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What is a Surge Protective Device or Surge Arrestor (SPD)?

A device designed by Prosurge to limit surge energy to electrical equipment. It does this by diverting or limiting surge current. A Prosurge’s SPD is wired in parallel to the equipment it is intended to protect. Once the surge voltage exceeds its designed rating it “begins to clamp” and starts to conduct energy directly to the electrical grounding system. An SPD has a very low resistance during this time and “shorts” the energy to ground. Once the surge is over it “opens” up,so it does not trigger upstream circuit breakers.

What is important when specifying SPD?

When specifying SPD, submit a clear, concise specification detailing the required performance and design features. A minimum specification should include:

• UL surge rating

• Suppression rating

• Short circuit rating

• Peak surge current per mode (L-N, L-G, and N-G)

• voltage and configuration of electrical service

What is the UL Short Circuit Current Rating (SCCR)?

SSCR-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.

UL 1449 and the National Electric Code (NEC) require the SCCR (Short Circuit Current Rating) to be marked on all SPD units. It is not a surge rating, but the maximum allowable current a SPD can interrupt in the event of afailure. The NEC/UL 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. Prosurge SP/SMTMOV* series have been verified in UL lab at the highest lever 200KA test, which means that Prosurge SP/SMTMOV *can be for Type1/Type 2 application.


SP:Type 2 (class II) Surge arresters designed by Prosurge, more detail information please contact us or visit our website www.spd-china.com.

SMTMOV: Thermally Protected MOV, also named as TPMOV, TMOV etc. more detail information please contact us or visit our website www.spd-china.com.

What are the UL SPD Type categories and what do they mean?

Type 1 SPDs are permanently connected devices which bear a UL listing for installation at any location between the secondary of the utility service transformer and the service entrance primary disconnect. A Type 1 SPDs can also be installed anywhere on the load side of the service entrance and allowed to be installed anywhere on the low-voltage electrical system without requiring a dedicated fuse or breaker.

Type 2 SPDs are permanently connected devices which bear a UL listing for installation on the load side of the service entrance primary disconnect. Type2 SPDs may, or may not require the use of a dedicated fuse or breaker.

Type 3 SPDs are installed at a conductor length of 10 meters (30 feet) or greater from the electrical panel. These devices are typically cord connected, direct plug-in, receptacle type SPDs installed at the load equipment being protected.

Type 4 SPDs are components and assemblies that do not have a UL Listing.They might carry the UL recognized mark. Examples are Metal Oxide Varistors, MOV and gas tubes.

Prosurge SP/SMTMOV series have been verified in UL lab at the highest level 200KA SCCR test, which means that Prosurge SP/SMTMOV can be for Type1/Type 2 application.


SP:Type 2 (class II) Surge arresters designed by Prosurge, more detail information please contact us or visit our website www.spd-china.com.

SMTMOV: Thermally Protected MOV, also named as TPMOV, TMOV etc. more detail information please contact us or visit our website www.spd-china.com.

How does a Type 1 SPD compare to a Type 2 SPD?

A Type 1 SPD does not require a means of disconnect and can be connected to either side of the service entrance. A Type 2 requires a means of disconnect such as a circuit breaker. Prosurge provides UL listed Type1/ type 2 SPD, more detail information please contact us or visit our website www.spd-china.com.

What is the UL Nominal Discharge Current (In) rating?

The UL Nominal Discharge Current is defined as the peak value of 8/20 microsecond surge current conducted through the SPD. The SPD must still function after 15 applied surges. The highest nominal discharge current rating by UL is 20kA. A SPD for use at the service entrance in UL 96A Lightning Protection Systems must be UL Listed and have a Nominal Discharge Current rating of 20kA.

How are SPDs related to UL 96A?

UL 96A is the standard for Lightning Protection systems. For a building to meet UL 96A is must have a Type 1 SPD with a Nominal Discharge Current rating of 20kA installed at the service entrance.

What is Suppressed Voltage Rating (SVR) and Voltage Protection Rating (VPR) which marking on Prosurge’s SPD(Surge arrestor)?

SVR was part of an earlier version of UL 1449 Edition and is no longer used in the UL 1449 standard. The SVR was replaced by VPR.

VPR is part of the UL 1449 3rd Edition and is the clamping performance data for SPDs. Each SPD mode is subjected to a 6kV/3kA combination surge wave and its measured clamping value is rounded up to the nearest value based on table 63.1 from UL 1449 3rd Edition.

Do SPDs protect against direct lightning strikes?

A direct lighting strike is the most powerful and difficult surge to protect against. Prosurge recommend that Proper grounding and bonding of the electrical system and employing proper surge protection can protect sensitive equipment. A SPD with a higher single surge current rating will perform best against this type of event, if the unit is properly installed and the grounding system is adequate.The maximum single withstand surge current rating is defined in IEEE SPD Standard C62.62.

What are surges transients, and temporary over-voltages, and what are their typical characteristics?

Prosurge: 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.

Prosurge: Temporary over voltages (TOVs) are oscillatory phase-to-ground or phase-to-phase over voltages 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.

What is a practical surge rating for service entrance protection?

Prosurge: This is a difficult question and depends on many aspects including – site exposure, regional is okeraunic 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 anumber 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.

How does installation affect the performance of SPDs? (What is the effect of lead length, what size conductor isrequired, 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. Prosurge suggests that 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 bondpoint, 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.

Is “Response Time”a valid specification?

Prosurge: Response time specifications are not supported by any standards organizations overseeing Surge Protective Devices.IEEE C62.62 Standard Test Specification for SPDs specifically mentions it should not be used as a specification.

