Webinar. What SPDs are designed for?, page 2

TThe third webinar of the Grounding and lightning protection: issues and problems arising in the design series

Webinar text. Page 2

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Interference generating current goes outside the shield

Shielding factor of cable shields

Now there's the second scenario: the current does outside the metal screen, i.e. the shield is just a shield. In this case, the shielding effect heavily depends on the shield material. If you shield is made from a copper braiding, an aluminum tape, or a steel tape as shown in the graph to the right, then the order of EMI attenuation is somewhere between 50 and 20, i.e. your electromagnetic interference is weakened by 20 to 50 times. But if you use steel reinforcement bars of a concrete structure as shielding, or you building is made of brick, the magnetic field is shielded by about 3 to 10 times anyway. And this shielding is very substantial, so the electromagnetic interferences in the internal volume of the building are reduced quite significantly thanks to the shielding. And you need to take into account this reduction when you calculate EMI and decide whether it is necessary to apply SPDs.

The physical basis

Once again, as a rule, two versions of the SPD are in use today. One is varistor-based SPDs, which have a current-voltage characteristic shown on the left in the picture. The second are spark gaps, which have a characteristic shown to the right. Both device types are operational, and three classes of such devices are commercially available. The first class is devices designed for the transmission of the full lightning current. They are installed at the entrances to the electrical circuit. Such SPDs are tested by a total lightning current pulse of 10 * 350 microseconds. These devices are most capable and most expensive. The only drawback that these devices have is as follows: while transmitting a rather big current, they still retain a decent voltage on their terminals. This voltage is somewhere around 4 kV. And the voltage of 4 kV is too high for the electrical circuits of buildings because the insulation of internal circuits there usually does not withstand more than 2.5 kV. The second class of devices is also designed to transmit the lightning current, but only partial lightning current on the one hand and electromagnetic interferences on the other hand. As a rule, these devices are tested by a pulse of 8 * 20, not 10 * 350. And these devices are used for protection in places where the full lightning current cannot flow. Finally, there is a third class. Such SPDs are usually installed directly at the input of the protected equipment.

Selecting SPDs for a particular electrical circuit

Let's go on. Knowing the place where the device is installed, I can calculate a voltage that may occur in the circuit of this device. And I can calculate the electromagnetic interference. I don't have to know, which zone this device is installed in. This is not important for me. I'm concerned about the geometrical pattern I can use for my calculation. The third thing is that I can calculate what amount of the lightning current will get into the protective device, i.e. the SPDs if I install them. I can also calculate this, and I don't need the zone theory for that. Finally, I can determine whether I can put the SPD I want to use in my electrical circuit. Will it match with its frequency characteristics? Will it match in terms of its housing reliability? Will it match in terms of some other operational functions? And I do not need zones for this either. And when I'm done with all the calculation, I take product catalogs supplied by various companies and start looking for a device suitable for my needs. If one device satisfies all my needs, then I will install only one device. If one device is not enough, then I can consistently install a series of devices.

Rated withstanding surge for different categories of surge

Now let's go on. What do I have? Class 1 SPDs have the voltage of 4 kV on their terminals, and for Class II SPDs it is about 1.5 to 2.5 kV. Finally, for Class 3 SPDs, the voltage may be less than 1 kV. Low-voltage insulation is also divided into categories. There, too, we have four categories. External insulation is for a voltage of about 6 kV. The next class of insulation is for 4 kV. Then comes 2.5 kV followed by 1.5 kV, and the device itself may also withstand a lower voltage. And I need to harmonize SPDs I want to install with the dielectric strength of insulation I have to protect. I have to do this, and I need to know the SPD characteristics for this purpose.

Calculation of lightning pulse current going through SPDs

The matter is that each company is trying to justify its products. It says, for example, that it manufactures SPDs unable to withstand the current greater than 5 kA. And this is OK because the current greater than 5 kA cannot get into the SPD under any circumstances. Another firm says "Come on, guys, stop telling this!" A very significant part of the lightning current can enter the SPD, e.g. 20% of the current. That's why you need to by only powerful SPDs, and this is me who manufactures them. And the struggle of these firms suddenly translates into different methodologies proposed to calculate the current flowing through SPDs. Without mentioning the names of such companies, I'll give you two examples. The first case is a lightning strike into a lightning rod installed on a protected building. Let's assume this is someone's cottage. What will the firm that wants to minimize the current say? It will say that the current will flow into the building's ground electrode system through at least two down conductors. But let's assume that this building has a transmission power line, which has four wires: three phase-carrying wires and a neutral wire. Furthermore, a water line made of steel pipes enters the house. In addition, a gas supply line also enters there. And a cable phone line, too. So the current will spread between all these lines in such a way that each such individual utility line accepts approximately the same current. That means that if I take for lines (power supply is one, water supply is two, the telephone line is three and the ground electrode system is four), then I have to divide the current into four parts. Each part is approximately 1/4. But I additionally need to take into account that I have four wires. This means that in each wire the current will be less than 5% of the total lightning current. So when selecting an SPD, I will proceed from the fact that a current of just 5 kA would flow through it (assuming that the lightning current is about 100 kA, as this is the rated current for the third level of protection). So I do not need a powerful SPD, and this is only one aspect. Now, here's the second aspect. Another firm asks the following: does a power line enter the house? Yes, it does. Can a lightning strike into the power line? Yes, it can. How many wires are there in the line? Four. So, 1/4 of the current will flow through each wire. So what rated current do you need for your SPD? You need 25 kA rather than 5, right? What should you choose?

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