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 suitable properties are available for this. Essentially, these components differ in terms of their response behavior and discharge capacity.

Suppressor diodes

Graphic symbol for and U/I characteristic curves of a suppressor diode  

Graphic symbol for and U/I characteristic curves of a suppressor diode

Properties:

  • 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.

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 = Peak pulse current
IR = Reverse current

Varistors

Graphic symbol for and U/I characteristic curves of metal oxide varistors  

Graphic symbol for and U/I characteristic curves of metal oxide varistors

Properties:

  • The function is commonly referred to as medium protection.
  • Response times in the lower nanosecond range.
  • Respond faster than gas-filled surge arresters.
  • Do not cause line follow currents.

Special features:

Varistors with a nominal discharge current of up to 2.5 kA are used as a medium protection stage 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 arresters 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 arresters with up to 80 kA are also available.

Key:

A = High-resistance operating area
B = Low-resistance operating area/limiting area

Gas-filled surge protective devices

Graphic symbol for and characteristic ignition curve of a gas-filled surge protective device  

Graphic symbol for and characteristic ignition curve of a gas-filled surge protective device

Properties:

  • 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.

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

Spark gaps

Graphic symbol for and characteristic ignition curve of a spark gap  

Graphic symbol for and characteristic ignition curve of a spark gap

Properties:

  • Key component of a lightning current arrester
  • High extinguishing capacity for line follow currents
  • Relatively high response speed
  • Stress-dependent ignition behavior

Special features:

In most cases, the key component of a powerful lightning current arrester is a spark gap. This component contains two arcing horns that face one another and are positioned closely together. Surge voltages cause a sparkover between the arcing horns and an electric arc is created. This plasma field 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. 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 surge voltage is no longer present.

Key:

UZ = Response 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.

Two-stage protective circuit with ohmic decoupling (left) and three-stage protective circuit with inductive decoupling (right)

Two-stage protective circuit with ohmic decoupling (left) and three-stage protective circuit with inductive decoupling (right)

The desired component-specific advantages can be brought together by combining various components. Combined gas-filled surge arrester 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 stages. In other words, an ohmic or inductive decoupling element is looped in between the components. This causes the staggered protection stages to respond at different times.

The protective circuits differ primarily according to:

  • Number of protection stages
  • 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-stage protective circuits

Voltage distribution in a two-stage protective circuit  

Voltage distribution in a two-stage protective circuit

When a surge voltage 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 response voltages of the suppressor diode and the gas-filled surge protective device.

In this way, the response voltage of the gas-filled surge arrester 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-filled surge arrester. The residual voltage via the gas-filled surge arrester 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-stage 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 stages around the 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.

Key:

UG = Response 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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Rydalmere NSW 2116

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