DPS: the airbag of photovoltaic systems

Let's talk about safety and quality. Two things that always go together and are important when we talk about DPS (Surge Protection Devices), the air bag of photovoltaic installations.

Imagine the following scene: you go to a car store and the salesperson offers two options of the same vehicle with different safety accessories.

Option 1 comes equipped with a air bag of a well-known brand and with an international certificate of conformity. Option 2 has a air bag generic, slightly cheaper and without any certificate.

The seller argues that the air bag It's going to be used, so you can take it with you. You are satisfied with the recommendation and leave the store happy with the car from option 2, with the certainty of having made a good deal.

The scene narrated above seems impossible, but it happens every day in the photovoltaic solar energy market: the seller pushes a string box which has a surge protector device that, in addition to being harmless, can be dangerous.

DPS can be considered the air bag of photovoltaic systems for two reasons. Firstly, it is a safety item. It is the device that handles the shock and protects the equipment when an atmospheric discharge occurs close to the installation.

Secondly, it is an item that cannot be tested. You can't fire the air bag to find out if it works. If you have a air bag in your car, it better be the best air bag from the market. The same thing can be said about the DPS that equips the string boxes (boxes strings) of photovoltaic systems.

What's inside string box? Basically it is a box or an electrical panel where connections are made to the strings photovoltaics, that is, direct current circuits.

One of the string box basic has at least one disconnector switch and one DPS. If the string box is prepared to receive three or more strings, fuses will also be needed to protect the photovoltaic modules against reverse current.

Why is the market flooded with string boxes assembled with generic components?

Many photovoltaic kit suppliers provide good quality modules and inverters but slip up when it comes to string box, whose cost corresponds to a tiny fraction of the total value of the system.

If we look inside the string box Let's realize that DPS corresponds to a small part of the cost of this box. Moral of the story: the cost of DPS is practically negligible in a photovoltaic system and the savings are not justified.

The DPS seen from the inside

The DPS is a relatively simple device. All DPS on the market are (almost) the same, except for small details that make all the difference. The device is basically a plastic box that contains a varistor or in some cases a spark gap. The most used in photovoltaic systems only has the varistor.

The function of the varistor is to divert the electrical current and absorb the energy from voltage surges, thus protecting the equipment that is connected to the electrical installation. Voltage surges are normally caused by atmospheric discharges, a phenomenon that often causes unprotected electronic devices to burn.

Differentiating the surge protectors on the market

Much of what can be said about surge protectors from one brand or another, without proper regulatory guidance and without laboratory evidence, can border on simple “guesswork”.

According to information I gathered from my colleague Sérgio Santos, electrical engineer at Lambda Consultoria, the installation standards involving DPS are the ABNT NBR 5410: 2008, Low voltage electrical installations and ABNT NBR 5419: 2015, Protection against atmospheric discharges.

The DPS product standard is ABNT NBR IEC 61643-1:2007 Low Voltage Surge Protective Devices Part 1: Protective Devices Connected to Low Voltage Power Distribution Systems – Performance Requirements and Test Methods. In Brazil we only have the first part of this standard, but there are several other parts in the corresponding IEC standard.

He also added that DPS are not products with compulsory certification by INMETRO (National Institute of Metrology, Quality and Technology), like circuit breakers, for example.

For this reason, several DPS manufactured in Brazil are said to 'comply with the ABNT NBR IEC 61643-1:2007 standard', 'according to the ABNT NBR IEC 61643-1:2007 standard'. But the manufacturer who certified his product should write 'certified according to ABNT NBR IEC 61643-1:2007' and present the corresponding certificate.

In summary, according to the above, there is an international IEC standard that is partially adopted in Brazil, but certification is not compulsory. So, what we observe is the entry of products into the Brazilian market without any criteria. The manufacturer may simply declare that it meets the standards or may declare nothing.

It is important for consumers to request a certificate of international conformity for products, as we do not have a compulsory certification program for these devices in Brazil. Good manufacturers, who have nothing to fear, will certainly volunteer to provide this documentation.

Below we see an example of a certificate of conformity for a DPS for photovoltaic applications, issued by an accredited international laboratory.

Arc extinguishing and fire protection device

An important aspect of commercially available surge protection devices is their ability to extinguish the electric arc and prevent fires.

Devices that do not have this feature would, in principle, require the use of external fuses, which makes installations more complex. If these fuses are not available, it is recommended (if not mandatory) to use an internal device.

This problem of arc extinction and the possibility of fire is related to the end of the device's useful life. The DPS internal varistor has a limited lifespan and can only be used a limited number of times.

After several actuations absorbing electrical surges, the varistor begins to have its impedance reduced, causing a current leak through the device.

This leakage current will at some point reach a critical level and could cause a short circuit and melt the DPS, as occurs quite frequently in installations.

When melted, the DPS may cause an electric arc, creating a principle of fire. This situation must be avoided in photovoltaic installations.

