Selection of Surge Protective Devices

Selection of Surge Protective Devices

Created by: Glen Zhu | Updated Date: April 12th, 2024

Selection of SPDs

How to select surge protective devices that fit my demand has been a question that bothers so many. Choosing a suitable surge protective device requires some effort to access the target protective equipment and learn knowledge of surge protective devices.

This blog offers a comprehensive guide to making a suitable selection of surge protective devices.

How surge protective devices are classified

Surges can arise due to various sources, with some of the most common causes being lightning strikes, power switching, short circuits…

They are primarily classified into 3 types and surge protective devices are made according to these 3 types of waveforms. Waveforms are important parameters to consider upon making the selection of surge protective devices.

Type 1 surge protective devices (SPDs) are based on 10/350 waveform standards. The 10/350 waveform refers to a surge waveform with a 10-microsecond rise time to peak current and a 350-microsecond decay time to half value.

10/350 waveform represents a specific type of surge event commonly associated with lightning strikes and is used as a standard for testing and rating surge protective devices. This waveform stimulates high energy surges that could greatly damage the electric devices.

Figure 1 – Iimp 10/350 µs waveform of type 1 surge protective device SPD

Type 2 SPDs are based on the 8/20 waveform standard. The 8/20 waveform refers to a surge waveform with an 8-microsecond rise time to peak current and a 20-microsecond decay time to half value.

Type 2 SPDs are commonly used for secondary protection at the main electrical panel to safeguard against moderate surges typically originating from internal sources or nearby lightning strikes.

Figure 2 – In and Imax 8/20 µs waveform of type 2 surge protective device SPD

Type 3 SPDs are based on the 1.2/50 waveform standard. The 1.2/50 waveform represents a surge waveform with a fast rise time of 1.2 microseconds to peak current and a slower decay time of 50 microseconds to half value.

They are designed for point-of-use protection, offering additional defense against low-level surges that may occur within a facility’s electrical system, providing supplementary protection beyond Type 2 devices.

Figure 3 – Uoc 1.2/50 µs waveform of type 3 surge protective device SPD

The 3 distinct waveforms offer great valuable guidance in the selection of surge protective devices.

Selection of surge protective devices: type 1 vs type 2 vs type 3

Due to the disparity of waveforms, different types of surge protective devices are varied in many aspects.

Type 1 SPDs are typically installed on the load center at the service entrance with high energy dissipating capacity. As the primary surge protective device in the system, type 1 SPDs are capable of absorbing the majority of surge energy, providing first-line defense for all the devices connected.

They are widely seen in applications such as industrial sites, power stations, Internet centers and important facility sites. A type 1 selection of surge protective devices could be your first choice in facing high lightning strike likelihood.

Type 2 SPDs are commonly positioned downstream from the main electrical panel, offering secondary protection against moderate surges originating from internal sources or nearby lightning strikes.

Acting as a crucial layer of defense, Type 2 SPDs help mitigate transient voltage spikes before they reach sensitive equipment. Type 2 SPDs handle moderate voltage surges and are commonly placed at distribution panels or specific branch circuits within a facility.

Type 3 SPDs are typically installed closer to individual equipment, such as power strips or plug-in devices, to provide supplementary protection beyond Type 2 devices.

Offering localized defense against low-level surges within a facility’s electrical system, selecting a type 3 SPD enhances the overall surge protection resilience by targeting specific equipment susceptible to transient events.

While not mandated by IEC 61643, incorporating Type 3 SPDs can significantly improve the resilience of sensitive electronics and ensure comprehensive surge protection in diverse environments.

Important parameters for optimizing the selection of surge protective devices

Surge Current Rating: Different types of SPD have different maximum discharge current ratings and there are distinct ratings for one particular type of surge protection device. The rating is often indicated as Iimp and Imax depending on which type of SPD it is. The chosen SPD must possess a surge current rating that exceeds the current capacity of the circuit that you design to protect.

A proper Imax rating guarantees that the target-protected equipment gets the fastest and most effective protection. SPDs with low ratings potentially may risk being overwhelmed when a surge occurs while high ratings may not respond quickly enough to provide protection.

