Surge protector
A surge protector, spike suppressor, surge suppressor, surge diverter, surge protection device, transient voltage suppressor, or transient voltage surge suppressor is an appliance or device intended to protect electrical devices in alternating current circuits from voltage spikes with very short duration measured in microseconds, which can arise from a variety of causes including lightning strikes in the vicinity.
A surge protector limits the voltage supplied to the electrical devices to a certain threshold by short-circuiting current to ground or absorbing the spike when a transient occurs, thus avoiding damage to the devices connected to it.
Key specifications that characterize this device are the clamping voltage, or the transient voltage at which the device starts functioning, the joule rating, a measure of how much energy can be absorbed per surge, and the response time.
Definitions
The terms surge protection device and transient voltage surge suppressor are used to describe electrical devices typically installed in power distribution panels, process control systems, communications systems, and other heavy-duty industrial systems, for the purpose of protecting against electrical surges and spikes, including those caused by lightning. Scaled-down versions of these devices are sometimes installed in residential service entrance electrical panels to protect equipment in a household from similar hazards.Voltage spikes
In an AC circuit, a voltage spike is a transient event, typically lasting 1 to 30 microseconds, that may reach over 1,000 volts. Lightning that hits a power line can cause a spike of thousands of volts. A motor when switched off can generate a spike of hundreds of volts. Spikes can degrade wiring insulation and destroy electronic devices like light bulbs, battery chargers, modems, televisions, and other consumer electronics.Spikes can also occur on telephone and data lines when AC main lines accidentally connect to them or lightning hits them, or if the telephone and data lines travel near lines with a spike and the voltage is induced.
A long-term overvoltage surge, lasting seconds, minutes, or hours, caused by power transformer failures such as a lost neutral or other power company error, are not protected by transient protectors. Long-term surges can destroy the protectors in an entire building or area. Even tens of milliseconds can be longer than a protector can handle. Long-term surges may or may not be handled by fuses and overvoltage relays.
Surge currents
A building's wiring adds electrical impedance that limits the surge current that reaches the loads when a voltage transient arrives at the service entrance. There is less surge current at longer wire distances and where more impedance is present between the service entrance and the load.Category A loads are more than 60 feet of wire length from the service entrance to the load. Category A loads can be exposed to and surge currents. Category B loads are between 30 and 60 feet of wire length from the service entrance to the load. Category B loads can be exposed to and. Category C loads are less than 30 feet from the service entrance to the load. Category C loads can be exposed to and.
A coiled extension cord can be used to increase the wire length to more than 60 feet and increase the impedance between the service entrance and the load.
Protectors
A transient surge protector attempts to limit the voltage supplied to an electric device by either blocking or shorting current to reduce the voltage below a safe threshold. Blocking is done by using inductors that inhibit a sudden change in current. Shorting is done by capacitors which inhibit a sudden change in voltage or by spark gaps, discharge tubes, Zener effect semiconductors, or metal-oxide varistors, all of which begin to conduct current once a certain voltage threshold is reached. Some surge protectors use multiple elements.In the shorting method, the electrical lines are temporarily shorted together or clamped to a target voltage, resulting in a large current flow. The voltage spike is reduced as the shorting current flows through the resistance in the power lines. The spike's energy is dissipated in the power lines or the ground, or in the protector, converted to heat. Since a spike lasts only tens of microseconds, the temperature rise is minimal. However, if the spike is large enough or long enough, the protector can be destroyed and power lines damaged.
Surge protectors for homes can be in power strips used inside, or a device outside at the power panel. Sockets in a modern house use three wires: line, neutral and ground. Many protectors will connect between all three in pairs, because there are conditions, such as lightning, where both line and neutral have high voltage spikes that need to be shorted to ground.
Additionally, some consumer-grade protectors have ports for Ethernet, cable television and plain old telephone service, and plugging them in allows the surge protector to shield them from external electrical damage.
The characteristic of a TVS requires that it respond to overvoltages faster than other common overvoltage protection components such as varistors or gas discharge tubes. This makes TVS devices or components useful for protection against very fast and often damaging voltage spikes. These fast overvoltage spikes are present on all distribution networks and can be caused by either internal or external events, such as lightning or motor arcing.
