Frequently Asked Questions
From our friends over at NEMA...
What is the difference between the terms “Surge Arrester”, “Transient Voltage Surge Suppressor (TVSS)” and now “Surge Protective Devices (SPD)”?
Up until the third edition of the ANSI/UL 1449 standard was introduced and put into effect in 2009 there were various terms used when referencing devices intended to limit the effects of transient surge events. Surge arresters less than 1000 volts, commonly referred to as secondary surge arresters, were originally developed for and applied to the line side of the power distribution system to protect utility supplied equipment and building wiring. Surge arresters were intended to protect the system structure and not necessarily the connected equipment and loads.
A TVSS as it was defined must be applied to the load side of the main service overcurrent disconnect device. Unlike a surge arrester, a TVSS was intended to protect the sensitive electronics and microprocessor based loads by having a tighter ‘clamp’ or limiting effect on transient voltages.
With the adoption of ANSI/UL 1449 – Third Edition, both the terms “Secondary Surge Arrester” and “TVSS” were done away with and replaced with a more general term of “Surge Protective Devices (SPD)”. With this new term there was a need to identify the proper SPD for its intended application so UL also introduced the different Types of SPDs. For those applications similar to where a surge arrester would have been used on the line side of the system, it would now require a Type 1 SPD. In those applications where you once placed TVSS devices on the load side of the system, these same installations require a minimum of a Type 2 SPD. Bringing these devices under one ‘SPD umbrella’ and one test standard of ANSI/UL 1449 insures a consistent product.
The electrical industry has followed these changes. The NEC 2008 Sec 285 has updated their terminology eliminating the term TVSS and describes the proper application of Type1 and Type2 SPDs.
What do the various product markings on a surge protective device (SPD) really mean, and which are important?
Underwriters Laboratories (UL) requires certain markings be on any UL listed or recognized SPD. Some parameters which are important and should be considered when selecting an SPD include:
SPD Type – used to describe the intended application location of the SPD, either upstream or downstream of the main overcurrent protective device of the facility. SPD Types include:
Type 1– A permanently connected SPD intended for installation between the secondary of the service transformer and the line side of the service equipment overcurrent device, as well as the load side, including watt-hour meter socket enclosures and Molded Case SPDs intended to be installed without an external overcurrent protective device.
Type 2– A permanently connected SPD intended for installation on the load side of the service equipment overcurrent device, including SPDs located at the branch panel and Molded Case SPDs.
Type 3– Point of utilization SPDs, installed at a minimum conductor length of 10 meters (30 feet) from the electrical service panel to the point of utilization, for example cord connected, direct plug-in, receptacle type SPDs installed at the utilization equipment being protected. The distance (10 meters) is exclusive of the conductors provided with or used to attach SPDs.
Type 4– Component Assemblies -,Component assembly consisting of one or more Type 5 components together with a disconnect (internal or external) or a means of complying with the limited current tests.
Type 1, 2, 3 Component Assemblies – Consist of a Type 4 component assembly with internal or external short circuit protection.
Type 5 – Discrete component surge suppressors, such as MOVs that may be mounted on a PWB, connected by its leads or provided within an enclosure with mounting means and wring terminations.
Nominal system 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 typically 15-25% higher than the nominal system voltage.
Nominal Discharge Current (In) – is the peak value of current, through theSPD having a current waveshape of 8/20 where the SPD remains functional after 15 surges. The peak value is selected by the manufacturer from a predefined level UL has set. I(n) levels include 3kA,5kA, 10kA and 20kA and may also be limited by the Type of SPD under test.
VPR – Voltage Protection Rating. A rating per the latest revision of ANSI/UL 1449, signifying the “rounded up” average measured limiting voltage of an SPD when the SPD is subjected to the surge produced by a 6 kV, 3 kA 8/20 µs combination waveform generator. VPR is a clamping voltage measurement that is rounded up to one of a standardized table of values. The standard VPR ratings include 330, 400, 500, 600, 700, etc. As a standardized rating system, VPR allows the direct comparison between like SPDs (i.e. same Type and Voltage).
SCCR– Short Circuit Current Rating. The suitability of an SPD for use on an AC power circuit that is capable of delivering not more than a declared rms symmetrical current at a declared voltage during a short circuit condition. SCCR is not the same as AIC (Amp Interrupting Capacity). SCCR is the amount of “available” current that the SPD can be subjected to and safely disconnect from the power source under short circuit conditions. The amount of current “interrupted” by the SPD is typically significantly less than the “available” current.
Enclosure rating – ensures that the NEMA rating of the enclosure matches the environmental conditions at the location where the device is to be installed.
What are surges, transients, and temporary over-voltages, and what are their typical characteristics?
Although often used as separate terms in the surge industry, Transients and Surges are the same phenomenon. Transients and Surges can be current, voltage, or both and can have peak values in excess of 10kA or 10kV. They are typically of very short duration (usually >10 µs & <1 ms), with a waveform that has a very rapid rise to the peak and then falls off at a much slower rate. Transients and Surges can be caused by external sources such as lightning or a short circuit, or from internal sources such as Contactor switching, Variable Speed Drives, Capacitor switching, etc.
