Spark-gaps are composed of 2 electrodes separated by a gas (oxygen or a noble gas such as argon or neon).
In encapsulated spark-gaps, the electrodes are connected to each other through an isolating tube made of glass or ceramic where the gas can be found.
The main advantage of spark-gap SPDs is a low arc resistance similar to a short circuit combined with a great discharge capacity as well as a minor stray capacitance adequate to protect high-frequency data signals. The main disadvantage is the required time to ionize the gas. For slow voltage rises (up to 10V/μs), the ignition voltage is equal to the nominal ignition voltage whereas for fast voltage rises the “dynamic ignition voltage” must be taken into account. Subsequently, the protection level is not clearly defined and can be far higher than the initial nominal ignition voltage.
An other drawback of the spark-gap SPDs is the follow-through current. When ignited, the electric arc is maintained because of the main voltage until the current decreases. In industrial facilities, this follow-through current can reach high levels thus requiring the installation of an adequate disconnector.
A varistor is a component whose resistance fluctuates according to the voltage applied at its terminals: the higher the voltage, the lower the resistance. It is generally composed of ceramic and zinc oxide grains. Once a certain voltage level is reached, the layer lets the current flow meaning that a varistor is a non-polarized element which can be used in DC and AC installations.
Varistors outmatch spark-gaps in several aspects: the triggering time is shorter (25 ns only when the connections are the kept as short as possible to reduce the inductance) and no follow-through current.
However, the varistor’s major flaw is the limited amount of energy it can deflect based on the volume of the component. A strong lightning impulse (Imax) or an accumulation of smaller overvoltage surges (In) are both ways to reach this limit. In such case, the absorbed surges impact on the varistor’s reference voltage as it declines to the level of the main voltage and generates high leakage current. In the absence of a thermal disconnector, the power loss increases causing the potential explosion of the element in extreme cases (which is not the case for MOV SPDs complying with the NF EN 61643-11 standards which have an open circuit end-of-life).
Dedicated Zener diodes (TVS, Transil, “avalanche”) are designed to clamp overvoltage surges very quickly and to withstand high curents to protect sensitive equipment.
The diodes used in the FUSADEE® SPDs share similar aspects with the MOVs but possess 4 main advantages:
• Thanks to their triggering time, they are the only SPDs that can safely protect a device exposed to fast transient surges (triggering time in picoseconds).
• The maximal peak current that can be handled by the component without damage can reach hundreds of amperes and its lifespan is unlimited as long as this value is not exceeded.
• The non-linearity of the PSD offers lower (and better) protection levels.
• The EOL of the FUSADEE® SPDs always occurs in short circuit (fail-safe mode) with a continuity of protection against type of overvoltage surge.