Voltage funnel

The voltage funnel or potential funnel refers to the course of the electrical potential near the point where lightning strikes the ground or a high-voltage conductor comes into contact with the ground.

Potential funnel (red line), which forms around the contact point if the overhead line is torn off and comes into contact with the ground

High potential differences occur at the point where contact with the ground is made. The equipotential lines are approximately circular, the potential decreases exponentially with increasing distance from the contact point . If the potentials are displayed graphically as a potential curve, as shown in the adjacent illustration in a sectional view as a red line, the name-giving funnel shape results. The "depth" of the funnel describes the potential curve on the earth's surface. The electrical voltage on the conductor is , which at the same time represents the peak value. ${\displaystyle U_{L}}$

If there is a person in the area of ​​influence of the voltage funnel, their feet stand on areas with two different electrical potentials during one step; in the graphic these are the intersection of the soles of the feet on the floor with the voltage funnel, so that a voltage with height between the legs occurs, which is also known as step voltage . If this voltage exceeds the limit value of a low voltage of 50 V AC or 120 V DC, there is a risk of an electrical accident . ${\displaystyle U_{S2}-U_{S1}}$

Due to the exponential decrease in potential, the step voltage is higher the closer you get to the contact point, with the same step length. For this reason, a minimum distance of 20 meters is recommended for faulty high-voltage systems that come into contact with the ground. The safety distance of 20 m is designed for an earth resistance of 1  kΩ with an earth fault current of 132  A and a maximum permissible step voltage of 120 V, which requires conductor voltages ( ) of more than 100 kV. In many cases, as is the case with the frequent medium-voltage lines , the safety distance is reduced to a few meters due to the lower voltages. There is therefore no need to maintain a safety distance in low-voltage networks. ${\displaystyle U_{L}}$

The safety distance of 20 m represents a distance if it is not known or cannot be assessed how the so-called neutral point treatment of the affected system is carried out. Networks with low-ohmic neutral point earthing (NOSPE), as they are common in 220 kV or 400 kV systems, are switched off within fractions of a second in the event of an earth fault , which means that a voltage funnel only occurs for a short time. At the 110 kV distribution network level, overhead lines are usually operated with resonance neutral point earthing (RESPE) and remain in operation in the event of a simple earth fault in order to avoid national power failures . In this case, a residual earth fault current flows at the point of contact between the conductor and the ground. This residual earth fault current is significantly lower than a short circuit current , but high enough to be able to cause a dangerous potential funnel.

The term “voltage cone ”, which is also used, is misleading, since with a geometric cone the potential would drop linearly and the step voltage would be the same everywhere regardless of the distance to the contact point.

Individual evidence

1. ↑ Deutsche Gesetzliche Unfallversicherung eV (DGUV) 2011: Information on electrical hazards at the scene of operations - Lecture for emergency services (BGI / GUV-I 8677), July 2011 edition. Internet source: http://publikationen.dguv.de/dguv/pdf/10002 /i-8677.pdf