Electrosafe private residential building and cottage. Part 3. Lightning protection

Beginning of the article:

Electrosafe private residential building and cottage. Part 1

Electrosafe private residential building and cottage. Part 2

External lightning protection

Let's start with the simplest. Suppose we have a residential building (cottage) which is powered by a power line (overhead line) and in which metal communications (gas, water supply, etc.) are not connected. We list the dangers that may await us in this case and then how to deal with them.

1. Direct lightning strike into the house.

2. Direct lightning strike into the antenna.

3. Direct lightning strike into the overhead lines.

4. Lightning strike in the ground near the house.

5. A lightning strike in the ground near the overhead line.

In case No. 1, a direct lightning strike can destroy the building itself, cause a fire in it, damage the electrical equipment of the house and electrical appliances included in the outlets. In this case, there is only one protective measure - the installation of external lightning protection on the house.

In case No. 2, the TV will fail, possibly igniting it. Protection measures: - installation of the antenna in the external lightning protection zone and / or disconnect the antenna cable from the TV.

In case No. 3, a surge voltage of tens of kilovolts is brought into the house, which will cause damage to the electrical wiring insulation and damage to electrical appliances connected to outlets. Protection measures: - disconnect the power supply at the entrance to the house at the time of departure or thunderstorm OR - install an SPD (arrester) at the power input to the house.

In case No. 4, induced voltage (tens of volts) in the wiring is possible, which can cause damage to sensitive electronic equipment (computer, etc.) connected to outlets. Protection measures: - at the time of departure or during a thunderstorm, disconnect such equipment from sockets OR - install an SPD to protect such equipment.

Case No. 5 is similar to Case No. 3.

Fig. 1. Hazardous events during a thunderstorm

First you need to understand that lightning protection is divided into external and internal. External lightning protection protects our house from the outside from direct lightning strikes into the house, and internal lightning protection protects our house from lightning strikes near the house, from lightning strikes in and near power line wires. In this part, we will talk about external lightning protection, that is, protection against direct lightning strikes into the house.

The task of external lightning protection is to take the lightning strike and it is safe for people and the house to divert its current to the ground. At the same time, you need to understand that all modern lightning protection does not give a one hundred percent guarantee.

The ideal solution would be to protect our home by analogy with the famous Faraday cage, but for obvious reasons this is not suitable for us. Which next solution is almost perfectly possible? There are two options: - Install a high lightning rod far from our home. Then our whole house will be in its protection zone, and the lightning current flowing from it to the ground “will not reach” our house.

- Pull the lightning protection cable over our house, and also make access to the ground for lightning current as far as possible from the house. The general principle here is this: to protect the house from direct lightning and to divert its current as far as possible from our house. Alas, these two options are also not suitable for us due to the enormous cost. But I described them so that there was something to be guided by (as an ideal).

So what should one do if an ideal is unattainable? It remains to do a "simple" external lightning protection. What does perfect, almost perfect and simple external lightning protection mean? Perfect, one hundred percent is Faraday cage. Almost perfect - this means that the probability of protection will be 0.9 (one out of ten lightnings will burst into the house, or 0.99 - one out of a hundred lightnings will burst).

Simple - it’s fifty to fifty - or protect, or not. After all, what remains for us in this case? Pull the cable over the roof ridge and ground it.Connect small vertical lightning rods to this cable to protect the antenna, chimneys, etc. The probability of a lightning breaking through such protection still remains, but what should please us is that the most vulnerable places on the roof of the house will still be reliably protected (this applies primarily to antennas, chimney pipes, etc.).

After all the above, you can get confused - what to do? Personally, my opinion is - you need to look at the circumstances. If, for example, thunderstorms occur in your area every day, then it’s stupid to sit without light, without a TV almost every day (why, during a thunderstorm, you must turn off the power to the house and remove the antenna cable from the TV, I said above).

Then you need to see what is built around your home. If there are tall buildings near your home, etc. - you can hope that these high-rise buildings will protect your home. If nothing like this is observed around your house, then it is better to make your own lightning protection (both external and internal). If in your area there is a thunderstorm two to three times a year, then you can not do it, although the risk still remains. So, still you decided to make your lightning protection.

First, let's deal with grounding. According to PUE 1.7.55, the grounding device for protective grounding of building electrical installations and lightning protection of categories 2 and 3, as a rule, should be common.

What is 1,2,3 and even now a new 4 category has appeared? Category 1 - this is lightning protection made by a stand-alone rod or cable lightning rods. If lightning protection is performed on the building itself and is not isolated from it, then this is lightning protection of category 2.3. By standards, our residential building belongs to 3 (4) categories, though nothing prevents us from making lightning protection in 1 category.