Applications of transient voltage suppression diodes are used for unidirectional or bidirectional electrostatic discharge protection of transmission or data lines in electronic circuits. MOV-based TVSs are used to protect home electronics and distribution systems and may accommodate industrial-level power distribution disturbances, saving downtime and damage to equipment. The level of energy in a transient overvoltage can be equated to energy measured in joules or related to electric current when devices are rated for various applications. These bursts of overvoltage can be measured with specialized electronic meters that can show power disturbances of thousands of volts amplitude that last for a few microseconds or less.
It is possible for a MOV to overheat when exposed to overvoltage sufficient for the MOV to start conducting, but not enough to destroy it, or to blow a house fuse. If the overvoltage condition persists long enough to cause significant heating of the MOV, it can result in thermal damage to the device and potentially start a fire.
Comparison of transient suppressors
Domestic use
Many power strips have basic surge protection built in; these are typically labeled as such. However, in countries without regulations, there are power strips labeled as "surge" or "spike" protectors that only have a capacitor, an RFI circuit, or nothing at all and do not provide surge protection.Lightning and other high-energy transient voltage surges can be suppressed with pole-mounted suppressors by the electricity utility or with an owner-supplied whole-house surge protector. A whole-house product is more expensive than simple single-outlet surge protectors and often needs professional installation on the incoming electrical power feed; however, they prevent power line spikes from entering the house. Damage from direct lightning strikes via other paths, such as telephone lines, must be controlled separately.
Industrial use
A surge arrester, surge protection device or transient voltage surge suppressor, is used to protect equipment in power transmission and distribution systems. The energy criterion for various insulation materials can be compared by impulse ratio. A surge arrester should have a low impulse ratio so that a surge incident on the surge arrester may be bypassed to the ground instead of passing through the apparatus.To protect a unit of equipment from transients occurring on an attached conductor, a surge arrester is connected to the conductor just before it enters the equipment. The surge arrester is also connected to ground and functions by routing energy from an over-voltage transient to ground if one occurs, while isolating the conductor from ground at normal operating voltages. This is usually achieved through use of a varistor, which has substantially different resistances at different voltages.
Surge arresters are not generally designed to protect against a direct lightning strike to a conductor, but rather against electrical transients resulting from lightning strikes occurring in the vicinity of the conductor. Lightning striking the earth produces ground currents that can pass over buried conductors and induce a transient that propagates outward towards the ends of the conductor. The same kind of induction happens in overhead and above-ground conductors, which experience the passing energy of an atmospheric electromagnetic pulse caused by lightning flash.
The common assumptions regarding lightning specifically, based ANSI/IEEE C62.41 and UL 1449, are that minimum lightning-based power line surges inside a building are typically 10,000 amperes or 10 kiloamperes. This is based on 20 kA striking a power line, the imparted current then traveling equally in both directions on the power line with the resulting 10 kA traveling into the building. These assumptions are based on an average approximation for testing minimum standards. While 10 kA is typically good enough for minimum protection against lightning strikes, it is possible for a lightning strike to impart up to 200 kA to a power line with 100 kA traveling in each direction.
Surge arresters can only protect against induced transients characteristic of a lightning discharge's rapid rise-time, and will not protect against electrification caused by a direct strike to the conductor. Transients similar to lightning-induced, such as from a high voltage system's fault switching, may also be safely diverted to ground; however, continuous overcurrent is not protected against by these devices. The energy in a handled transient is substantially less than that of a lightning discharge; however, it is still of sufficient quantity to cause equipment damage and often requires protection.
Without very thick insulation, which is generally cost prohibitive, most conductors running more than approximately will experience lightning-induced transients at some time during use. Because the transient is usually initiated at some point between the two ends of the conductor, most applications install a surge arrester at each end just before the conductor lands in each piece of equipment to be protected. Each conductor must be protected, as each will have its own transient induced, and each SPD must provide a pathway to earth to safely divert the transient away from the protected component.
The one notable exception where they are not installed at both ends is in high-voltage distribution systems. In general, the induced voltage is not sufficient to do damage at the electric generation end of the lines; however, installation at the service entrance to a building is key to protecting downstream products that are not as robust.