A surge protector, sometimes called a surge protection device (SPD) or transient voltage surge suppressor (TVSS), is a device typically installed in power distribution
panels, process control systems, communications systems, and other heavy-duty industrial systems, for the purpose of protecting electrical equipment from spikes in
voltage by limiting the voltage supplied to an electric device by either blocking or shorting to ground any unwanted voltages above a safe threshold.
Surge protectors provide critical protection for expensive motors, sensitive electronic instrumentation, computers and communication networks. Without this protection,
surges can affect power cables, data/telephone cables and instrumentation wiring, causing anything from data loss to the total destruction of equipment leaving facilities
exposed to the associated costs of repairs, replacements and downtime.
Surges
Voltage surges are quick, temporary increases in voltage within an electrical circuit. Sometimes referred to as ‘spikes’, ‘overvoltages’ or ‘transients’, these surges
can be caused by faulty or damaged electric wiring, blown fuses, switching of large motors or fluorescent lighting, short circuits, and lightning. Of these, lightning is
potentially the most damaging.
Lightning induced voltage surges can rise from zero to 6kV in about 1μs, far too fast for fuses and circuit breakers to provide adequate protection. The job, then, falls
to surge protectors to add the protection necessary though it is important to note that surge protectors cannot protect equipment against direct lightning strikes. The
power of such strikes is simply too great. Instead, surge protectors dissipate voltage surges on cables caused by inductive or resistive coupling from nearby lightning strikes.
Effective surge protection needs to be comprehensive in nature. When considering surge protection, the ability of the entire system to withstand voltage surges must be weighed
and the protection must be capable of limiting any surge voltages to a level considered safe for the most vulnerable piece of equipment served by the system. Failure to do so
leaves the entire system at risk.
Types of Surge Protection
Numerous types of surge protection have been developed over the years though the current state of surge protection technology compels users to choose between high current
handling capability and high speed operation. This limitation makes hybrid systems, using multiple technologies within a single surge protection system, a common strategy when
protecting expensive equipment.
Though there are many types of surge protectors available, there are two types that are most commonly found in commercial and industrial settings.
Gas Discharge Tube (GDT)
Gas discharge tubes (GDT) are sealed glass tubes that contains a gas mixture trapped between two or more electrodes. When encountering a voltage surge, the trapped gas ionizes
and conducts the surge through the electrodes to ground, safely separating equipment from the surge.
Gas discharge tubes are capable of handling very high surge currents, in excess of 25 KA. They also have an extremely low capacitance and high insulation resistance, making them
almost invisible in normal operation. Because of their high current handling capability, they are capable of protecting power lines.
Though they can effectively handle high voltage surges, gas discharge tubes are somewhat slow to start and can let through some of the surge—sometimes more than 500 V over 100 ns
in duration—before they trigger. Sometimes additional protective components are used to prevent this let-through voltage damage a protected load.
Gas discharge tubes have a finite life expectancy and can be expected to handle a few very large transients or a greater number of smaller transients. Many GDTs are also light-sensitive
and exposure to light lowers their triggering voltage. Therefore, GDTs should be shielded from light exposure.
Metal Oxide Varistor (MOV)
Metal oxide varistors are voltage-dependent resistors in which the resistance material is a metallic oxide, primarily zinc oxide. Metal oxide varistors are a very common type of
voltage clamping device and are available in a wide range of voltages and currents.
Varistors are variable resistors which starts conduction at a specific voltage and stops conduction when the voltage falls below a threshold voltage. The use of a metallic oxide in
their construction means that MOV’s are extremely effective in absorbing short term voltage transients and have higher energy handling capabilities. Metal oxide varistors also have
a very low leakage current as compared to other types of varistors—such as silicone carbon varistors—and its speed of operation in clamping transients is much faster.
Metal oxide varistors offer ery accurate and rapid voltage clamping performance and typically limit voltages to about 3 to 4 times the normal circuit voltage by diverting surge
current away from the protected load. MOVs may be connected in parallel to increase current capability and life expectancy. A thermasl fuse is often incorporated so that the fuse
disconnects before catastrophic failure can happen.
MOVs have finite life expectancy and "degrade" when exposed to a few large transients, or a higher number of smaller transients. As a MOV degrades, its triggering voltage falls lower
and lower.
Important Specifications
There are a number of ways to quantify surge protectors. Among the most important are:
Clamping Voltage
Clamping voltage, also known as the let-through voltage, indicates how much voltage a spike must contain to make the protective components inside a surge protector divert unwanted
energy from a protected line. A lower clamping voltage indicates better protection, but can sometimes result in a shorter life expectancy for the overall protective system. The lowest
three levels of protection defined in the UL rating are 330 V, 400 V and 500 V. The standard let-through voltage for 120 V AC devices is 330 volts.
Joules rating
The Joules rating defines how much energy a MOV-based surge protector can theoretically absorb in a single event, without failure. A commonly misunderstanding is that a lower joule
rating equates to less protection since the total energy in harmful spikes can be significant. In reality, however, well-designed surge protectors should not rely on MOVs to absorb
surge energy, but instead to survive the process of harmlessly redirecting it to ground. A properly installed surge protector dissipates between 4 and 30 joules into ground for every
joule it absorbs. The higher this ratio, the longer the life expectancy of the device.
Response time
Surge protectors don't operate instantaneously; a slight delay always exists and the longer that delay, the longer the connected equipment will be exposed to the surge. However,
surges don't happen instantly either. Surges usually take around a few microseconds to reach their peak voltage, and a surge protector with a nanosecond response time would kick in
fast enough to suppress the most damaging portion of the spike.
Metal oxide varistor surge protectors have response times measured in nanoseconds providing adequate protection from fast voltage spikes. Gas discharge tubes, on the other hand, are
slower-responding and often combined with faster-acting MOVs, to provide more comprehensive protection.
Standards
A number of certifying and regulatory agencies including the International Electrotechnical Commission (IEC), European Committee for Electrotechnical Standardization (CENELEC),
American National Standards Institute (ANSI), Underwriters Laboratory (UL), Institute of Electrical and Electronics Engineers (IEEE) and others have created standards which
define different protector characteristics, test vectors, or operational purpose of surge protectors. None of the standards guarantee that a surge protector will provide proper
protection in a given application. Rather, each standard defines what a surge protector should do or might accomplish, based on standardized tests that may or may not correlate
to conditions present in a particular real-world situation. A specialized engineering analysis may be needed to provide sufficient protection, especially in situations of high lightning risk.
Regulatory compliance may require surge protectors meet a certain standard. If so, it is crucial that the device you choose be marked as such.
If you have any questions regarding surge protectors, please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
