Components and protective circuits
When surge voltages occur, the affected devices and cables must be short-circuited with the equipotential bonding within a very short time. Various components with appropriate properties are available for this.
These components mainly differ with regard to their response behavior and discharge capacity.
- 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, for example. However, these versions are considerably larger and are therefore hardly ever used in combined protective circuits.
- Medium protection
- Response times in the lower nanosecond range
- Respond faster than gas-filled surge arresters
- Do not cause line follow currents
Varistors with a nominal discharge surge current of up to 2.5 kA are used as a medium protection stage in measurement and control technology. In the field of power supply, varistors with a nominal discharge surge 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 arresters are considerably more powerful. In this area of application, the standard version supports nominal discharge surge currents of up to 20 kA. For special applications, type 2 arresters with up to 80 kA are also available.
Gas-filled surge arresters
- Coarse 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.
- Core of a lightning arrester
- High extinguishing capacity for line follow currents
- Relatively high response speed
- Ignition behavior is not stress-dependent
In the majority of cases, the core of a high-performance lightning arrester is a spark gap. In this component, two spark horns face each other at a short distance. Surge voltages cause a sparkover between the spark horns and an arc occurs. This plasma path short circuits the surge voltage. Very high and steep rising currents flow here, with values into the three-figure kA range. There are open and closed spark gaps. The discharge and extinguishing capacity of open spark gaps are greater due to their nature.
Arc chopping technology has proven to be particularly effective for spark gaps. In this technology, there is also a baffle plate opposite the electrodes. The arc is forced between the electrodes toward the baffle plate and is chopped up when it hits it. Arc fragments then form and are blown out of the area of the spark gap and then easily extinguished. The spark gap can then return to a high-resistance state when the surge voltage is no longer present.
Combined protective circuits for signal interfaces
Various components are used depending on the application. They can be used individually or also combined in complex protective circuits.
Using a combination of different components means that the desired component-specific advantages can also be combined. For example, combinations of circuits comprising gas-filled surge arresters and suppressor diodes represent a standard protective circuit for sensitive signal interfaces. This combination provides high-performance and fast-responding protection with the best possible protection level.
The components are connected indirectly in parallel as protection stages. In other words, an ohmic or inductive decoupling element is looped in between the components. This causes a staggered response from the protection stages, which have an offset arrangement.
The various protective circuits differ primarily according to:
- The number of protection stages
- The direction of action of the circuit (common/normal mode voltage protection)
- The nominal voltage
- The damping effect on signal frequencies
- The protection level (clamping voltage)
Function of multi-stage protective circuits
When a surge voltage occurs, the suppressor diode responds first as the fastest component. The discharge current flows through the suppressor diode and the decoupling resistor connected upstream. A voltage drop occurs via the decoupling resistor, which corresponds to the difference between the various operate voltages of the suppressor diode and the gas-filled surge arrester.
The operate voltage of the gas-filled surge arrester is therefore reached before the surge current overloads the suppressor diode. That means that when the gas-filled surge arrester has responded, almost the entire discharge current flows through the gas-filled surge arrester. The residual voltage via the gas-filled surge arrester is a maximum of 20 V, which relieves the suppressor diode. If the discharge current is low and does not overload the suppressor diode, the gas-filled surge arrester will not respond.
UG = Operate voltage of gas-filled surge arrester
UD = Clamping voltage of suppressor diode
UW = Differential mode voltage via the decoupling resistor
The above-illustrated circuit offers the advantages of fast response with low voltage limitation as well as a high discharge capacity. A three-stage protective circuit with inductive decoupling operates 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 arrester.
The principle of voltage distribution also generally applies between the various protection stages in the field of power supply. UW drops via the cable between the type 1 and type 2 arresters and between the type 2 and type 3 arresters. However, there are also arrester concepts for the power supply where coordination between the protection stages is possible without lengths of cable.