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.
Properties:
Special features:
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.
Key:
UR = Reverse voltage
UB = Breakdown voltage
UC = Clamping voltage
IPP = Surge current pulse
IR = Reverse current
Properties:
Special features:
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.
Key:
A = High-resistance operating area
B = Low-resistance operating area/limiting area
Properties:
Special features:
With this component, stress-dependent ignition behavior leads to residual voltages that can reach several 100 V.
Key:
1) Static response behavior
2) Dynamic response behavior
Properties:
Special features:
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.
Key:
UZ = Sparkover voltage/strike voltage
tZ = Response time
Various components are used depending on the application. They can be used individually or combined in complex protective circuits.
Two-level protective circuit with ohmic decoupling (left) and three-level protective circuit with inductive decoupling (right)
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:
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 across the conductor 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.
Key:
UG = Sparkover voltage of gas-filled surge protective device
UD = Clamping voltage of suppressor diode
UW = Differential mode voltage via the decoupling resistor
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