Webinar «Frequently asked questions about SPDs», page 3

The thirteenth webinar of a series "Grounding and lightning protection: issues and problems arising during design"

Webinar text. Page 3

Is there any danger connected with the use of SPD?

- Second question - is there any danger connected with the use of SPD and limiting its use? Yes. There is such a danger, of course. Put hundreds of SPDs into an electric circuit and you will reduce its reliability by one presence of these SPDs. Is it clear? Second question. Every SPD has a parasitic capacity and it is necessary to activate additional inductivity for each SPD. And it brings to the fact, that frequency response of the circuit you are protecting, its upper boundaries of the frequency characteristics go down, if it is inadmissible, you can't apply SPD. And, finally, the last thing. Increase of number of SPDs is always an increase of operational expenditures, because you need to control SPDs. And it is necessary to control them seriously, so I come to the same conclusion, SPD is a powerful, but not safe, drug.

Does grounding resistance matter at the SPD operation choice?

- Third question- Does grounding resistance matter at the SPD operation choice? The answer to this question is ambivalent again. It all depends on what surges we are talking about. If it is a direct strike overvoltage into a line between the substation and the protected object. Here, grounding resistance of the substation and the object definitely influences the distribution of current between them. And it is necessary to count this current distribution and SPD operation mode depends on that. Another option. Another option is at the bottom. Induced surge in an electric circuit. The level of this induced overvoltage doesn't depend on the grounding resistance and the mode of SPDs operation which you install, it can depend on the grounding resistance. Why? Because the upper SPD, which is between the phase and the neutral wire is not connected with the grounding and won't affect operation of SPD, if there is one. But if you connect the neutral to the ground wire of the object, then the current which goes through this SPD, will depend on the ground resistance of this circuit and then this current value will have to be considered at the choice of SPD. So the answer is again as follows: look what you are protecting.

For what grounding systems it is correct to use SPDs?

- Fourth question- For what grounding systems it is correct to use SPDs, with the connection circuit of poles 2+0 (4+0) and 1+1 or (3+1)? We are talking about the following. On the top, there is the circuit, on the top left there is SPD 1+1 connection circuit. What is good in this circuit? In this circuit it is good that overvoltages between the neutral wire and phase one will be limited to a full extent, because there is nothing more than one single SPD. Let's see the left circuit. To limit overvoltage between the phase wire and the neutral, you now have two consecutively activated SPDs and the voltage drop on these two SPDs will be more than on one. It is clear, that if you want to limit overvoltage between the phase wire and the neutral, you better use 1+1 circuit.

Does it make sense to install SPDs of one class?

-Let's move on, does it make sense to install SPDs of one class or a set of different classes of SPDs is required? The answer to this question is as follows. Guys, count the overvoltage you have and what current will flow? Count, what the device which you protect can stand. If one SPD of class one is enough, install it. If not enough, add an SPD of class 2. If it is not enough, add an SPD of class 3. Always keep the following in mind. When you install the sequence of SPDs, you will have to divide the inductivity of their wire, or coil, set inside the bodyframe. So the question is: this additional increase of circuit inductivity, on which a useful signal runs, is it admissible by frequency characteristics or not? You will have to count, otherwise, your SPD installation won't be accepted by the user. Its frequency characteristics will change.

How effective are SPDs of class 1+2?

- So let's go on. How effective are SPDs of class 1+2 in comparison to installation of SPD class 1 and then class 2? In fact, I already answered this question and the reason is the following. The thing is that in order to achieve selective operation of SPD, inductivity between them is obligatory. It doesn't really matter how you will make this inductivity. Whether you make it with a wire between these SPDs of the length which are recorded in the manual or the manufacturing company will put a coil, which will have the same inductivity value. SPDs operation doesn’t depend on that if the coil is made correctly, but it is up to the conscience of the manufacturing company. And we need to check if this inductivity is admissible by the frequency characteristics of this device?

How to choose safety device in SPD circuits?

