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WHAT IS TRANSIENT VOLTAGE SURGE SUPPRESSION? (TVSS)

Transient voltages and surges on power lines are caused by faults, switching and other interruptions of steady state power consumption. Transients and surge durations are less than a half-cycle of the normal voltage waveform and generally less than a millisecond. These fast changes in voltage and power can stress electronics and electrical equipment. Electrical stressing can interfere with the normal equipment operation and can also cause premature failure. Thus increase operational costs and company down time during corrective actions. Depending on a buildings electrical distribution, transients and surges can be amplified at different locations inside the building. This amplification is due to the various loads causing reflections of the energy throughout the wiring. These reflected transients and surges will sometimes cancel each other out while lowering the threat to equipment. Other times these reflected waves will add together while increasing the threat. TVSS devices are used to control the abnormal current flows caused by such reflections. A TVSS device diverts the energy away from the electrical and electronic equipment through the grounding system. The key to TVSS device operation is first having a good grounding system.

TVSS devices can be grouped into four categories: Metal Oxide Varistor (MOV), TransGuards (silicon device), Thyristor, Arc Gap.

Arc Gap Device

The arc gap is one of the oldest protection devices. It is typically made up of two sharp points of metal enclosed in a ceramic case. The case is then filled with a gas. The combination of gap distance and the type of gas used, determines the breakover voltage of the device. Arc gap devices can handle high currents easily. However once the arc is formed, all energy sources must be removed from the device for arc to stop. This extinguish time allows the removal of the plasma path formed during an arc event. Otherwise the current will continue to flow through the arc as normal power returns to the circuit. The typical failure modes for arc gap devices are breakover voltage changes when the sharp points melt or the gas leaks. Then the device opens and will no longer function. Arc gap devices are typically used for mid to high voltage, low capacitance applications.

TransGuards Device

A transguard device is a silicon-based device similar to a zener diode. These devices are enclosed in plastic. The device is reversed biased in the circuit. At the breakover voltage, the device conducts the energy away from the circuit it is protecting. These devices will clamp the voltage to a certain level for currents up to the failure point. A transguard device fails open due to melting of the PN junction. This failure is usually due to overheating of the device under high current or repeated transient events. TransGuard devices are typically used in low to mid voltage application that can operate with some capacitive loading.

Thyristor

A Thyristor device is a semi-conductor device that acts like a high impedance resistor under normal conditions. However, when the device sees a fast voltage change, it will conduct and divert the energy away from the circuit it is protecting. These devices are usually enclosed in a plastic case and fail open due to the same characteristics as transguards. Thyristor are usually used for low voltage, low capacitance situations.

Metal Oxide Varistor (MOV)

MOV’s are made up of compacted metal oxide particles between two plates. The oxide crystal size and distance between the plates determines the breakover voltage characteristics. The cross sectional area determines the current handling capability. These devices are usually enclosed in a plastic case. The clamping voltage of an MOV is proportional to the current that flows through it up until failure. This means that a higher transient current will have a higher clamping voltage. This is not like the silicon-based devices. The MOV’s first failure mode is a short due to the melting of oxide particles. As the device continues to conduct under the short condition, it will eventually open due to the complete melting of the particles. This usually only happens in extreme conditions. The MOV is designed to handle very high currents, and is used in mid to high voltage applications that can operate with some capacitive loading.

CONCLUSION

The most common TVSS used in mid to high voltage applications is the metal oxide varistor. Given the devices failure mode it is typically packaged with a thermal fuse and some sort of indicator, so one can tell that the device is still operational. The arc gap is used in very high current applications or ones that require very low capacitive loading. The thyristors and transguards are used in low to mid voltage applications, typically on circuit card internal to electronic equipment. The combination of these devices at the proper locations can greatly reduce equipment damage and company downtime.