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Surge protection FAQ

Eaton manufactures a comprehensive range of surge protective devices (SPDs) designed to meet the needs of virtually any environment or application. Before determining the optimal device for your facility, it is helpful to gain a general understanding of the importance of surge protection and the key factors to consider.  

Need help determining the best surge protection solution for you?

What is a surge?

Defined as a very fast, short-term, high voltage variation above 110% of nominal, a surge is most often sparked by lightning, line or capacitor switching, or the disconnection of heavy loads. Also sometimes referred to as transient voltages, these random, high-energy, electrical disturbances occur for just 1 to 10 microseconds, as illustrated in Figure 14. As such, surges should not be confused with longer-duration events such as swells or temporary over-voltages. 

Watch this video by Electrical Safety Foundation International (ESFI) on how surge protective devices protect electronics in your home. 


Why do you need surge protection?

Surges can destroy electronic components and result in data processing errors, data loss, equipment damage and electromagnetic interference. Before blaming your utility company, you should know that up to 80 percent of power surges occur on the customer side of the electric meter. When high-powered electrical devices within a facility — such as elevators, air conditioners, refrigerators, pumps, compressors and motors — are switched on and off, or operate in cycles, they can generate internal surges.

Because modern day electronics consist of microprocessors that rely on digital signals, even slight distortions on power or signal lines may disrupt the sensitive signal sequence. As electronic components have become smaller and more powerful, they have also become more sensitive. In fact, almost every electronic device now features a high circuit density with microchips that contain thousands of transistors on a single chip.  For these reasons, surge protection represents the standard technology to bolster reliability and uptime of microprocessors.

    What is the most effective way to use surge protection with a panelboard?

    Integrated surge protection is the most effective installation method for panelboards and switch boards, as it provides a number of key benefits over externally mounted applications. These include:

    1. Enhanced performance—Integrating the surge protective device into the electrical distribution equipment eliminates installation lead length, significantly improving performance through much lower let-through values.

    2. Ease of installation—The surge suppressor being factory installed and tested saves time and money for the specifier, as well as reduces future claims and problems for both the engineer and the customer.

    3. Reduced wall space─ Integrating the surge protective device eliminates two to three feet of wall space required by an externally mounted suppressor.

    4. Single source for warranty claims─ Should a problem occur, the customer eliminates potential conflicts between different manufacturers.

    5. Reduced installation costs─ Elimination of contractor fees for mounting surge protective devices.

    Eaton offers an aftermarket, integrated surge protector that can be retrofitted into an existing FD frame breaker slot. The RSPF is the only retrofit integrated surge unit in the commercial surge industry that fits into an FD frame breaker. 

    What is the difference between a surge protector and an uninterruptible power system (UPS)?

    Surge protectors (or suppressors) provide just that: a line of defense against surges, which are short-term high voltages above 110 percent of nominal.  They are often associated with lightning strikes and utility switching, but in fact 80% of surges originate inside a facility. These occur due to electrical switching or other disturbances created by various devices within the building.  Regardless of the source, the increased voltage from surges can damage the components of electrical systems such as computers, networks, and process control equipment. 

    Even if nothing is immediately destroyed, over time the increased strain can cause premature failure of expensive components. It’s important to note that surge protection will not keep your equipment operational during a blackout, but damaging surges occur much more frequently than power outages. A properly designed backup power system should always incorporate a cascaded approach to applying surge protection (i.e. a two layered approach) working in conjunction with a UPS.  The first surge unit, (upstream SPD) mitigates the brunt of the surge energy while the second unit (the UPS) reduces any remaining surge energy to an inconsequential level.

    A UPS delivers second-level protection against surges; it should never be considered a primary surge protection device. It also continually regulates incoming voltage and provides an internal battery that allows connected equipment to continue running even if the power supply is cut.  In order for your electronic devices to continue to function even if power is unavailable, you need a UPS, and often a backup generator. 

    How are surge protective devices categorized/differentiated?

    UL Listing

    Any surge protective device (SPD) you purchase should be certified by The Underwriters Laboratories (UL). This label ensures that the device has not only passed stringent UL safety requirements, but has been manufactured by a UL-certified company. The UL listing label allows contractors and electricians to install surge protection devices without concern for deviating from the National Electric Code (NEC) or impacting the listing of the panelboard or switchboard.

