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Surge protection – basics Technology, standards, and directives for surge protection
The answers to the following questions can be found here:
- How do surge voltages occur and what effect do they have?
- How do you create an effective surge protection concept?
- What is the technology behind the protection concept and products?
- What do you need to bear in mind?
Causes of overvoltages
Overvoltage – 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. Below you will find 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:
Lightning discharges (LEMP)
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 (SEMP)
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.
Electrostatic discharges (ESD)
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.
Surge voltages can get into circuits in various ways. These are known as coupling types.
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 an overvoltage.
This voltage affects all cables that are connected to the central equipotential bonding. An overvoltage also occurs along conductors carrying lightning current. Due to the fast rate of current rise, 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 overvoltage 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 an overvoltage in other cables in its vicinity, as is the case in the secondary winding of a transformer. The coupled overvoltage 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 an overvoltage in the affected cables and devices.
Direction of action of surge voltages
Surge voltages act in two directions in affected circuits.
Common mode voltage
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 voltage
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.
Effects of overvoltages
More often than not, overvoltages which couple into a circuit cause considerable damage to equipment and devices. Devices that are in constant use are at particularly high risk. Here, this damage can result in extremely high costs.
It is not just the replacement or repair of damaged devices that costs money. Even more expensive are long system downtimes or even the loss of software or data.
Incidence of loss
Every year, the statistics from insurers show high figures for the incidence of loss caused by overvoltages. In the majority of cases, operators of electronic systems are compensated by their insurance for damage to the hardware. However, software damage and system failure frequently remain uninsured, leading to great financial burdens.
According to statistics from German insurers for 2019, the proportion of lightning and surge damage alone makes up a notable proportion. Even though the number of claims has fallen slightly in recent years, around 200 million euros have been paid out annually for household contents and residential building insurance claims. (Source: German Insurance Association, GDV)
Each circuit works with its own specific voltage. Therefore, any voltage increase that exceeds the upper tolerance limit is an overvoltage.
The extent of the damage depends largely on the electric strength of the components used and the energy that can be converted in the affected circuit.
The protective circuit principle describes a concept for complete protection against overvoltages. An imaginary circle should be drawn around the item to be protected. Surge protective devices should be installed at all points where cables intersect this circle. The nominal data of the relevant circuit must be taken into consideration when selecting the protective devices. The area within the protective circuit is therefore protected in such a way that conducted surge voltage couplings are prevented.
The protective circuit concept can be broken down into the following areas:
- Power supply
- Measurement and control technology
- Information technology
- Transmitter and receiver systems
In order to achieve effective protection, it is important to determine where devices that are in danger are located and what influences represent a danger to the devices. The following figure shows a typical single-family home used as an example to illustrate the location of the individual protection zones.
The abbreviation LPZ stands for lightning protection zone and refers to the various danger zones. A distinction is made between the following zones:
- LPZ 0A (direct lightning strike): Refers to the danger zone outside the building.
- LPZ 0B (direct lightning strike): Refers to the protected danger zone outside the building.
- LPZ 1: Refers to a zone inside the building where high-energy overvoltages represent a danger.
- LPZ 2: Refers to a zone inside the building where low-energy overvoltages represent a danger.
- LPZ 3: In this zone, overvoltages and other influences caused by the devices and cables themselves represent a danger.
Effects of surge currents in cables
Overvoltages are limited by discharging high-frequency currents and therefore transient processes. This means that it is not the ohmic resistance but the inductive resistance of a cable that is of primary importance.
According to Faraday’s law of induction, when these types of surge currents are discharged to ground potential, overvoltages are created again between the coupling point and ground.
u0 = L x di/dt
u0 = induced voltage in V
L = inductance in Vs/A in H
di = current change in A
dt = time interval in s
The inductive resistance can only be reduced by shortening the cable length or connecting discharge paths in parallel. To minimize the total impedance of the discharge path and therefore the residual voltage, mesh-shaped equipotential bonding that is as tightly meshed as possible is the best technical solution.
Complete protection can only be achieved through complete isolation or through complete equipotential bonding. However, since complete isolation is impossible for many practical applications, only complete equipotential bonding remains.
To achieve this, all electrically conductive parts must be connected to the equipotential bonding system. Protective devices are used to connect live cables to the central equipotential bonding. In the event of an overvoltage, they are conductive and short circuit the overvoltage. Damage from overvoltages can therefore be prevented effectively.
Various equipotential bonding systems can be created:
- Line-shaped equipotential bonding
- Star-shaped equipotential bonding
- Mesh-shaped equipotential bonding
Mesh-shaped equipotential bonding is the most effective method, as all electrically conductive parts have a separate cable here and additional cables connect all end points via the shortest route. This type of equipotential bonding is suitable for particularly sensitive systems, such as computer centers.