- Now the question - how to choose safety device in SPD circuits? The safety device is necessary to eliminate short circuit, when SPD is broken and can't switch off the accompanying current. Manufacturing company is obliged to write of what level should be the fuse link for this type of SPD. This is obligatory to be written. What is within your responsibility? Compare the value of this fuse link with those which will be set into the electric circuit due to security of this circuit from short circuit currents. If short circuit current, which has to be included in the circuit, is less than the short circuit, which SPD can stand, in this case, it is necessary to include resistance, in this case it is possible not to add fuses into SPD circuit. Keeping in mind that you sacrifice something in any case. Imagine, that you need to protect the circuit with weaker fuses, than those which is guaranteed by SPD manufacturing company. You put the fuses, they are shown on the top in the scheme. They will response, they don't allow short circuit to develop, when they response they will leave your protected device without energy supply. Energy supply of this device will simply cease. Now imagine, that to protect SPD, we need less fuses, than to protect electric circuit. And you put fuses consecutively with SPDs into the circuit. If SPDs are damaged or they manage t disable short circuit, if they are arresters, your fuses will response and the manufacturing company tells you:" Everything is fine, your fuses responded, the thunderstorm ended, you will find the damaged fuses and will change them, everything will be ok". It's a trick again. A trick from the manufacturing company. Imagine yourself a lightning strike. The fuses responded incorrectly and put an SPD out of the electric circuit. Now the protected object is left without SPD protection. You are told: "It's ok! One strike happens in several years, you have time to replace it!". It is not the case. According to the existing statistics, about 85% of lightning are multi-component.

What SPD classes must be installed and where?

-Now, the next question. An entry is done in the distribution board in the garage. A house with the installed external lightning protection is at the distance of 45 meters from the garage, there is also a distribution board in it. What SPD classes must be installed and where? The question that I can't answer at all. I don't know what is inside the garage. I don't know what SPDs are installed in the house. And the most important thing, I don't know how 40 meters of circuit between the garage and the house are implemented and what can occur in this circuit due to induced pickups? I just don't know that. If this circuit is overhead, then at a 40-meter distance, the EMF can be very high and then SPDs will have to be installed in the garages in addition. If this is done by an underground cable, for example, then we can countenance. That is, every time we need to see what we are protecting, on what level and what overvoltages can appear on these parts of electric circuit, which remains unprotected. It is impossible to answer that question without that.

Are there factors influencing SPD operation?

- Are there any external factors, which can negatively influence the operation of SPD? Yes, there are. First of all, it is environmental temperature. Secondly, humidity. Thirdly, environmental pressure. Because there are moments, when SPDs are put inside, for example, of high pressure pipes. In this case, it is necessary to choose a special SPD. Special SPD must be found. There is no other way out. There are factors damaging SPDs. On SPDs on the base of variable resistors are destruction of zinc oxide bead. Or bead of other materials. It is destroyed at the expense of a local overheat. On SPDS on the base of an arrester - damage of an interrupting chamber. If your chamber is damaged, the accompanying current won't be switched off and SPD will be out of service. So we insist on choosing a reliability manufacturing company.

What is the probability of SPD "malfunction"?

- How to define if SPD did not response, what is the probability of malfunction, if to consider that it is correctly made and operates correctly too? I have no answer to this question. Any company very thoroughly hides the information about the probability of their SPDs malfunction. I did not find such info. And probably, nobody ever will. The companies are not going to reveal their defects. Is it possible to check efficiency of SPD? Manually?

Are there any peculiarities for choosing SPDs?

-Are there any peculiarities for choosing SPDs in Russia and abroad? First of all, there is nothing to compare here, because there is no methodology for choosing SPDs, but they can be no difference. SPDs are chosen by the level of calculation of overvoltages and by the level of current and accompanying current which flows via SPD. All these questions lie in the classical theory of electric circuits. And this theory of electrical circuits has not changed neither in Russia, nor abroad and will hardly change ever. That's all I wanted to say and I exceeded the time of five minutes I was given.

Questions and answers

— I am ready to answer questions if there are any, Alexey.

— Thank you, Eduard Meerovitch for your report. It was interesting to listen to you. We have questions. Perhaps three or four. We will try to read them now. The first question is wide, true-life and probably very relevant for single-family houses owners. "Eduard Meerovitch, is it not easier to protect equipment with the help of SPD for an existing building or some small object, like a single-family house, than to protect it with the help of shielding down conductors. How an ordinary man, not a designer, but an owner of the house can estimate electromagnetic impact?".

