TRANSIENT VOLTAGE SURGE SUPPRESSORS (TVSS)

Thursday, May 19, 2011

TVSS (Transient Voltage Surge Suppressor) is a power quality device designed to protect electrical and electronic equipment from surges. In other words, the purpose of a TVSS is to minimize or eliminate damages to sensitive and critical equipment due to transient voltages and currents. The source of these surges can either be internal (e.g. inductive switching), or external (e.g. lightning).

A TVSS is a relatively inexpensive mitigating device reducing production losses due to transients (if properly installed). TVSS is important since the excess energy can cause damages to equipment, especially to process control instruments, data processing and other critical equipment. Other benefits include, but not limited to:

Ø  Data processing and digital equipment protection
Ø  Reduction of motor stress and overheating
Ø  Longer lighting lamp and ballast life expectancy

The basic TVSS forms are the hardwired and the plug-in or point-of-use. Hardwired TVSS units are utilized as a primary protection for the entire facility, group of equipment or sensitive circuits. They are commonly applied at the service entrance or circuit breaker panels. A TVSS installed at the mentioned locations shall provide protection for all downstream electrical components from all upstream transient disturbances. On the other hand, plug-in or point-of-use TVSS are inserted into outlets to protect electronic devices or certain appliances. 
Plug-in TVSS
Plug-in TVSS
Hardwired TVSS
Hardwired TVSS
The term TVSS is usually used interchangeably with surge arresters. Nonetheless, it should be made clear that the two devices are not the same, although they both provide surge protection. According to the National Electrical Code (NEC), surge arresters are generally applied in the line side of the meter. Meanwhile, the TVSS is classified as a Type 2 and Type 3 Surge Protective Device and is commonly installed on the load side or the end-user equipment. Some are listed for use in either location; however, most surge protection devices are designed for use in only one or the other.

Applications

Electronic components in modern electrical devices are more sensitive to sudden changes in supply voltage and current. The microprocessor, which is an integral part of computers are particularly sensitive to transients. Electrical equipment are exposed to surges from AC power, telephone and signal lines. 

The TVSS is suitable in the following applications, but not limited to:

Ø  Computers and peripherals such as printer and monitor
Ø  Telephone lines (modem and fax machine)
Ø  PABX and communication equipment
Ø  Data processing
Ø  Medical and scientific equipment
Ø  Weighting bridges and measuring equipment
Ø  Security systems
Ø  Entertainment components
Ø  Other sensitive electrical equipment

Transient Voltage Surge Suppressors are to be installed at points prone to surges and limit them below the withstand level of the equipment. ANSI C62.41 defined three categories of surge level, based on strategic location within a facility's wiring network, where transients could be encountered.

Basic Operation

A TVSS primarily functions to divert the excess voltage and current from transients into ground. It acts as a voltage sensitive switch that constantly monitors the voltage. It is not activated during normal conditions, but when the voltage difference between the protected line and the power supply exceeds threshold, the TVSS will divert the transients away from the equipment. This switching voltage is often called the clamping voltage. This process prevents transients to flow through the electrical and electronic equipment, while simultaneously allowing normal voltage to continue along the circuit.

Basic Components

The three basic components used to reduce or limit high voltage usually includes: 

1.    Metal Oxide Varistor (MOV) 

It is compose of zinc oxide, which is a semiconductor with a variable resistance value. The MOV presents itself as a high impedance device during normal operating conditions. However, when incoming voltage exceeds the threshold, the resistance of the MOV decreases to provide a low impedance path to ground, thus, diverting transients. 

2.    Gas Discharge Tube (GDT) 

GDT can divert the excess current from line to ground by using inert gas as conductor to ground. The inert gas acts as a poor conductor during normal conditions, but it is ionized when the voltage is above limits. Subsequently, it becomes an effective conductor to pass on current to ground until voltages are within limits. Gas Discharge Tubes conduct more current for their size than other components. 

3.    Silicon Avalanche Diode (SAD) 

SAD provides the limiting action of protective component, but has a lower energy capability. If the voltage increases above the threshold level, SAD will tolerate avalanche breakdown with resulting voltage conducted to ground.

The TVSS specifications will provide information as to what component is used. Higher quality devices will use multiple components to benefit from their different characteristics (hybrid type). Moreover, resistors, capacitors and/or inductors are used in conjunction with the components mentioned above.

References:

BC Hydro Power Smart. Transient Voltage Suppressors (TVSS) for Commercial/Residential Applications
Data Center Journal. What is a TVSS?
Leonics Company. Surge Protection Devices

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.