Causes of surge voltages

Surge voltage – what is it exactly? How do surge voltages occur? How do surge voltages get into your devices and systems? You've probably wondered about the answers to these questions. The following pages provide comprehensive information on surge protection technology.


Surge voltages only occur for a fraction of a second. For this reason, they are called transient voltages or transients. They have very short rise times of a few microseconds before they drop off again, relatively slowly, over a period of up to 100 microseconds.

Surge voltages occur as a result of:

The technical term for lightning discharge is LEMP. This stands for lightning electromagnetic pulse.

Lightning strikes during storms cause extremely high transient overvoltages. They are much higher than surge voltages that are caused by switching operations or electrostatic discharge. However, they occur a lot less frequently than other causes.

Switching operations are referred to with the abbreviation SEMP. This stands for switching electromagnetic pulse.

In this context, switching operations mean the switching of powerful machines or short circuits in the power supply network. During such operations, significant current changes occur in the affected cables in a split second.

The abbreviation ESD stands for electrostatic discharge.

Here, an electrical charge is transferred when bodies with a different electrostatic potential approach or come into contact with one another. A familiar example of this is when a person becomes charged while walking over a wall-to-wall carpet and then discharges to a metal grounded object, such as a metal rail.

Coupling types

Surge voltages can get into circuits in various ways. These are known as coupling types.

Galvanic coupling (left), inductive coupling (center), and capacitive coupling (right)

Galvanic coupling (left), inductive coupling (center), and capacitive coupling (right)

This refers to surge voltages which are directly coupled into a circuit. This can be observed during lightning strikes, for example. In this case, lightning current amplitudes at the grounding resistance of the affected building cause a surge voltage.

This voltage affects all cables that are connected to the central equipotential bonding. A surge voltage also occurs along conductors carrying lightning current. Due to the fast current increase rate, this can mainly be traced back to the inductive component of the cable resistance. Faraday's law of induction is used as the basis for calculating this: u0 = L x di/dt.

This process occurs through the magnetic field of another current-carrying conductor, following the transformer principle. A directly coupled surge voltage causes a surge current with a high rate of increase in the affected conductor.

At the same time, a strong magnetic field is created around this conductor, as is the case in the primary winding of a transformer. The magnetic field induces a surge voltage in other cables in its vicinity, as is the case in the secondary winding of a transformer. The coupled surge voltage is channeled along the cables into the connected device.

This type of coupling primarily occurs via the electric field between two points with a large potential difference. A high potential occurs via the down conductor of a lightning arrester due to a lightning strike. An electrical field is created between the down conductor and other parts with a low potential.

These may be, for example, cables for power supply and signal transmission or devices inside the building. The charge is transferred through the electrical field. This leads to a voltage increase or ultimately a surge voltage in the affected cables and devices.

Direction of action of surge voltages

Surge voltages act in two directions in affected circuits.

Common-mode voltage (left) and normal-mode voltage (right)

Common-mode voltage (left) and normal-mode voltage (right)

Common-mode voltages [UL] occur in the event of interference caused by surge voltages or high-frequency interference voltages between active conductors and ground. The term asymmetrical is also often used.

Asymmetrical voltages primarily endanger components that are located between active potentials and a grounded ground, as well as the insulation between active potentials and ground. This results in sparkovers on PCBs or between voltage-carrying equipment and grounded housing parts.

Normal-mode voltages [UQ] occur in the event of interference caused by surge voltages or high-frequency interference voltages between the active conductors of a circuit. The terms symmetrical and differential-mode are also used.

Symmetrical surge voltages endanger the voltage and signal input of devices and interfaces. This results in direct overload and destruction of the affected equipment, e.g., in the power supply or signal-processing components.


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