?
See the filtering FAQs.
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