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.
Surge arresters less than 1000 volts have been called secondary surge arresters. Going forward they are now considered a Type 1 SPD and listed in Article 285 of the NEC. Surge arresters where originally developed and applied to 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.
Under ANSI/UL 1449-2006, the main difference between a TVSS and a secondary surge arrester is the location within the electrical distribution system which their respective Listing allows them to be installed. Secondary surge arresters (now known as Type 1 SPDs) are generally intended to be installed on the line side of the main service disconnect overcurrent device (service equipment) Their main purpose is to protect insulation levels of the electrical system. The Type 1 SPD is now subjected to additional safety test as dictated by the recent updates included in ANSI/UL 1449-2006 (3rd Edition). Further, these devices must pass a nominal discharge current level of 10 kA or 20 kA.
A TVSS (now known as a Type 2 SPD), by code (NEC 2008), must be installed on the load side of the main service disconnect overcurrent device (service equipment) and be 1000 volts or less. Their main purpose is to protect the sensitive electronics and microprocessor based loads by limiting the transient voltage.
Under ANSI/UL 1449-2006, both Type 1 SPDs and Type 2 SPDs are marked with a Short-Circuit Current Rating (SCCR). This rating is required by code to determine if an SPD is suitable for installation at a given location within the electrical system. Prior to ANSI/UL 1449-2006, there was no requirement for this rating on Type 1 SPDs. Under ANSI/UL 1449-2006, all SPDs are tested to evaluate the suitability of their end-of-service condition given various levels of available fault current when subjected to abnormal overvoltages. Prior to ANSI/UL 1449-2006, only Type 2 SPDs were required to undergo this evaluation.
Underwriters Laboratories (UL) specifies certain minimum requirements. Some parameters which are important and should be considered when selecting an SPD include:
- SPD Type - is 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 disconnect overcurrent device, as well as the load side, including watt-hour meter socket enclosures and 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 disconnect overcurrent device, including SPDs located at the branch panel.
- Type 3- Point of utilization SPDs, installed at a minimum conductor length of 10 meters (30 feet) from the electrical service panel; e.g. 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 the SPD.
- Type 4- Component SPDs, including discrete components as well as component assemblies. Must be tested for the installed location, i.e. Type 1, Type 2, Type 3.
- 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.
- Nominal Discharge Current (In) – is the peak value of current, selected by the manufacturer, through the SPD having a current waveshape of 8/20 where the SPD remains functional after 15 surges.
- VPR – Voltage Protection Rating. A rating per UL 1449 Third edition, 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.
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 UL 1449 (3rd Edition) ensures that the SPD will not create a safety hazard under these conditions, SPDs are not designed to protect against TOVs.
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.
While conceptually an 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.
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.
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.
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.
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.
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 can not travel far on power wiring, thereby limiting their occurrence in practice.
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.
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.
UL 1449 is the standard required to List (or Recognize) a Surge Protective Device 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 SPD, indicating Voltage Protection Ratings (VPR).
For further information on sustained overvoltage, see IEEE C62.72
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.
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.
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.
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.
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.
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.
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!
There is no single answer to this question. From a small facility to a large 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. Each faculty 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 (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.
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.
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/phase being typical.
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.
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.
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.
National Electrical Manufacturers Association (NEMA) (www.nema.org) Standards Publication 250-2008, ” 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.
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.
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”.
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.
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.
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.
See the filtering FAQs.
*(Note: UL 1449 Third Edition went into effect September 2009. Updates pending.)