Components and protective circuits

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:

U_R = Reverse voltage
U_B = Breakdown voltage
U_C = Clamping voltage
I_PP = Surge current pulse
I_R = Reverse current

Properties:

• 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.

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:

• 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

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 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:

U_Z = Sparkover voltage/strike voltage
t_Z = Response time

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. U_W 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:

U_G = Sparkover voltage of gas-filled surge protective device
U_D = Clamping voltage of suppressor diode
U_W = Differential mode voltage via the decoupling resistor