Webinar «Surge protection of low-current lines, part 1», page 2

The 4th webinar about Surge Protection

Webinar text. Page 2

Quick slide navigation:

— The main ones are the rated and maximum allowable voltage of the transmitted signals, the maximum current in the signal lines, the threshold frequency, the signal transmission mode (symmetrical or unsymmetrical), the allowable signal attenuation in the concerned line. If we look at the first one, rated and maximum allowable voltage of the transmitted signals, if the designer takes into account only the rated voltage, then we will have the same situation, that we had at the time when we got an order for an element for loudspeaker systems. And the loudspeakers stopped working at a large stadium at a certain volume. This means that the voltage was higher than rated voltage, not 110 V, but 115-120-130 V, and this simply led to the activation of protective elements.

— The next elements are, for example, the wave impedance of signal transmission that is necessarily related to a coaxial minimum. We must understand that this is either 50 or 75 Ohm, or invented - this is very rare, the wave line impedance. If we set the impedance orher than the rated, then we can just suppress the signal. The maximum impedance that we can include into this line. Very simple, but very important information, when we design protection, we need to consider the type of connectors used in the protected line. Anyone who has worked with coaxial lines knows that there are different standards for connectors NB, BNC, and so on. If we install the wrong type or if we install not female, but male type, then the system just will not work. I heard about this case when the customer first ordered about 1500 pieces, and these 1500 pieces could not be connected, only because the connectors did not fit. Someone did not know what he was doing. It seems simple, but you can always make a mistake. A very important protection type or protection circuit is the one included into the protected device itself, when the manufacturer of the protected device applied protection. So, we must remember that we must coordinate the energy between the next stages of protection. And if we do not have this information, which happens often, it may turn out that the protection circuit we have not event known about in this device will trigger, and the circuit that we have installed earlier because of the inductance mismatch between these two cascades of surge protection system will not work. Therefore, we must always aim to get this information. If we do not have this information, we must apply a circuit with an additional inductor at the output of the SPD.

— When we already have this information, we proceed to the next stage, in other words, we initially select the parameters of the protective device and look at the parameters, we find them in the manufacturers catalogs. For example, we must understand that if we are dealing with partial lightning currents then we always look at a 10/350 μs pulse. If the object does not have a lightning protection system, then we look at an 8/20 µs pulse. The question is: what do we do with the railway, where there's no such lightning protection? Here, a little more knowledge is needed because it seems that along the railway the protection system is not installed, but if it is electrified then the traction wire itself and the entire wire construction that hangs over the rails is already a lightning protection system, whether we want it or not. This is when we need to figure out a little more about where our device will be found or accept such a systematics of 8/20 μs, or just stick to our knowledge and expect that a partial lightning current will flow. In an object that has a lightning protection system, knowing that we have determined that lightning can get into the object, we must protect it, we always use elements with a pulse of 10/350 μs at the cable entry. And today, if we take a look at the catalogs, then 2.5 kA 10/350 μs are the parameters that we can get for our overvoltage protection elements. Why is it not 10 kA, not 50 kA, not 20 kA, as in the power supply network?

— So, this scheme, even if it is affected by a parial lightning current, gives a very small pulse at the input, which leaks to our protected device. Its energy is negligible and in no way can damage our device. We select the voltage level by the limiting level, basically a diode is installed here at the end, and as we know, the diode is semiconductor element made of monocrystalline silicon and it is very easy to select the voltage parameters. It is possible to buy a diode for any voltage and very carefully select the right one for our protected circuit, so we must take into account whether we are designing single-stage or multi-stage schemes in the current object.

— At the next stage we define something that I have mentioned earlier at the beginning of today's webinar. How many stages will our overvoltage protection system have? And the principle is simple here, if we have a zone protection concept, then we install separate protection elements on the boundary of each zone. If we do not use such zone concept, but we take into account the length of the cables, I will give you an example of such object where I use such cascades, although I haven't managed to construct a zone concept of lightning protection in such object. When we had the border points upgraded, we adapted the existing facilities for customs. As you know we are at the border of Schengen zone. And in such customs facility there is a lot of computer systems now that process a lot of signals that will remember forever what Ivan or Miroslav transported today or tomorrow and what he declared. To somebody it is not clear, but the systems are huge, expensive. And in this case the object of length is about 100 m, because trucks are examined inside, has received a new lightning protection system. But this object was shared between customs and the existing woodworking plant where they simply cut the tree into boards.

— If we look at signal transmission we just have to understand that it mainly matters for the manufacturer, not that much for the designer. But you must remember that such operating current can flow in a wire-ground circuit. And in practice it is important at the SPDs design stage.

— The frequency range of devices is a whole another story. In practice, the maximum frequency range for the 3 dB level is indicated. And for an ordinary electrician or designer this information is enough. For special systems it is necessary to have more knowledge, and full characterization is necessary, but basically this data are sufficient. Here we have such parameters that at the level of 1-10-100 MHz we get too much signal attenuation and our device is not suitable abouve this frequency, so we should always check the frequencies of this signal are. Those who know this understand that this can be expressed by the data bit rate in Mb must be able to recalculate and select the parameters, having seen them in the manufacturer's catalog, so that the operation of our protected system was not affected.

Useful materials for designers:

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