Are joule ratings a specification used for SPD?

Prosurge: In the past, some manufacturers have used joule ratings in their specifications. They are not considered a good indicator for SPD performance and not recognized by any standard organizations. Prosurge doesn’t support this specification also.

Can lightning destroy the photovoltaic system?

Photovoltaic systems are technologically highly sensitive and a direct lightning strike would definitely destroy it. There is also yet another hazard,as a lightning strike could create surge voltage near the solar power system and these surge voltages can also destroy the system. The inverter is the primary point in need of protection. Usually, inverters will integrate surge-voltage protectors into their inverters. However, since these components only discharge small voltage peaks, you should consider using surge protective devices (SPD) in individual cases. Prosurge provide high quality  PV SPDs for photovoltaic system.

How do I know which is the correct surge protection device that must be installed?

Choosing the right surge arrester(s) is a key factor to guarantee correct protection of the installation. A poorly designed Lightning & Surge protection system may lead to early ageing of the SPD and potential failure of the protective devices in the installation allowing damage to the primary systems up stream, thus defeating the rationale behind the protection being installed

Prosurge does not provide a set of rules and guides to support acorrect design of the protection system according to the application. However we follow the IEC and UL lightning and surge protection standards. With this in mind we provide a cascaded system as laid down in the rules of the standard, not the rules of Prosurge.

In the field of industrial applications, a standard practice is to install a cascaded protection system based on several coordinated protective devices installed at different stages (LPZ’s). The benefit of this strategy is the fact that it allows a high discharge capacity close to the installation entrance along with a low residual voltage (level of protection) at the main incomer of installation of sensitive equipment.

The design of such a protective system is, amongst other factors, based on the assessment of information such as existence of a lightning rod (Lightning Protection System) and type of incoming power supply lines, secondary primary equipment and data systems.

The solutions provide protection against either Transient or Permanent (TOV) overvoltages or against both of them (T+P) simultaneously.

The final product choice depends on parameters such as: type of installation, type of network disconnection (operation on MCB or RCD), auto reclosing, breaking capacity, etc.
Usually you can refer IEC61643- Low-voltage surge protective devices - Part 12:Surge protective devices connected to low-voltage power distribution systems -Selection and application principles

If you require further in-depth information or a design please contact us directly.

What are ANSI/UL 1449 Third Edition Versus IEC 61643-1 – Key Differences In Testing

The following examines a few of the key differences between Underwriters Laboratory’s required test for surge protective devices (SPDs); ANSI/UL 1449 Third Edition and the International Electrotechnical Commission (IEC) required test for SPDs, IEC 61643-1.

Short Circuit Current Rating (SCCR): The capacity of current with which the tested SPD can withstand at the terminals where connected, without breaching the enclosure in any way.

UL: Tests the full product at twice the nominal voltage to see if entire product is completely offline. The entire product (as shipped) is tested; including metal oxide varistors (MOVs).

IEC: Test only looks at the terminals and the physical connections to determine if they are robust enough to handle the fault. MOVs are replaced with a copper block and a manufacturer’s recommended fuse is placed in-line (external to the device).

Imax: Per IEC 61643-1 – The crest value of a current through the SPD having an 8/20 waveshape and magnitude according to the test sequence of the class II operating duty test.

UL: Does not recognize the need for an Imax test.

IEC: An operating duty cycle test is used to ramp up to an Imax point (determined by the manufacturer). This is meant to find “blind points” within the design when subjected to a high level impulse. This is conducted as a life expectancy or robustness test. The fuse needs to withstand Imax, and the test checks thermal stability of the SPD (after each duty cycle impulse bringing the SPD up to its maximum continuous operating voltage MCOV) and its physical condition。

I nominal: The crest value of the current through the SPD having a current waveshape of 8/20.

UL: I nominal test is similar to the IEC’s, however, the I nominal results do not link to a Up value (a value used internationally for electrical coordination). Instead, UL uses I nominal to determine a product’s Voltage Protection Rating (VPR). Levels are limited to a maximum of 20 kA. The SPD remains functional after 15 surges.

IEC: Does not limit I nominal testing to 20kA, however, the manufacturer’s selected In level is used to get a Up value, a value considered to be the protective performance of the SPD. This value is used for electrical coordination(ratings of building wire, equipment).

Therefore the goal of the manufacturer is to try to attain the highest Inominal level with the lowest Up results. Many manufactures opt to only test as high as 20 kA so they will appear to have a low Up.

Class versus category

UL: UL Type designation is a location designator with a difference to the way I nominal is tested (for device which provides SCCR needs to be included and survive when doing I nominal testing).

IEC: Designates certain tests as a class I, II, or III. Class designation between I and II has to do with the impulse applied – Class I; an I imp test (10x350) and a class II – 8 x 20 μs.

The IEC designates certain tests as a class I, II, or III and can beconfused with UL’s Type I, II, III, or IV designations. There is some validity to both identifying the product’s approved installation location (UL) and applying a more robust impulse/waveform to those products that will be installed in harsher locations (IEC).

Waveforms: A graph of an impulse wave that shows its shape and changes in amplitude with time.

UL: Recognizes the 8 x 20 μs waveform.

IEC: The IEC incorporates 2 waveforms into their test, the 8 x 20 μs which is used for class II testing to represent surges induced onto power lines. And the 10 x 350 μs waveform which is used for class I testing that represents partial or direct lightning currents (due to building or power line strikes).The IEC also uses other ring wave type waveforms for point of use (class III) tests.