In most cases, such impacts are characterized by the influence of the electromagnetic field of lightning on the electrical wires from which the protection object is powered, or on its internal electrical network. If we assess the degree of impact hazard level, then first of all it is worth considering which type of power lines are most vulnerable.
Today's buildings, in order to ensure their power supply, mainly have shielded cables, which are underground, coming from a substation located nearby. In these places, there is a minimum of induced pickups. Such an impact is not always taken into account, since it is practically not dangerous. Any pick-up is only activated in a situation where, during an impact, almost all the current is directed to the shell of the underground electro-communications.
Lightning protection of overhead lines in low structures
In the countryside detached cottages and the private sector, the air method of cable laying (380/220 V) is predominantly used. And only recently insulated wires have been started to be used in the Russian Federation. In interphase distances, it is identical to the ordinary underground wiring. The size of the area of the contour from wire to wire, which is penetrated by the electromagnetic field of lightning is relatively small. Therefore, the induced EMF of magnetic induction also has a small value. Only lightning, getting on such a cable in a straight line, can be dangerous.
The vast majority of aerial electrical communications 380/220 V still consists of bare, unprotected wires. Between them there is a distance d=0,3-0,5 m. Some lines can have a length (lВЛ from the object under protection, equal to hundreds of meters. In these cases, the contour that is pierced by the magnetic field when exposed to lightning, that is, s=lBLd, will be equal to several tens of square meters. This provokes the occurrence of a high induced voltage UЭM, even if the lightning is located far enough away from the line at the distance r (Fig. 1).
To calculate the induced voltage for the purpose of revealing an elementary approximate estimate, use the formula № 1:
Formula № 1. Induced voltage
where μ0=4π×10-7 H/m is the magnetic permeability of vacuum, and AI=dIM/dt is the growth rate of the lightning electric current at the pulse front. The parameters of the electric current lightning, including the rate of its increase, can vary in a chaotic order in the range of two orders of magnitude. Based on the standard IS-153-34.21.122-2003, the maximum growth rate is 2x1011 A/m.
In the derivation of the estimate, the application of this characteristic would result in an overestimated indicator, therefore, it is more appropriate to use the most real value of 4x1010 A/m. For a line of 200 m length, substituting this average value, we obtain UЭM equal to 2500 V, which will be a clear overvoltage for the intranet system of the building.
So, we get an unusual conclusion: some low-rise buildings with a small area, which are powered by traditional overhead lines with bare electrical wires, are at risk of ignition not from direct lightning strikes, but from voltages induced by these lines.
Formula № 1 shows the most productive way of limiting them. To do this, it is necessary to reduce the distance from wire to wire, taken according to the standard of 30-50 cm. It is this method that is used in networks with SIW wires. This makes it possible to reduce the induced voltage up to 20 times. They will become safe at a remote impact of lightning. Only a direct strike is an exception. We calculate the area of contractions of two wires according to the formula № 2:
Formula № 2. The square of contraction of two wires
where hВЛ - height, lВЛ - length. At the height of 6 meters and an original length of 200 meters, we have 0.0072 km2 contraction square, which is ten times less than in the line with bare wires.
The conclusion is unambiguous: the SIW wires for aerial lines effectively struggle with the overvoltage in these communications when exposed to lightning.
When organizing lightning protection of private sector buildings, it is important to take into account important nuances, namely:
- connection of a low-rise building to a substation by means of SIW wires by air will significantly increase its security against lightning. At the same time, a SIW cable sveral tens of meters in length, which is sometimes recommended, is absolutely unnecessary in the building;
- SIW wire cannot withstand direct lightning. Impacts will be identical to its getting into the building.
What are the effects of lightning in multi-storey buildings
Electrical communications of high-rise buildings are mainly underground. Here, one should not be afraid of stimulation of induced overvoltages. In reality, overvoltage can come from a lightning current that propagates through the metal elements of the house structure and its down conductors.
In addition, phase wires for supplying the substation, conducted through the air, can also serve as a source of excess voltage at lightning strikes. It is quite possible for it to get into the transformer on the low side by connecting the capacitances between the windings. It should be noted that in practice this situation practically does not occur, because different measures are applied at the substations. Experts on lightning protection of buildings outside do not work on this aspect.
Their task is to give a correct estimate of the number of direct strikes. In multi-storey buildings, mainly not having a large area occupied on the ground, the height parameters play a significant role. In this case, the number of direct strikes is assumed to be not less than a few dozen during one storm season. The elementary estimate is calculated by the formula № 3:
Formula 3. Estimation of the number of direct lightning strikes
The material was created on the basis of an article by Professor Eduard Meerovich Bazelyan "Remote lightning effects".
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