Temporary over voltages (TOVs) are oscillatory
phase-to-ground or phase-to-phase overvoltages that can last as little as a few seconds or as long as several minutes. Sources of TOV’s include fault reclosing, load switching, ground impedance shifts, single-phase faults and ferroresonance effects to name a few. Due to their potentially high voltage and long duration, TOV’s can be very detrimental to MOV-based SPD’s. An extended TOV can cause permanent damage to an SPD and render the unit inoperable. Note that while ANSI/UL 1449 ensures that the SPD will not create a safety hazard under these conditions; SPDs are typically not designed to protect downstream equipment from a TOV event.
What are multi-mode SPDs – why do I need L-L (line-to-line) and N-G (neutral-to-ground) protection?
Multi-mode surge protective 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. Having protection in each mode provides the protection for the loads particularly against the internally generated transients where ground may not be the preferred return path. In some applications such as applying an SPD at a service entrance where both the neutral and ground points are bonded there is no benefit of seperarate L-N and L-G modes, however as you go further into the distribution and there is separation from that common N-G bond, the SPD N-G mode of protection will be beneficial.
Is the Joule rating of an SPD important?
While conceptually a surge protective 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 misleading the end users.
Because Joule ratings can easily be manipulated many of the industry standards (UL) and guidelines (IEEE) do not recommend the comparison of joules. Instead they put the focus on actual performance of the SPDs with test such as the Nominal Discharge Current testing, which tests the SPDs durability along with the VPR testing that reflects the let-through voltage. With this type of information a better comparison from one SPD to another can be made.
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.
What about repetitive low-level transients?
Majority of all transients are being generated internal to a facility, 80%. These type of transients are typically of lesser energy and repetitive as they are generated from the different loads within a facility cycling on and off. It is important to understand where these low-level transients can be generated from, and to have a level of surge protection at that point.
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. However, with the increasing use of solar and wind generation there are growing needs for SPDs to provide a level of protection on the DC side. The SPD industry recognizes this and more and more SPDs intended for DC are being made available. When applying an SPD to a DC bus, the SPD needs to be marked and approved for these types of applications.
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 measured limiting 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 cannot travel far on power wiring, thereby limiting their occurrence in practice.
What is distributed protection?
Distributed protection is the process of coordinating protection between the primary service entrance to a large facility and the internal branch distribution panels. This is commonly referred to in the industry as layering, or cascading of surge protection. 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.
Where is the best place to fit protection?
Ideally, protection should always be installed at the main service entrance. This will ensure that externally generated 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 another layer of protection closer to the equipment to be protected. Point -of-use protection will provide the maximum level of protection possible.
What is ANSI/UL 1449?
ANSI/UL 1449 is the standard required to List (or Recognize) a Surge Protective Device to Underwriters Laboratories, Inc. specifications. UL labels are required on every UL Listed or Recognized SPD, indicating Voltage Protection Ratings (VPR), Short Circuit Current Rating (SCCR), the TYPE of SPD, the Maximum Continuous Operating Voltage (MCOV) and Nominal discharge Current Rating I(n)..
What is the concern about sustained overvoltages?
Sustained overvoltages are not Transient events and are the leading cause of SPD failures. For further information on sustained overvoltage, see IEEE C62.72
Are underground cables protected from lightning?
Underground cables do 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/or underground, power feeders.
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.
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.
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.
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.
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.
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!
Are primary SPDs alone sufficient for equipment protection?
No, from a small facility to a large facility, it is usually necessary to adopt a cascaded, or layered approach where primary protection is installed at the service entrance panel, and secondary protection at branch panels. Each facility requires individual analysis to determine the right protection to meet the needs of the equipment being used.It can even be necessary to include additional point-of-use SPDs if this equipment is located some distance from the supplying panel. A cascaded approach is recommended by the IEEE and this type of approach will provide the most effective surge protection throughout a facility. For more information on where to apply SPDs, select the type of environment you’re interested in:industrial, commercial, or residential.
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. Also, keep in mind the majority of transients are being generated by the loads within the facility, on the load side of these transformers. To minimize the effects of these internally generated transients from one piece of equipment to another an SPD should be placed.
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 C62.41.1-2002 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/mode or 200kA/phase being typical. Extremely large kA rated SPDs will not provide a better level of protection to the downstream equipment, they will however provide the same level of protection as a ‘smaller’ rated SPD for a longer period of time.
Do SPDs devices need to be coordinated when several are applied in one system?
Yes, since ANSI/IEEE C62.41.1-2002 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. For further information see IEEE C62.72-2007.
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.
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 discrete 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 unless the SPD has been specifically identified and approved for that.
What are NEMA and IP enclosure (or environmental) ratings?