    Type 1 vs Type 2

    Surge protective devices are rated as either Type 1 or Type 2. The basic difference is that Type 1 devices are installed before the main device in the load center, while Type 2 are deployed after the main equipment. A Type 1 SPD is permanently connected at any location between the secondary of the utility service transformer and the service entrance primary overcurrent disconnect. Type 1 rated SPDs can also be installed anywhere on the load side of service entrance or on the low-voltage electrical system without requiring a dedicated fuse or breaker.

    Conversely, a Type 2 Surge protective device is a permanently connected on the load side of the service entrance primary overcurrent disconnect. Type 2 SPDs may or may not require the use of a dedicated fuse or breaker.

    It is important to note that Type 1 devices are dual-rated for Type 2 applications, as well, providing the highest ratings available for installation at the service entrance.


    An MOV has a maximum continuous operating voltage (MCOV), also known as the “threshold voltage,” this is the voltage at which the MOV resistance begins to drop. The MCOV is sized to be slightly higher than the system voltage so the unit can operate normally without shunting energy to ground.  Under a surge condition the resistance of the MOV rapidly drops becoming the path of least resistance. Conduction begins when the voltage across an MOV reaches the MCOV. As the voltage increases, the MOV's resistance drops dramatically, eventually approaching zero. A properly designed transient suppression device will divert transient current through itself and away from sensitive loads, splitting evenly between L-N and L-G MOV’s due to MOV matching and same MCOV. (For example, 100kA per mode = 200kA per phase.) When the applied voltage returns to normal, the MOV resistance rises, and the circuit returns to its original state.


    Short Current Circuit Rating

    Short Current Circuit Rating (SCCR) is the highest symmetrical fault current at the nominal voltage that equipment is rated to safely withstand (200 kA). Every electrical system has an available short circuit current. This is the amount of current that can be delivered by the system at the point of installation in a short circuit situation Typical available short circuit currents for the following types of devices are:

    • Residential – 5-10kA
    • Small commercial – 14-42kA
    • Large commercial/industrial – 42kA-65kA
    • Large industrial/utility/downtown in large cities – 100kA-200kA

    Peak Surge Current – kA Rating

    Peak surge current is the listed maximum current a device would be able to mitigate without suffering irreversible damage. The kA rating is a value that can be related to surge protector life expectancy. Increasing the kA rating of the surge suppressor does not significantly increase the protection performance of the surge device. A good analogy for this is the tread of a tire - as tires are used, the tread is worn off until the tire has reached its end of life. Surge devices that could be subjected to higher levels of surge should be sized for larger kA ratings. The kA rating is determined by the number of MOVs in the SPD. Increasing the number of MOVs in the surge protector increases the number of potential paths to ground.

    How does high resistance grounding impact surge protective devices?

    In today’s manufacturing facilities, ground faults can wreak havoc on production and process equipment, leading many organizations to deploy a high-resistance grounding (HRG) system.  Connecting between the neutral of the transformer secondary and earth ground, these systems effectively limit the fault current to 10A or less, allowing the system to continue operating normally, even under the ground fault condition.

    In today’s electrical systems, with many different grounding systems and various voltages, determining which SPD voltage configuration to specify can be confusing. Following are several guidelines to follow when specifying SPDs:

    • Only apply a wye (three-phase, four-wire) configured SPD if the neutral is physically connected to the SPD and if the neutral is directly and solidly bonded to ground
    • Use a delta (three-phase, three-wire) configured SPD for any type of impedance (resistive, inductive) grounded system
    • Use a delta (three-phase, three-wire) configured SPD for a solidly grounded wye system where the neutral wire is not pulled through to the SPD location
    • Use a delta (three-phase, three-wire) configured SPD if the presence of a neutral is unknown

    What are the applicable standards for surge protective devices?

    This table is a summary of applicable UL and IEEE standards for surge protective devices.


    (Current revision date)

    Purpose of standard/comments
    UL 1449 (1987)— Transient voltage surge suppressors (TVSS)

    1. Safety test (constructed of approved components in a safe manner).

    2. Suppressed voltage rating (let-through voltage using the IEEE C62.41 C1 test wave). Other IEEE recommended waveforms such as the C3 and B3 Ringwave are not tested by UL. 