— The situation is very simple, it seems to me. 

— OK. The second part of the same question concerns the fact, that SPD can be chosen individually from the catalogue, but it is hard to make a calculation. As far as I understood from your answer, it is recommended to address to the specialists who will make such a calculation and will chose a justified variant. Am I right?

— Of course, a specialist will make a justified calculation, it is not that hard as it seems. It is not hard at all. But there is no other variant. Don't take it unkindly, there are things which can be only specifically answered. And this specific answer is a calculation of current distribution in such a circuit. I don't know, maybe I am guided by the seminars we held, maybe the project "Grounding and lightning protection on ZANDZ.com" will think to make a simple guidance for the calculation of overvoltages, which will be oriented not on a computer but engineer calculator. So a designer could make such a thing. Probably, Alexey can bring this idea to the management. And the management will device if it is reasonable or not. In any case, it is not very hard to make such a study guide.

— The idea is very good and useful. I will raise this question, but won't promise anything for now. We are interested to help designers in calculations and single-family houses owners. Thank you.

— I see a question about a map of thunder activity, right?

— Yes. There is such a question.

— You know what?

— Thanks for your answer. I want to add from myself. I was reading literature about the climate change, global warming and all like that. It was stated that such phenomena will influence the frequency of lightning strikes, is it true? How it will be in the future? Could you comment this?

— You know, I really do not want to answer that question. I am not a specialist on generation of atmospheric electricity. And that is why I don't know where these changes might bring to. The dependence is as follows - in humid regions, in Southern humid regions, thunder activity is higher. And the more humid and warmer the region is, the activity is higher. But on the other side, in dry region, not looking at hot temperatures, thunder activity is much weaker and nobody knows where this temperature change will bring to. Because the forecasts are so that part of regions, including European, will turn into dry ones. Where to move? I can't answer this question.

— Thank you. Next question from Igor. He is asking what manufacturer writes requirements on fuse links? Igor is indicating that he never met such requirements anywhere. Eduard Meerovitch, please do not name certain brands. We can send you the catalogue with all the requirements. But it is not good to tell the brands to the public.

— By all means.

— The second part of this question. Is it possible to install automatic breaker instead of a fuse?

— Great question. Thank you very much, Alexey. Thanks to who asked it.

— I believe, yes. Igor, If you have any comments to my answers, please send them to the chat. I think we can answer the next question. Denis is asking:" Eduard Meerovitch, how effective is the use of SPD in DC control circuit. Is installation of such SPDs required and what are the recommendations on their choice?".

— You know, there is nothing special in a DC in comparison to AC, except one thing. DC circuit is usually low-voltage. It can be, I know today's devices of mobile connection- they are 48 V. In many systems, say, perimeter protection - there is 24 V. And the only thing to be done is put SPDs consecutively from 1 to 3 level in order to reduce the overvoltage value to the level which the supply equipment of this electronic device of DC will stand. The technology of making such SPDs is acquired. Many companies produce such SPDs, produce them specially for 48 and 24 V and there are no special problems here. It is necessary to chose SPDs which are intended for that.

— Next question. Olga Borisovna is asking a question about manufacturing companies, which recommended themselves as good SPDs, but we agreed not to name specific models and brands, so let's switch to another question. Vasiliy is asking a question, the situation is as follows: "We put a surge arrester in a transformer substation and on the high side, on the side of 0.4 kV we install fuses, as you said. And on the side of 6-10 kV we connected a surge arrester directly the buses without any switching devices. It is a violation?".

— I am not ready to answer this question because I don't know EIC requirements to 6-10 kV circuits in this relation. I am afraid it is not good to install SPD on the base of an arrester without a fuse. But it is better to read EIC to see the requirements. I prepared myself to a low-voltage seminar and did not check these requirements. So I am afraid and don't want to answer this question. I don’t know.

— Eduard Meerovitch, can we get some information for Vasiliy and answer him later?

— Yes, of course.

— OK. Vasiliy, we will put your question down, contact you via email and answer you. I think we will answer it on the site, in the form of a news, we will publish the answer to everybody could see. There is a comment from Viktor about solar electric stations of up to 1000 V of DC. I don't understand if it is a question or a comment to some answer. As far as I understand, we answered all questions. Do you have any other questions, colleagues?