National Electrical Manufacturers Association (NEMA) (www.nema.org) Standards Publication 250-2014, ” Enclosures for Electrical Equipment (1000 Volts Maximum)” provides a comprehensive definition of NEMA Enclosure Types (www.nema.org) for interested parties.
NOTE : the enclosure Type rating should not be confused with the SPD Type rating. These Type ratings are completely unrelated.
The enclosure Type and installation environment must be considered when choosing a “permanently connected” SPD. Cord-connected, Direct Plug-in and permanently connected receptacle type SPDs do not require an enclosure Type rating. The enclosure Type must coincide with the environmental conditions at the location where the device is to be installed. Enclosure Types 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 2008 edition, Article 285.6, has a requirement that the SPD be tested and labelled with a SCCR equal to, or greater than the available fault current at that point in the system. The ANSI/UL 1449-2006 (3rd Edition). Standard provides both the performance test requirements and the marking requirements for the SCCR.
Surge Protection with Electrical Filtering
Should I be concerned about really fast transients?
Switching pulses and subsequent re-strikes in multi-stroke lightning can produce transients, with very fast rise times in the fraction of microseconds. These transients can capacitively and inductively couple to wiring and cause over-voltages. To mitigate the potential damage from such fast transients, it is common to incorporate additional electronic components within an SPD to serve as a wave-shaping circuit (aka, electrical filtering or filter). This ‘filter’ can simply be a capacitor connected in parallel to the SPD’s surge protective components, or it might be as complex as a series multi-stage filter – often called a two-port SPD where there are distinct sets of input and output terminals with the power or data signal running through electronic componentry. A multi-stage filter within a two-port SPD may include a combination of parallel and series wired components designed to function as an electrical filter. Typically, for AC or DC power SPDs, these multi-stage filters use both capacitors (C) and inductors (L). SPDs incorporating series LC filters generally provide better filtering performance than parallel only filters; however, they are more expensive and need to be selected for the continuous load current (higher load currents will require physically large inductive components). It should be pointed out that SPDs with “filters” are more accurately described as wave-shaping devices as the filter’s prime role is to slow and attenuate the very fast rate of voltage rise (dv/dt) rather than to “filter” or remove.
What is meant by dB attenuation in an SPD?
There are two basic types of SPDs where this dB data typically appears. One is in SPDs for use in information & communication technology (ICT) and the second is for use in AC or DC power SPDs.
The dB of attenuation applies to SPDs used in ICT. In these products, the SPD should have a low dB value (attenuation) at the operating frequency range of the data system in use. This low dB value could indicate little to no adverse impact on the desired data signal. The dB number without any reference to a specific frequency or frequency range is of no value; therefore, one should always look for both the dB value and the referenced frequency or frequency range. In ICT SPDs it is common to see an operating frequency range stated with a dB value noted as ‘insertion loss’. For example, a coaxial SPD may have a frequency range of 0 to 3 GHz with an insertion loss of < 2 dB. The engineer or technician familiar with the ICT system in question must determine if the dB value and operating frequency range of the SPD is of concern.
Secondly, the dB of attenuation applies to AC or DC power surge protective devices (SPDs), which incorporate filters or filtering. The dB of attenuation represents a value that numerically shows the filter’s ability to reduce the transient and is typically stated from the point at which the filter has reduced the incident transient by 3 dB (or the voltage by a factor of 20) at a specific frequency. A more effective surge filter will have a higher dB at a lower frequency. For example, an SPD with 60 dB attenuation at 30 kHz is more effective than a filter with 60 dB at 100 kHz.
For SPDs protecting ac power circuits, it is common for SPD manufacturers to quote the dB value at 100 kHz, rather than the frequency at which 3 dB attenuation occurs. Rather than quote a single performance figure, a graph of frequency response from 1 kHz to 1 MHz is more useful. Performance above 1 MHz is of little value as at these higher frequencies large variations will occur between installations. While many specifications call for 60-80 dB at 100 kHz, little practical performance benefit is obtained beyond 30 dB.
What is Sine Wave Tracking technology?
This is a marketing term given to a surge protective device (SPD) which includes filtering. SPDs with capacitive filtering may exhibit sine wave tracking abilities. SPD manufacturers may offer products with varying filter performance that can exhibit better mitigation against fast-rising, low-level transient voltages.
Why should filters have air cored inductors?
Series-installed surge protective devices (SPDs), which include LC (inductive(L) capacitive(C)) networks with a series ferrite inductor in the line-side conductor, can experience saturation under the high current levels during surge activity. Saturation simply stated is where the inductive component loses is characteristic and desired inductance. It is often undesirable for the SPD filter to lose its inductive characteristic as the filter circuitry performance will degrade as the inductor approaches and reaches saturation. Air core inductors do not suffer from problems of saturation; however they are more expensive to build and are physically larger for the same value of inductance than ferrite wound inductors.
What is the difference between shunt protection, parallel filters, and series filters?
See the filtering FAQs.
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