Webinar «Surge protection of low-current lines, part 1», page 3

The 4th webinar about Surge Protection

Webinar text. Page 3

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— In practice, the question always comes up, what to do with the cable screen? Today, everybody who knows about an outdoor cable in an object with the lightning protection system, if it is not screened, it can carry tremendous energy. Therefore, all external cables must be screened. And what to do with the screen? If you remember studying at the university, if you remember your technical work experience, we have met different approaches to cable screens. First, screen in order to be a screen, the screen must divert the currents induced in it to the ground. Simply put, it must be grounded. The question which side is grounded is always very interesting. I always say that whichever side they taught you at school is the side to ground the screen. But it all started from signal standards when we had analog systems. There has always been a problem with interference that went through the screen when we would ground it in places where large short-circuit currents flowed through ground, for example. Today we deal with his problem better. And from our point of view on overvoltage protection, it would be good to ground the screen in one centimeter steps. Just kidding.

— The second question is always: how to install SPDs protection elements in the power supply network and low-current lines. I think that after we talked about the zone protection concept, it was clear to everyone that these elements should not be intstalled far from each other, but in the same shield where we place all surge protection devices. These shields happen to be very big. Then we must always fight to ensure that these distances are not bigger than 0.5 meters, because the voltage drop across the inductor when these currents the reverse currents leak to the ground will be too high. We install them in the same shield at the entry to this zone and at the entry to this protected device, for example, at the bottom of the protected device shield, but everything is still in the shield. I will show it later. We must install a special metal structure to shield the reverse current that go to the ground and those that come from our sensitive device outside.

— Let me try to read Dmitrii's question: "Is there a necessity (for the stability) to lay shielded communication cables (data transmission) in metal sleeves/pipes along a metal tower (lightning conductor) and in the ground or is it enough to install it on a cable run laid from the tower to protect overvoltage limiter?" The question is not that simple to answer without a figure. I am afraid that without a special figure I can not explain this to you.

— We all understand that it is possible to weld a very serious high-level tool at any point of our system. Question: how will it end? I'm afraid that from time to time by a nuclear explosion, pardon my humour. And in this case, the zone protection concept about which we have already spoken many times, makes it easier for us to protect and reduces overall designing time. Here we see an example of a low-current system, a securite and alarm system. The system, and when it is the alarm system, is characterized, as we have already said, by a very large number of lines that go from the central point to point A. From statistics, this means that a large interference can come to one of the lines and it can damage or simply change the working conditions of our system. In this case, at the cable entry to the object, all the wires are protected from the leaking overvoltage. If the cores are free, then we close to the electrical bonding bus that is always connected to the ground at the one-storey building. And if it is inside the object, then inside the equalization system that is usually indicated by yellow and green colours. So, this bus is always connected in this place at one point from the power supply side and from all signal lines. And at one point we divert this energy to the ground. Let's agree for now that we are talking about external systems that are outside the object. The same for antenna cable. At the cable entry to the object, not at the central control desk, we install overvoltage protection. Now, our friend in Warsaw was building an antenna system on a 100 meter object in the center of Warsaw. And imagine that there is protection installed inside the object a hundred meters down the cable. You have to understand that when lightning strikes this cable, this current will flow to the protection element and somehow this current will be diverted to the ground inside the object. Maybe it will go back to the roof, if someone designs it this way, or down. So all this energy inside this cable chamber will interfere with other cables. In other words, he can protect his device, but damage the other ones. So, all the elements are at the cable entry to the object. And what do we do with the elements that give signal to our system?

— And so, it turns out that at the cable entry to the object all signal wires are protected in one SPD. They are connected to the common bus that comes in on top of the cable. The grounding goes in the same direction, so that the energy of these cables did not leak further and we did not let the object this energy. And the already laid protected cores go to the central point of the alarm system.

— If we analyze such microwave barrier, in practice we have to study its scheme, what signal output standards are. We select those elements for them that we see on the right. "EnerPro" for 12 V, "DataPro" for 90 V is already a spark gap. And we see that there are fuses of 1 and 0.5 A, these are our devices. Therefore, our elements must be able to pass such currents in the nominal mode, as indicated here. All lines are protected, take a look at the power supply and sabotage. Sabotage also includes cores, they will pass high potential as well. If we do not protect these cores, then again we will not have complete protection.

— An example from the American company called "PolyPhaser", with which we have been cooperating for many years. It works mainly in coaxial lines. Here we see that many years ago for the cable point we installed protection at the camera and at the cable entry where the conductors are laid to the dispatcher, to the central observation point.

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