— You know, I'd like to say. I understand, that our seminar should not have any promotional purpose. We should not state that some companies are good and some are bad. But to understand for yourself what companies are good and which are not good, I would do the following. First of all, I would compare the catalogue, which is given in the net in Russian with the catalogue of the same company in English. And if these catalogues are seriously different in output indexes of SPDs, I would be cautious with this company. I'd do it. If there is a chance to compare catalogues, I would do it. I know, there were cases when in Russian materials the data was one, and in English materials - absolutely other, they counted on the fact that nobody notices in Russia. Is it clear? I would check these things. And the second matter. How detailed the indexes of a certain type of SPD are indicated in the catalogue? If it is excessive information, it is easier to trust it. I would check these two things before choosing some SPD. There is one moment on which we should pay attention. Imagine that all companies are selling SPD for 1 ruble, and there are some companies that sell the same SPD for 50 kopecks. I would ask this company why their SPD costs twice less. Today, everything is so conditioned on the market, that it is hard to reduce the price to 10 % and to 50% is a serious problem, you need to think it over. I would give such recommendations.

— Thank you. I agree with you, that our webinar is an educational event and we can't give any promotion.

— Finally, I would like to say "Answers on questions about SPD", what will be at the future conference on lightning protection about the regulatory document on internal lightning protection and on SPD. I wouldn’t doubt it. And let's ask Alexey Korytko, as the representative of the company, to make an offer about current calculation method via SPD, if the company management says yes, we would put this method on the Internet.

Is it possible, Alexey?

— It is possible to discuss it. We will report about the results.

— Thank you very much again.

— Eduard Meerovitch, two more questions have appeared, while we were answering the last ones. Igor is asking a question:" Is there some general recommendation on the nominal current of the fuse link, for example 20A, so not to count?".

— No. There is no recommendation like this and can't be at all. The first thing is that the arc-suppressing ability of each SPD is different. And the mechanical strength, namely heat strength from each SPD is different, that is why, it is impossible to orient some certain SPD current, but we can orient on the following. We can orient on the fact, that if you have an admissible current in your electric circuit, short circuit current is very low, then you can orient on this current, firmly realizing that the resistance, or current of fuse link response, which will be at the SPD, is higher than the current in the circuit. Then, of course, it can be done, but unfortunately the possibilities of SPD will be underused.

— Thank you for the answer, Eduard Meerovitch. Question from Maxim:" Is it true that it is recommended to install SPD into a metal bodyframe board?".

— I don't know that and I don't think a shield with a metal bodyframe is required. Here I will have to refer to a certain company. I know that arresters produced by DEHN+ SOHNE guarantee that the emission of a gas jet in an interrupting chamber doesn't exceed 10 cm in length. And it means, that if you have some plastic, which may burn and this plastic is at the distance more than 10 cm, there is no danger in this case and then the bodyframe can be plastic. I do not know any SPD which would have a metal bodyframe. Their bodies are usually plastic. And why to put it in a metal board? If it is done for safety reasons, electric safety, then it is not our question, because we can't talk about electric safety of personnel at this seminar, I am not a specialist.

— Thank you. Question from Viktor, question about fire security of variable resistor SPD on thermal releaser built into SPD. What can you say about that?

— Probably, nothing. Nothing because these thermal releases are the same thing as fuses which we enable from outside. They have no advantages. If this releaser responded, then SPD with a destroyed zinc oxide or any other coil won't limit this overvoltage.

— OK. Thank you. I understand, there are no more questions. If you have any questions, please contact us by email, dear colleagues. I sent the e-mail to the chat. Please, copy it and send us any questions on calculations, selection of SPDs, lightning protection systems, grounding. We will answer them with pleasure. I think we can finish here. Eduard Meerovitch, thank you very much for the webinar, for the presentation. It was very substantial. Colleagues, thank you for visiting the webinar. You are welcome to sign up for the next ones on our site.

— Alexey, can you contact us by phone?

— Yes, of course. We can stop here. Thanks everyone. See you soon!

— Best regards! Good bye!

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