Multi-level protection concept for the power supply
The measures required to protect devices and systems are divided into two or three levels depending on the protective devices chosen and the environmental influences to be expected. The protective devices for the individual levels differ with regard to the discharge capacity level and the voltage protection level depending on which protection level they belong to.
Three-level protection concept with separately installed protection levels:
- Type 1: Lightning current arrester
Voltage protection level <4 kV, typical installation location: main distribution
- Type 2: Surge protective device
Voltage protection level <2.5 kV, typical installation location: subdistribution
- Type 3: Device protection
Voltage protection level <1.5 kV, typical installation location: upstream of the end device
Protection levels 1 and 2 can also be implemented in a type 1+2 combined lightning current and surge arrester. This protective device meets the same requirements as type 1 and type 2 arresters. The main advantage is the easy installation. In addition, no special installation conditions have to be taken into consideration.
Three-level protection concept with type 1+2 combined lightning current and surge arrester and separate type 3 arrester:
- Type 1+2 combined lightning current and surge arresters
Voltage protection level <2.5 kV, typical installation location: main distribution
- Type 3: Device protection
Voltage protection level <1.5 kV, typical installation location: upstream of the end device
Components and protective circuits
When overvoltages occur, the affected devices and cables must be short-circuited with the equipotential bonding within a very short time. Various components with suitable properties are available for this. Essentially, these components differ in terms of their response behavior and discharge capacity.
- The function is commonly referred to as fine protection.
- Responds very quickly.
- Low voltage limitation.
- Standard version with low current carrying capacity and high capacitance.
- At a nominal voltage of 5 V the maximum discharge capacity is approximately 750 A.
- At higher nominal voltages the discharge capacity drops significantly.
There are also diodes with a higher nominal voltage and greater discharge capacity. However, these versions are considerably larger and are therefore hardly ever used in combined protective circuits.
UR = reverse voltage
UB = breakdown voltage
UC = clamping voltage
IPP = surge current pulse
IR = reverse current
- The function is commonly referred to as medium protection.
- Response times in the lower nanosecond range.
- Responses are faster than gas-filled protective devices.
- Do not cause line follow currents.
Varistors with a nominal discharge current of up to 2.5 kA are used as a medium protection level in MCR technology. In the field of power supply, varistors with a nominal discharge current of up to 3 kA are a key component of protective circuits in type 3 arresters for device protection. Varistors used in type 2 surge protective devices are considerably more powerful. In this area of application, the standard version can withstand nominal discharge currents of up to 20 kA. For special applications, type 2 protective devices with up to 80 kA are also available.
A = high-resistance operating area
B = low-resistance operating area/limiting area
Gas-filled surge protective devices
- The function is commonly referred to as medium protection.
- Response times in the medium nanosecond range.
- Standard versions discharge currents of up to 20 kA.
- Despite its high discharge capacity, the component has very compact dimensions.
With this component, stress-dependent ignition behavior leads to residual voltages that can reach several 100 V.
- Static response behavior
- Dynamic response behavior
- Key component of a lightning current arrester
- High extinguishing capacity for line follow currents
- Relatively high response speed
- Stress-dependent ignition behavior
In most cases, the key component of a powerful lightning current arrester is a spark gap. This component contains two spark horns that face one another and are positioned closely together. Overvoltages cause a sparkover between the spark horns and an electric arc is created. This plasma field short-circuits the overvoltage. Very high and steep rising currents flow here, with values into the three-figure kA range. There are open and closed spark gaps. Physically, the discharge and extinguishing capacity of open spark gaps is greater.
Arc chopping technology has proven to be particularly effective for spark gaps. In this case, a so-called baffle plate is also positioned opposite the electrodes. The electric arc between the electrodes is channeled toward this baffle plate, where it is dispersed. This results in the formation of electric arc fragments, which are blown away from the spark gap and can then be easily extinguished. This enables the spark gap to exhibit a high resistance again once the overvoltage is no longer present.
UZ = sparkover voltage/strike voltage
tZ = response time
Combined protective circuits for signal interfaces
Various components are used depending on the application. They can be used individually or combined in complex protective circuits.
The desired component-specific advantages can be brought together by combining various components. Combined gas discharge tube and suppressor diode circuits, for example, represent a standard protective circuit for sensitive signal interfaces. This combination provides high-performance and fast-responding protection with the best possible voltage protection level.
The components are indirectly switched in parallel as protection levels. In other words, an ohmic or inductive decoupling element is looped in between the components. This causes the staggered protection levels to respond at different times.