Uc: Uc stands for maximum continuous operating, indicating the highest voltage that an SPD can handle without compromising its protective function. Similar to the surge current rating, the Uc is preferred slightly higher than the maximum continuous voltage of the protected system. An inappropriate rating can weaken the performance of your surge protective devices. This rating is typically the primary factor in the selection of surge protective devices.

Up: Up means the voltage protection level of a surge protective device. It represents the maximum voltage the surge arrester can clamp a surge down to. Typically, the selected Up is slightly lower than that of the protected system. For example, to protect equipment with a maximum operating voltage of 320V, then the Up rating of SPD should be chosen from the range of 280-300V.

In: In is another important parameter in surge protective devices, indicating the highest amount of current an SPD can safely handle. Surges not only bring high voltages but a substantial number of currents, a higher In rating brings about a greater ability to dissipate surge energy.

How surge protection works

Surge protection devices typically incorporate one or two nonlinear components, with metal oxide varistors (MOVs) and gas discharge tubes (GDTs) as two of the most common types.

Figure 4 – Cutaway Section Metal Oxide Varistor MOV

They can efficiently dissipate extra surge energy by altering their resistance. Under normal conditions, MOVs are in high impedance mode, letting current go through with minimal resistance.

When there is a surge event, their resistance dramatically drops, which allows SPDs to effectively dissipate surge voltage to the ground and protect equipment from potential damage.

MOVs have now become the most widely adopted in the component selection of surge protective devices.

In this process, a very important matter is that the utilized MOVs have to endure the high temperature that transient surges bring, otherwise SPDs could catch fire and cause catastrophic results.

At the same time, when MOVs reach the limit of temperature and pressure, they are designed to activate the trigger mechanism, extinguishing the arc to cut the connection with the surge protection device to avoid potential electrical hazards.

Figure 5 – MOV trigger-releasing mechanism

Selection of surge protective devices: the right MOVs

The selection of the diameter and thickness of MOVs depends on the required surge rating and voltage rating. The higher the rating, demanding larger and thicker MOVs.

In most cases, In (nominal discharge current) is half of the number of Imax (maximum discharge current) ratings. For example, A 34 mm MOV is rated approximately In: 20 kA – Imax: 40 kA.

MOV model selection guide:

If we are choosing MOVs for the AC power system operating at 385V, check the MOV data sheet and search 385V in the Maximum Allowable Voltage column, the matched MOV model number is 54S621-K-1.

  Model Number Maximum Allowable Voltage Varistor Voltage Clamping Voltage (max.) Rated Voltage 8/20μs Peak Current 10/350us Maximum Energy (Joule) Typical Capacitance (Reference)
Normal ACrms DC V1.0mA (V) VC Ip Reference In Imax Iimp 10/1000 @1kHz
(V) (V) min. max. (V) (A) (V) kA kA kA μs (pf)
54S241K-1 150 200 216 264 395 400 AC 125 25 60 12.5 1200 9375
54S431K-1 275 350 410 496 710 400 AC 250 2300 4900
54S511K-1 320 415 459 561 845 400 2650 4400
54S621K-1 385 505 558 682 1025 400 3100 3650
54S711K-1 440 585 644 786 1180 400 11.0 3150 3170
54S821K-1 510 670 738 902 1355 400 AC 380 or G 8.0 3450 2700
54S911K-1 550 745 819 1001 1500 400 3650 2500
54S951K-1 575 760 855 1045 1570 400 PV 20 6.5 3800 2400
54S102K-1 625 825 900 1100 1650 400 4000 2280
54S112K-1 680 895 990 1210 1815 400 4200 2050

If it’s a U-configuration SPD, the MOV for Type 2 DC Surge Protector SLP-PV600 (UCPV = 600V dc, In = 20kA PV, Imax = 40kA):

Check the MOV data sheet and search 670V in the Maximum Allowable Voltage column, the matched MOV model number is 34S281K.