    UL 1449 (2nd Edition 1996) 

    1. Additional safety tests. Test for other standards used to improve safety of products.

    2. Surge test. Let-through voltage tested at lower current than 1st Edition. 10 kA (IEEE Cat. C3) used for the first time; however, it was used only to see if products fail safely

    UL 1449 (2nd Edition 2007) 

    1. Stringent new safety requirements. New tests subject TVSS units to prolonged AC overvoltage conditions to ensure safe failure modes

    2. UL label changes to the wording of the short circuit current rating. 3. New Testing at 10, 100, 

    UL 1449 (3rd Edition 2009) 

    1. TVSS will now be referred to as SPD (surge protective devices).

    2. UL 1449 is now ANSI/UL 1449.

    3. Addition of four types of SPDs to cover surge arrestors, TVSS, surge strips and component SPDs

    UL 1283 (1996)— Electromagnetic interference filters  This safety standard covers EMI filters connected to 600V or lower circuits. The UL 1283 is a safety standard and does not include performance tests such as MIL-STD-220A insertion loss or Cat. B3 Ringwave let-through voltage tests.
    UL 497, 497A, 497B  Safety standard for primary telephone line protectors, isolated signal loops and surge protection used on communication/data lines. No performance tests conducted for data/communication lines.
    IEEE C62.41.1 (2002)  IEEE Guide on the Surge Environment in Low-Voltage AC Power Circuits. This is a guide describing the surge voltage, surge current, and temporary overvoltages (TOV) environment in low-voltage [up to 1000V root mean square (Rms)] AC power circuits.
    IEEE C62.41.2 (2002)  IEEE-recommended practice on characterization of surges in low-voltage AC power circuits. This document defines the test waves for SPDs.
    IEEE C62.45 (2002)  Guide on surge testing for low voltage equipment (ANSI). This document describes the test methodology for testing SPDs.
    IEEE Emerald Book Reference manual for the operation of electronic loads (includes grounding, power requirements, and so on).
    NEMAT LS-1  NEMA Technical Committee guide for the specification of surge protection devices including physical and operating parameters.
    NECT  National Electrical Code Articles 245, 680 and 800
    NFPAT 780  Lightning protection code recommendations for the use of surge protection devices at a facility service entrance.

    More videos by Eaton Surge Protection

      What is surge current per phase/per mode?

      TVSS devices are classified by the unit’s maximum “surge current” measured on a per phase basis. Surge current per phase (expressed as kA/phase) is the maximum amount of surge current that can be shunted (through each phase of the device) without failure and is based on the IEEE standard 8 x 20 microsecond test waveform.

      The per phase rating is the total surge current capacity connected to a given phase conductor. For example in a WYE system, L-N and L-G modes are added together since surge current can flow on either parallel path. If the device has only one mode (e.g. L-G), then the “per phase” rating is equal to the “per mode” rating because there is no protection on the L-N mode.

      Note: N-G mode is not included in the surge current per phase calculation.

      Is a surge protective device the same thing as a transient voltage surge suppressor (TVSS) and a Surge Arrestor?

      A surge protective device (SPD) and a transient voltage surge suppressor (TVSS) are the same device; they both reduce the magnitude of transient voltages. The only difference is that Underwriters Laboratories (UL) uses the term TVSS, while NEMA, IEC and IEEE refer to the device as an SPD.

      However, SPDs are different than surge arrestors, which are primarily used along transmission lines and upstream of a facility’s service entrance.

      Surge protective devices offer the following advantages over surge arrestors:

      • Low let-through voltage (better performance)
      • Longer life expectancy
      • Improved safety (less destructive debris if damaged)
      • Full monitoring capability
      • Internal fusing
      • Filtering capabilities to remove low level surge/noise

      What criteria are important when specifying a suppressor?

      No matter which surge protective device you select, the installation requirements and inspection are the most important factors of the specification. However, it is also important to consider let-through voltage and surge current capacity.

      Let-through voltage is the amount of voltage that is not suppressed by the surge protector and passes through to the load. It is a performance measurement of a surge suppressor’s ability to attenuate a defined surge. IEEE C62.41 defined specified test waveforms for service entrance and branch locations. A surge manufacturer should be able to provide let-through voltage tests under the key waveforms.  Published let-through voltage ratings are for the device/module only, and do not include installation lead length, which is dependent on the electrician installing the unit.

      Surge current capacity, on the other hand, is dependent on the specific application and the amount of protection required. Consideration include the geographic location of the facility, its susceptibility to transients, and how critical connected equipment is to the organization.

      What is impact of lead length on surge protective device performance?

      Installation lead length (wiring) reduces the performance of any surge suppressor. As a rule of thumb, assume that each inch of installation lead length will add between 15V and 25Vof let through voltage. Because surges occur at high frequencies, the lead length from the bus bar to the suppression elements creates impedance in the surge path.

      The actual let-through voltage for the system is measured at the bus bar and is based on two factors: the device rating and the quality of the installation.

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