Webinar «Surge protection of low-current lines (signal, control, data transmission, telephone etc.), part 2», page 2

The 5th webinar about Surge Protection

Webinar text. Page 2

Quick slide navigation:

— If we look at the base itself, the element of which is mounted on the DIN rail, we will see that there are many varieties. They are selected for different types of protection circuits that are placed in a pluggable module and for this we need to be able to connect them when we select them in the course of designing. That is, MP Base designation simply means base, “MP” is a series of arresters with pluggable modules. (9, 10) terminals that are listed here, are numbered on our base. They are used to ground the cable screen, and they are isolated from the mounting rail in this case. If this is our GDT designation, these terminals for grounding the cable screen are connected to the mounting rail indirectly through a spark gap. And if this base is GDN-labeled - (9, 10) clamps directly ground the cable screen to the mounting rail that must necessarily be connected to earth, but in practice it is the PE conductor in our facilities. MP Base 1x2 designation simply means that we are dealing with a protection module for one signal pair, that is, we are protecting only two cores.

— In order to understand this catalog - you will not find, so use this hint. You see different bases, different bases that are mounted on a DIN rail. If you look at the first diagram, we see that in this case we are dealing with insulating (9, 10) terminals, that are used to connect the screen from the grounding joint. But this T-4 joint connects directly to a DIN rail. In another case, when the GND designation, which we talked about, we are dealing with a direct shorting of this core with the (9, 10) clips on the (3, 4) grounded below. And in this case we are dealing with a spark gap connected between these two cores. What does it mean? This means that here we have cable screen insulation from the ground. In this case, the direct connection of the screen with the ground, and in this case we use a spark gap that will turn on only when we exceed its breakdown voltage. And at this time, when overvoltage is in this place, the spark gap will work for a short time, microsecond, will connect two cores of the screen with the ground. Remember that manufacturers of different systems do not always allow us to ground the end of the screen and then in this place it is recommended to use this option with a spark gap. In another case, when we deal with two-cascade circuits, we see that in the base there will be elements padding the first cascade with the second. And these elements are characterized by some kind of impedance, we will call it resistance. And for the same kinds of elements, we have the same two elements included in the lines, which coordinate us with two levels of protection, we talked about this in previous webinars.

— If we look at the second base, which is already used for two signal pairs, then we protect 4 cores. So for them the designations are similar, as for signal ones, only here instead of the number 1 it has changed to the number 2. And we see that a PE conductor appears again. There is a cable screen and two signal lines that in this case are protected by a spark gap. Please look at this three-electrode arrester that protects, for example, 7, 8 conductor against the PE conductor of the screen. Depending on what we include here between the APE screen, we either insulate the screen or directly connect it, or connect it through a gas-filled arrester. So we get different ways to connect the screen. The most common method is a direct connection, that is the safest way, or an indirect connection through a cable screen discharger, then most of the energy goes to us immediately on the ground during an overvoltage. In the case when we have the letter R, there are elements that coordinate the second cascade with the first cascade. That is, we need to get a voltage drop on it so that with the sum of the voltage drop across the protective element would cause the arrester to work until our second stage is blown.

— Let's see what kind of element that is isolated from the cable screen from the PE it is. As always, we mount all the elements on the DIN rail - this is the same for everyone of them. This spark gap is gas-filled, sorry, this is a mistake, we should have insulation here. Insulated, excuse me, please. And the load current, when selecting 0.5 A, we must take into account that in this case, when we select an element, we must understand which system of signal standards we have - symmetrical or asymmetrical.

— In this case, when we have a cascade scheme and to coordinate this scheme in the base, we have placed the elements of coordination. And as we said, if they are in the base, we don’t pull them out when we change a module. In this case, they remain turned on all the time, then it is necessary until they fail, but this is a rare case. If we choose the electromagnetic environment correctly, we determine the equivalent environment for this place that we protect, so it will be very difficult to damage the base, but there are accidents. Then, in this case, we have a gas-filled spark gap connected between the screen terminals 9, 10, but the PE 4 conductor, that must be grounded, without this, our element will not be able to divert typical energy.

— These designations - different triggering schemes, connection circuits of these elements appear. The schemes MP 1x2 GDT ST and MP 2x2 GDT ST as we see are similar, have a high discharge capacity, they pass a large current, a partial lightning current. Regular circuits, besides their high discharge capacity, are characterized by a low limit level about 10 V and more that means that this is a very accurate voltage system. In this case, if we look at the diagrams, we will see that everywhere they have a case of insulation in order to show that for this case we have a lot of different types of elements - it makes sense when we select this system.

— All these elements can be connected using the geometrical parameters of the terminals. In such a simple way, the screen can be grounded and connected to the arrester depending on which base we use. But here you can see that the method is simple. For us, simply pull out a bigger screen and tie it up with a plastic clip. But it is necessary to remember that in conditions of high humidity such a solution is unacceptable, we need to insulate such places. If you use a box that has a good level of protection, a high EP - 65 - 66, this method may be sufficient.

— Another difference in practice is the method of wiring. We see here that decoupling, insulating an element from PE means that the element itself in a given place must have an ability to absorb such energy. If our PE is connected, we don’t need it here, it will not help, because we have everything insulated. The nominal voltages are very low for this element, because the second cascade has appeared, and it determines the nominal voltage that is visible in the next elements. They appear in the name of the element as 5, 12, 24 V. And at that moment, the output voltage in the cascade between the lines also changes. We see that the diode is connected between two lines, and the spark gap is connected between the line and the ground. This is of great importance to us; this line is also protected by the same spark gap. Since we have two lines, similarly connected. For us, the situation in this module is clear.

Useful materials for designers:

• Webinars with leading industry experts
• Everything for calculations of grounding and lightning protection
• Useful materials: articles, recommendations, examples