Figure 6 – U-configuration diagram

Model Number Maximum Allowable Voltage Varistor Voltage Clamping Voltage (max.) Voltage 8/20µs Peak Current Maximum Energy (Joule) Typical Capacitance (Reference)
Normal ACrms DC V1.0mA (V) Vc Ip Reference In 10 times lmax 10/1000 @1kHz
(V) (V) min. max. (V) (A) (V) (kA) µs (pf)
34S511K 320 415 459 561 845 300 AC 250 20 40 1060 2650
34S561K 350 460 504 616 925 300 1150 2450
34S621K 385 505 558 682 1025 300 1250 2200
34S821K 420 560 612 748 1120 300 1280 2000
34S711K 440 585 644 786 1180 300 1280 1950
34S751K 460 615 675 825 1240 300 20 1280 1820
34S781K 485 640 702 858 1290 300 1350 1750
34S821K 510 670 738 902 1355 300 AC 380 or G 1395 1650
34S911K 550 745 819 1001 1500 300 1475 1500
34S951K 575 760 855 1045 1570 300 1485 1430
34S102K 625 825 900 1100 1650 300 1550 1350
34S112K 680 895 990 1210 1815 300 1700 1230
34S122K 750 980 1150 1320 1980 300 1750 1135
34S142K 850 1120 1315 1540 2310 300 15 1750 970
34S162K 1000 1320 1550 1760 2640 300 2000 840
34S182K 1100 1485 1700 1980 2970 300 2000 800

If it’s a Y-configuration SPD, the MOV for Type 2 SPD SLP 40-275 (UCPV = 1200V dc, In = 20kA PV, Imax = 40kA):

Check the MOV data sheet and search 670V DC in the Maximum Allowable Voltage column, the matched MOV model number is 34S821K.

Figure 7 – Y-configuration diagram

Mode l Number Maximum      Allowable Voltage   Varistor Voltage   Clamping      Voltage (max.)   Rated Voltage   8/20µs Peak Current   Maximum Energy (Joule) Typical Capacitance (Reference)
Normal ACrms DC V1.0 mA (V) Vc Ip Reference In 10times Imax 10 / 1000 @1kHz
(V) (V) mm. max. (V) (A) (V) (kA) µs (pf)
34S621K 385 505 558 682 1025 300 1250 2200
34S681K 420 560 612 748 1120 300 1250 2000
34S711K 440 585 644 786 1180 300 1280 1950
34S751K 460 615 675 825 1240 300 20 45 1280 1820
34S781K 485 640 702 858 1290 300 1350 1750
34S821K 510 670 738 902 1355 300 AC 380 or G 1395 1650
34S911K 550 745 819 1001 1500 300 1475 1500
34S951K 575 760 855 1045 1570 300 1485 1430
34S102K 625 825 900 1100 1650 300 1550 1350
34S112K 680 895 990 1210 1815 300 1700 1230
34S122K 750 980 1150 1320 1980 300 1750 1135
34S142K 850 1120 1315 1540 2310 300 15 40 1750 970
34S162K 1000 1320 1550 1760 2640 300 2000 840
34S182K 1100 1485 1700 1980 2970 300 2000 800

Here the chosen larger kA ratings are for redundancy and longer life.

Learn more about MOV Surge Protector

How to determine the selection of surge protective devices?

The selection of surge protective devices is a result of a combination of several factors.

System Requirements: Accessing your system’s specific needs and vulnerabilities is the first step in the selection of surge protective devices. Analyze the type of power supply, operating voltage, and potential risks to determine the appropriate specifications that will better protect your electrical system.

Installation Location:

The installation location significantly impacts the selection of surge protective devices. For locations nearing the main circuit or power entrance, type 1 SPDs are recommended for enhanced protection.

Conversely, when surge protection is provided for individual devices, it is inclined to select type 3 surge protection devices. Practical cases can be more complicated, the power rating for installation locations is as important as any other factors in the selection of surge protective devices.

Type 1 Permanently connected SPDs intended for installation between the secondary of the service transformer and the line side of the service equipment Installed without the use of external overcurrent protective devices
Type2 Permanently connected SPDs intended for installation on the load side of the service equipment overcurrent device
Type 3 Point-of-utilization SPDs Installed with a minimum conductor length of 10 meters (30 feet) from the electrical service panel

Certification and Compliance: Always reach out to surge protective devices that are proven by international standards and certificates such as IEC 61643-11, TUV-Rheinland. This is essential to guarantee reliability, performance, and adherence to safety regulations. Compliance with recognized standards guarantees that the surge protection device has undergone rigorous testing and meets quality benchmarks.