To avoid the problems mentioned above, the leakage current through the varistor must be interrupted in some way. In other words, the varistor must be eliminated from the circuit until it can be replaced with a new one.

The varistor removal system found in practically all surge protectors on the market consists of a metal contact associated with a spring. The circulation of leakage current melts a solder point and allows the contact to open like a switch. The system is very ingenious, but there are some dangers here.

The melt rupture method is not very predictable and depends on the quality of the weld. The DPS devices I have had the opportunity to open appear to be handmade – they have a homemade look and inspire little confidence.

Okay, you can say “It’s just your opinion.” True, it's just my opinion, as I haven't carried out tests with these devices. And it's not even my job to do it. For this, there are technical standards and accredited certification laboratories.

After my opinion in the previous paragraph, let's analyze a fact (which is not an opinion, but a true fact): opening the contact shown in the figure may not be fast enough to prevent a fire.

Opening the contact, as seen in Figure 4A, will cause an electric arc due to the nature of the circuit. We are talking about a direct current photovoltaic system, with voltages typically between 500V and 1500V. We have the perfect scenario for a fire here and it will almost always happen, sooner or later.

No one buys a DPS thinking about the day it will stop working, just as no one buys a car thinking about using the air bag. But you have to consider: when you need DPS, it better work.

And it's also good that he doesn't put his life or assets at risk. I still don't know of reports of people who died because of poor quality DPS, but there are lots of systems burning, with great losses for their owners.

The figure below shows a DPS of the “D” brand, of superior quality.

NOTE: I will not mention brand names in this article, this way I maintain the impartiality of my opinions and avoid my email inbox receiving the usual legal notices from manufacturers who feel uncomfortable. Unfortunately, there are companies that invest more in their legal department than in their engineering department. Nothing against lawyers (they are necessary in many cases and, as with the airbag and DPS, you better have a good one), but engineering is key in this case. Companies that do not invest in research and development and are afraid of criticism and technical opinions should be banned from the market. And no one better than the consumer to provoke this ban.

In Figure 6 we see the internal organization of a “D” brand DPS. In photo 6A we see that the same method of loosening by melting solder is used. The difference is what happens afterwards, when contact is interrupted.

In this DPS model, a rotating part, moved by the force of the spring, pushes the metallic contact to the right side and fills the space through which the electric current could pass. When pushed to the right, the metal contact activates the auxiliary circuit. bypass (illustrated in the figure below), where we find a fuse.

This fuse will then interrupt the electrical current safely, as the electric arc will occur inside its housing, without the risk of causing the DPS to melt or fire.

Buying a pig in a poke

Respecting the popular maxim, you should be careful with what you find on the market. In addition to the differences shown in the previous section, we still have to deal with the lack of commitment to the truth from some manufacturers.

In the absence of compulsory certification for this product in Brazil, anything can happen in the free-for-all market – the rules of the game are: there are no rules. Below, we see two products from the brands “D” and “S”. They are both cute on the outside, but one of them is ordinary on the inside.

In Figures 9 and 10 we see excerpts from the “D” and “S” brand surge protector catalogs. The first catalog mentions the product's international certificates, provided by accredited laboratories.

The second mentions the IEC 61643 standard, without specifying whether the standard is followed and without providing any indication that the product has a certificate of conformity.

In Figures 11 and 12 we find information about the internal electrical current interruption devices used (or supposedly used) by the two manufacturers. Manufacturer “S” claims to have a system very similar to that of manufacturer “D”, which is patented.

In addition to the information being false, as the “S” brand product clearly does not have this feature – as is evident in the images in Figure 4 – the figures presented in this manufacturer’s catalog appear to have been shamelessly copied from its competitor “D”.

The similarities between the figures are striking. What can we say about a manufacturer that presents false information in its own catalog, claiming to have technology that it does not have?

Conclusion

For good experts, half words are enough. If words are not enough, we also have the certificate of conformity, the technical standard, the catalog data and the photographs of the products that were presented in this article.

I ask the reader these final questions:

• Is it worth using generic surge protectors in photovoltaic installations?
• Would you buy a vehicle with air bag generic brand just to get a discount less than the price of a pizza?
• So why do you accept using lower DPS?

A DPS for photovoltaic application under the “S” brand costs around R$125 on the market currently (value for the end consumer). The same product from brand “D”, with all the advantages it offers, costs only 30% more.

This represents a saving of around R$40, which certainly does not justify the use of an inferior product, which will put the photovoltaic system at risk.

There are many problems in photovoltaic systems found in Brazil, ranging from gross design errors to installation failures. Using poor quality surge protectors is yet another problem we encounter. This occurs largely due to consumer misinformation or even a lack of knowledge on the part of resellers and distributors.

This article has shown that surge protection devices are not all the same and the quality of this product is important for the safe operation of photovoltaic systems, without this representing a significant increase in the overall value of the installation.