Step voltage and why birds are not electrocuted on medium voltage electrical cables
Hello friends,
Today I want to talk to you about the step voltage and why birds are not electrocuted when they land on electrical distribution lines.
The step voltage is a concept in electricity that is often forgotten and yet it can be the cause of electrical accidents or even the death of both people and animals.
STEP VOLTAGE is defined as the voltage difference on the ground surface that a person could experience when their feet are separated by a distance of one meter and without the person touching any element that may be grounded.
A person could suffer electrical injuries, simply by standing with their feet apart near a ground contact point in a high voltage pole or substation or during a lightning strike.
This step voltage will vary as a function of the current injected into the ground and also the electrical resistivity of the ground.
Birds are not electrocuted when they perch on power lines because the step voltage is very low.
Normally the cables of the three-phase medium voltage power lines are separated enough so that a bird can only touch or perch on one of the cables.
In this case, under normal operating conditions of the line, the voltage difference or step voltage between the two support legs of the bird will be very small, since both the current and the resistivity of the cable, as well as the distance between the legs are relatively small.
For example, if we consider a bird with a distance between legs of 20 centimeters, perched on a medium voltage cable with a nominal current of 250 amps and a resistance of less than 0.45 ohms per kilometer, the calculation of the voltage drop in the cable between the two legs of the bird would give a really low value, which would make it impossible for a dangerous current to flow through the body of the bird.
Another very different thing is the voltage existing between two lines or between a line and the ground, that makes large birds such as some storks, electrocute if they make contact with their wings with two different lines simultaneously or between a line and the structure of a metal tower.
The situation of a bird perched on a medium voltage line is very different from the case of a person in the vicinity of a point where a lightning strike has occurred to the ground, or a ground fault from a pole or substation, since both the current and the resistance of the ground are higher than the case of birds on medium voltage power lines.
In these cases dangerous step voltages could occur. For example, if the ground resistance between two points one meter apart is 0.4 ohms and the fault current is 1000 amps, then we would have a step voltage of 400 volts.
In the case of quadruped animals such as cows, horses, deer, etc. the situation is even worse since the distance between the front and rear legs is greater than one meter, which means that higher step voltages can appear and consequently more current flows through their bodies. In addition, in these cases the situation is especially dangerous since the current would circulate throughout the body through the heart area, which can facilitate the death of the animal.
On the other hand, as these animals tend to group together, we can find cases where a lightning strike has caused the death of several animals. For example, in August 2016, in Norway, more than 300 dead reindeer were found electrocuted by lightning.
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