Another important consideration is that different countries may have unique regulations towards SPDs, so make sure your selection of surge protective devices aligns with these standards for optimal performance. By selecting a certified SPD, you can trust in its ability to mitigate surge-related risks and rest assured that your valuable equipment is cared for.

Customized or Standard: To choose between a customized or standard surge protective device depending on protective requirements and the level of protection needed. Customized solutions offer tailored protection based on specific needs, ensuring every aspect is coordinated for maximum effectiveness.

In contrast, standard SPDs provide general protection suitable for common scenarios. By evaluating your system’s demands and risk factors, you can determine whether to select a customized or standard solution.

While customized surge protection devices (SPDs) offer superior performance compared to standard SPDs, it is important to note that they come at a higher cost. In most cases, a standard selection of surge protective devices can meet most applications.

Budget: striking a balance between cost considerations and the need for protection is necessary when considering the selection of surge protective devices. Evaluating your budget constraints allows you to choose a device that offers optimal protection within financial limitations without compromising on quality or performance.

By aligning your budget with the level of protection required for the electrical system, you can make a cost-effective decision that ensures reliable surge protection while staying within budgetary constraints.

Warranty: SPD warranty showcases the product’s durability and provides long-term reliability, indicating confidence in the device’s ability to deliver sustained protection against transient surges.

By selecting surge protective devices with a long-term warranty, you can mitigate risks associated with equipment damage and downtime, knowing that you have reliable support in case of any issues arising during the warranty period

How to size surge protection devices

A critical rule in sizing surge protective devices is while oversizing an SPD for its application may not cause harm to a system, undersizing the SPD can lead to premature failure of the surge protection device.

Meanwhile, this does not translate to a higher rating bringing better surge protection. There is a common assumption that a larger panel size necessitates a higher kA device rating for protection. Another misconception is that doubling the kA rating from 100 to 200 implies twice the level of protection.

A key factor when determining the selection of surge protective devices is its “Uc” voltage rating. This value represents the voltage level at which the SPD activates and diverts surge currents away from your equipment.

Regardless of whether you have an AC or DC system, the ideal Uc voltage should be slightly higher (ideally within 0 to 10 volts) than the maximum continuous voltage that your equipment normally experiences, which ensures that the SPD remains inactive during normal operation but triggers protection during a surge event.

For better understanding, take this as an example: If we are going to protect a PV site operating at 550V, some may think selecting a Uc rating at 1500V SPD FLP-PV 1500 is suitable for pretty much any PV system that does not exceed that voltage.

The truth is the FLP-PV1500 will not damage your system, but you can’t get much benefit from it. In this example, if you are using a 1500V SPD on a 550V PV line going to your charge controller then the SPD will do nothing essentially even if you have a voltage spike of 1000 volts, and at that voltage, the 550V PV input will be very likely already be damaged.

In this scenario, the FLP-PV600 surge protective device is a better selection. Going Slightly over the 550V required rating, FLP-PV600 dissipates extra energy when the surge goes over 570 volts and provides timely protection effects better than FLP-PV1500.

When evaluating the selection of surge protective devices in PV systems, make sure the SPD has a Uc rating for DC voltage for that most of the SPDs are only designed to work within AC voltage.

Cascading different selections of surge protective devices if necessary

For in-depth surge protection, installing SPDs at all levels is a popular and effective method to provide layered defense.

Especially for areas that are more vulnerable to lightning strikes or SPDs that have a significant distance from installation location points, this practice is more necessary as a single SPD doesn’t always get you the required protection no matter how expensive it is.

As the device gets closer to the service entrance, the selection of surge protective devices is required for a higher rating for increased robustness. The layered defense-in-depth approach protects both the facility and critical loads.

A commonly used guideline for recommending kA per phase ratings is the “3-2-1 rule of thumb”: 300 kA for the service entrance. 200 kA for distribution panels, and 100 kA for branch panels per phase.

Figure 8 – Cascaded surge protection

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