The protective circuits differ primarily according to:
- Number of protection levels
- Direction of action of the circuit (common-mode/normal-mode protection)
- Nominal voltage
- Damping effect on signal frequencies
- Voltage protection level (clamping voltage)
Function of multi-level protective circuits
When an overvoltage occurs, the suppressor diode responds first as the fastest component. The discharge current flows through the suppressor diode and the upstream decoupling resistor. A voltage drop occurs via the decoupling resistor. This corresponds to the difference between the various sparkover voltages of the suppressor diode and the gas-filled surge protective device.
In this way, the sparkover voltage of the gas discharge tube is reached before the surge current overloads the suppressor diode. This means that when the gas-filled surge protective device has responded, the discharge current flows almost entirely through the gas discharge tube. The residual voltage via the gas discharge tube is a maximum of 20 V, which relieves the suppressor diode. In the event of a low discharge current that does not overload the suppressor diode, the gas-filled surge protective device does not respond.
The illustrated circuit offers the advantages of a fast response with low voltage limitation and has a high discharge capacity. A three-level protective circuit with inductive decoupling works according to the same principle. However, the commutation takes place in two steps: First from the suppressor diode to the varistor, and then on to the gas-filled surge protective device.
The voltage distribution principle also works between the different protection levels around the power supply. UW drops via the cable between the type 1 and type 2 protective devices and between the type 2 and type 3 protective devices. However, there are also surge protective devices for the power supply where coordination between the protection stages is possible without lengths of cable.
UG = sparkover voltage of the gas-filled surge protective device
UD = clamping voltage of the suppressor diode
UW = difference in voltage over the decoupling resistance
Standards and directives
The various standards explain the requirements for installation and safety as well as the use of products in various applications in detail. The main individual topic areas are listed below along with the associated international standards.
Classification of surge protective devices
Surge protective devices (SPDs) are items of equipment whose key components can be varistors, suppressor diodes, gas discharge tubes (GDTs), or spark gaps. Surge protective devices are used to protect other electrical equipment and electrical systems against impermissibly high transient overvoltages and transient currents. Surge protective devices are divided into “classes” in accordance with the relevant product and application standards for surge protective devices.
Surge protective devices are grouped according to their application and protective function:
Surge protective devices (SPDs) for use in low-voltage systems with a nominal voltage of up to 1000 V.
The national installation specifications for low-voltage systems, such as IEC 61643-12, IEC 60364-5-53 Part 534, and VDE 0100 Part 534, must be observed for product selection and installation. The product standard is EN (IEC) 61643-11. According to this standard, surge protective devices in IEC and EN standards are divided into three test classes based on their discharge capacity and typical installation locations:
Type 1 SPDs: Powerful surge protective devices for discharging high-energy surge currents/surge voltages caused by direct or nearby lightning strikes. Installation location: At the boundary between lightning protection zone LPZ 0A and lightning protection zone LPZ 1 – typically in main distributions. Type 1 SPDs are always recommended if the building has an external lightning protection system.
Type 2 SPDs: Surge protective devices for discharging surge currents/surge voltages caused by remote lightning strikes, inductive or capacitive couplings, and switching overvoltages. Installation location: At the boundary between lightning protection zone LPZ 0B and LPZ 1 or at the boundary between LPZ 1 and LPZ 2 – typically in main distributions and/or subdistributions.
Type 3 SPDs: Additional surge protective devices (device protection) for protecting sensitive end devices. Installation location: At the boundary between lightning protection zone LPZ 2 and LPZ 3 – typically in close proximity to sensitive end devices. These sensitive end devices may include devices for permanent installation in distributions or portable protective devices in the socket area directly upstream of the end device that is to be protected.
General information can be found in the IEC 61643-12 or DIN EN 61643-12 Application Guide (selection and application principles). The four parts of EN (IEC) 62305-…/VDE 0185-305-… cover the basics of lightning protection, the lightning protection zone concept, and risk analysis.
Surge protective devices for use in telecommunications and signaling networks designed to protect against the indirect and direct effects of lightning strikes and other transient overvoltages. These also include low-voltage data systems, measurement and control circuits, and voice transmission networks with nominal voltages up to 1000 V AC and 1500 V DC.
The product standard is EN 61643-21, VDE 0845 Part 3-1. According to this standard, the devices are divided into categories A1, A2, B1, B2, B3, C1, C2, C3, and D1, D2 in order to define the test requirements and performance classes. A protective device can be marked and tested for various categories and performances classes.
General information can be found in the IEC (TS) 61643-22 Application Guide. Parts of VDE 0800… and VDE 0845… provide additional information. Additional national specifications must be observed.