In this paper, we will focus on category 3, as the cheapest option. Then we need to make one common grounding device for home electrical installation and lightning protection.

Why is this necessary? Imagine that in our house there are two different “lands”. Firstly, it is not convenient, because there is no single reference point “earth”, but “earth №1” and “earth №2” will appear. Secondly, it is simply life-threatening, since a person touching, for example, with ground hand No. 1, and with his other hand land No. 2, can get an electric tram when it suddenly appears, for example, on earth No. 2 of high potential (let it be a lightning strike in land number 2). Further, with an unacceptable rapprochement between the ground No. 1 and No. 2 and with a lightning strike into one of them - a spark discharge between them is possible, etc.

External lightning protection

Consider Fig. 2 which shows 6 dangerous cases with a direct lightning strike (PUM).

Fig. 2. Danger of direct lightning strike

Case No. 1 is when lightning pierces our external lightning protection and hits the house. There is only one countermeasure - strengthening lightning protection (for example, instead of one cable, pull two, etc.).

Case No. 2. With PUM, a lightning current will flow into the lightning rod through the lightning rod into the memory. Flowing through a lightning rod, such a current will induce (induce) a high potential on all open circuits (which form conductive parts, such as metal water pipes, etc.), with the highest potential at the ends of such open circuits (in closed circuits, the potential difference between any two points on the circuit is zero). With unacceptable proximity between the lightning rod and such circuits, a spark breakdown is possible (which of course is bad).

In Fig. 2 it is seen that in parallel with the lightning conductor in the house there is a metal pipe. Then, with PUM, high potential will be induced on it. We equalized the potential at the lower end of the pipe with a lightning rod by connecting them to a PE bus. At the upper end of the pipe, the potentials of the lightning conductor and the pipe are not equalized and in this place a spark breakdown between them is possible. What can be done here?

The following options are possible:

1. Observe the minimum required distances between the lightning conductor and such a conductive part.

2. Equalize their potentials.

3.Insulate the lightning conductor with a polyethylene pipe (the most suitable option).

For clarity, let's count a little. Suppose we have a brick house (Km = 0.5), protection level 3 or 4 (Ki = 0.05) and one lightning conductor (Ks = 1). The length of the parallel lightning rods and pipes is 10 meters. Then D = Ki x Ks x L / Km = 0.05 x1 x 10 /0.5 = 1 meter, that is, the minimum allowable distance in this case should be 1 meter.

Now we isolate the lightning conductor with a polyethylene pipe (Km = 60). In this case, D = 0.0008 meters is what we need. An additional advantage of this solution will be the fact that accidentally touching such a lightning rod we will not get under the touch voltage, see case No. 5.

Case No. 3. Here we must firmly understand that with PUM, only 50% of the lightning current will drain into our memory. The remaining 50% through the PE bus will scatter throughout the house along the PE conductors (to everything connected with them). If our house is not connected to any external communications, then after some time the remaining 50% will flow back to the memory. If external communications are brought to the house, then the remaining 50% will run along them, distributed equally between them. Since high potential appears on the RE bus in this situation, we need to install arrester, and for the s.TN-C-S and TT the switching circuits of such arrester will be different.

Case No. 4. With PUM, a person falls under the step voltage. For our house, the best option would be the construction of a lightning rod and a charger in a pedestrian place.

Case No. 5. A person comes under the stress of touch. For our house, the best option would be, as in Case No. 4, and the plus to this is to isolate the lightning conductor with a polyethylene pipe.

Case No. 6. As I described in Sl. No. 3, on all the conductive parts connected with PE conductors during PUM some high potential will be found for some time. Installing arresters to limit this potential will only help wiring and electrical appliances, but not the person who is standing on the conductive floor has touched the conductive parts (for example, the water pipe). There is only one way out of this situation - to have good grounding and not to touch such conductive parts during a thunderstorm.

NOTE to Fig. 2. In the figure, I did not move away from the PUE and indicated the location of the separation of the lightning rod on the wall of the house. If you make a separation in this place, then you need to increase the cross-section of the wire going from this point to the memory at least twice. My opinion is that this should not be done. It is better to reach the charger with a lightning rod, and only then connect the charger and PE bus from the charger with another wire. In this case, the ground potential and PE tires will be as equal as possible.

To be continued.

Electrical Engineer S. Mironov

Continuation of the article: Electrosafe private residential building and cottage. Part 4. Surge Protection