Updated October 15, 2025
A SPD (Surge Protection Device) is essential for protecting your photovoltaic system from lightning strikes and surges. But do you know when it needs replacing?
In this article, we explain how to identify signs of wear, regulatory recommendations, and how to ensure your system continues to operate with maximum safety.
In technical discussions about standards and best practices for using string boxes in photovoltaic systems, one of the central points is the importance of the SPD (Surge Protection Device) in the installations. Experts frequently emphasize that using an external string box offers significant advantages, particularly regarding SPD maintenance.
One of the points addressed in these technical discussions is that, although the presence of an internal DPS in the photovoltaic inverter is beneficial, the adoption of an external DPS — properly coordinated with the internal one — provides an additional layer of security for the system.
Furthermore, the need for maintenance and eventual replacement of SPDs at the end of their useful life is always emphasized. A recurring question among professionals in the field is: how to identify the right time to replace the SPD? This is a fundamental question to ensure the continuous protection of equipment and the safety of photovoltaic installations.
What are the types of DPS?
Before we learn how to identify the right time to replace your SPD, let's remember that there are three types or classes of surge protectors that can be used in photovoltaic and general electrical installations: Class I, Class II, and Class III. Surge protectors are classified according to performance test methods, as per NBR IEC 61643-1 (Low-voltage surge protection devices).
Class I devices are recommended for locations with high exposure to direct electromagnetic surges, such as input lines to locations protected by SPDA (Atmospheric Discharge Protection System).
Class II and III devices are suitable for pulses of shorter duration and are recommended for places with less exposure, normally inside installations or places that will indirectly receive the effects of atmospheric discharges.
Class I surge protectors are typically of the “voltage switch” type, as defined in IEC 61643-1. These devices present high impedance (open circuit) when no surge is present and are quickly switched to the low impedance state (closed circuit) in response to a surge.
This category includes protectors based on spark gaps and other “short-circuit” switching devices, which have an open or closed type of behavior. Class II and III protectors are of the “voltage limiter” type, also according to the terminology found in the standard.
These devices present high impedance (open circuit) when no surge is present, but they continuously reduce impedance as the electromagnetic surge increases. These devices can be based on varistors (class II) or zener diodes (class III), which gradually close the bypass circuit to ground depending on the intensity of the surge present at their terminals.
Class II and III SPDs are used as additional protection in locations already served by a Class I SPD. In general installations, it is very common to use only Class II SPD in low exposure locations, with distances of no more than 10 meters from the equipment to be protected.
Class III SPD is always used close to the equipment to be protected. It offers the lowest level of protection and is used in situations of very low exposure or in coordination with devices from other classes.
Besides your solar system's DPS, what else needs to be replaced? Learn more about how this technology works: visit our blog and see all about solar energy!
Varistor: the active element of class II DPS
Class II SPDs are used in all photovoltaic systems, both in the DC string box and in the AC connection panel. Its primary function is to protect the inverter, an expensive and sensitive component of the photovoltaic system.
The DPS can also be found inside the inverter itself – in this case, it can be coordinated with an external DPS, which increases protection and makes maintenance easier, as we mentioned at the beginning of the article.
The active element of the class II SPD is the varistor, which is an electronic component that presents a variable impedance depending on the voltage level found at its terminals. The varistor practically appears as an open circuit when the voltage across it is low, but gradually converts to a closed circuit as the voltage rises.
This behavior makes it a voltage limiting device, capable of limiting overvoltages and diverting transient electrical currents present in electrical installations.
When to replace the DPS?
Now that we know what's inside a Class II DPS, let's answer the article's initial question: how do you know if your DPS needs to be replaced?
It is difficult to know when and how the DPS will act and how quickly it will wear out, as this depends on the frequency and intensity of surges caused by lightning strikes or other causes (such as electrical power distribution network maneuvers) in the location where the device is employed.
After repeated actuations, the varistor wears out and needs to be replaced, at which point its impedance and voltage-limiting capacity are reduced. A varistor-based SPD can conduct between 10 and 20 times its nominal current and one to two times its maximum current.
In commercial protectors, the varistor is located in a removable module, which can be replaced, as shown in the figure below. A status indicator window indicates the status of the device.
A green signal means that the device is still within its validity period, while a red signal indicates that the DPS no longer offers any protection and needs to have its varistor replaced.
The figure below illustrates how the DPS status signaling device works. More than signaling the status, the mechanism shown in the figure has the function of isolating the varistor from the electrical installation when it reaches the end of its useful life, preventing it from overheating and causing a fire.
The operating principle of the system is the opening of a contact by melting a solder point. When the varistor has its impedance reduced, a current begins to flow through the device even in the absence of voltage surges.
When the current intensity reaches a critical level, the contact opens due to the pressure of a spring, isolating the device and simultaneously causing the signal flag to change its position (green element in the figure below).
One detail draws attention in the figure above: the presence of a fuse for safe electric arc extinction next to the varistor. This element is not found in all surge protectors available on the market, but its presence is desirable to prevent the formation of an electric arc when the contact is opened. This feature provides additional protection for installations employing Class II DPS.
Conclusion
Knowing when to change the DPS is essential to ensure longevity of your photovoltaic system and avoid critical failures. Watch for signs of wear and follow the recommendations of technical standards. Want to learn more about electrical protection in solar systems? Check out our courses e specialized articles.
References
- NBR IEC 61643-1 – Low voltage surge protection devices – Part I
- DPS Specification – Obo Bettermann Bulletin – Eng. Sérgio Roberto Santos
- Surge Protection Devices (SPDs) – The Electrical Sector – October 2012 – Eng. Sérgio Roberto Santos
Comments (2)
Comments are moderated before publication.Comments should be respectful and contribute to a healthy debate. Offensive comments may be removed. The opinions expressed here are those of the authors and do not necessarily reflect the views of the author. Canal Solar.
The DPS has a red LED on, what could it be?
Thank you very much for this article, it is very enlightening and also highlights the importance of its use and its ability to protect circuits and their users and it also becomes very advantageous due to all its technical characteristics and, always observed its protections when prospecting electrical projects.