thumb|upright|Surge protection device (SPD) for installation in a low-voltage distribution system
A surge protector, spike suppressor, surge suppressor, surge diverter, surge protection device (SPD), transient voltage suppressor (TVS), or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect electrical devices in alternating current (AC) 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 (SPD) and transient voltage surge suppressor (TVSS) 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 (TVs), 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, is 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 (the point where the supply company's wiring enters a property). 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
alt=|thumb|A [[power strip with built-in surge protector and multiple outlets]]
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 (MOVs), 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 (as by a spark gap) or clamped to a target voltage (as by a MOV), 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 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 (line–neutral, line–ground and neutral–ground), 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 (CATV) 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 (GDT). 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.
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
{| class="wikitable sortable sort-under"
|-
! rowspan="2" class="unsortable" | Type
! colspan="2" | Surge capability (A)
! rowspan="2" | Lifetime
! rowspan="2" | Response time
! rowspan="2" | Shunt
! rowspan="2" | Leakage current ()
! rowspan="2" class="unsortable" | Note
|-
! Min
! Max
|-
| TVS diode
| 1
| 15,000
|
| ≈1 ps (limited by pin lengths)
| <1 pF (small surface-mount device) to >10 nF (large through-hole device)
| 1 μA
| SMD to 15 kA (large through-hole device)
|-
| Metal-oxide varistor (MOV)
| 1
| 70,000
| @100 A, 820 μs pulse shape, 1,000 surges
| ≈1 ns
| Typically 100–1,000 pF
| 10 μA
|
|-
| Avalanche diode, Zener diode
| 50
|
| @50 A, 820 μs pulse shape, infinite
| <1 μs
| 50 pF
| 10 μA
|
|-
| Gas discharge tube
| 20,000
|
| @20 kA, 820 μs pulse width, >20 surges
| <5 μs
| <1 pF
| <1 nA
|
|}
Domestic use
thumb|upright=1.2|A whole house surge protector installed on a household electrical circuit breaker panelboard in 2012; this surge arrester provides protection to all the household electronic devices. Two green lamps indicate that the metal-oxide varistors in the unit are still functional.
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
thumb|Surge arresters
thumb|upright|Large surge arrester
A surge arrester, surge protection device (SPD) or transient voltage surge suppressor (TVSS), 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 overvoltage transient to ground if one occurs, while isolating the conductor from ground at normal operating voltages. This is usually achieved through the 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 a lightning flash.
The common assumptions regarding lightning, specifically, based on ANSI/IEEE C62.41 and UL 1449 (3rd ed.), are that minimum lightning-based power line surges inside a building are typically 10,000 amperes or 10 kiloamperes (kA). 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.
Types
; Low-voltage surge arrester : Apply in Low-voltage distribution system, exchange of electrical appliances protector, low-voltage distribution transformer windings
; Distribution arrester : Apply in 3, 6 and 10 kV AC power distribution systems to protect distribution transformers, cables and power station equipment
; The station type of common valve arrester : Used to protect the 3 ~ 220 kV transformer station equipment and communication systems
; Magnetic blow valve station arrester : Used to protect the 35 ~ 500 kV communication systems, transformers and other equipment
; Protection of rotating machine using magnetic blow valve arrester : Used to protect the AC generator and motor insulation
; Line magnetic blow valve arrester : Used to protect 330 kV and above communication systems and circuit equipment insulation
; Direct current (DC) or blowing valve-type arrester: Use to protect the DC system's insulation of electrical equipment
; Neutral protection arrester : Apply in motor or the transformer's neutral protection
; Fiber-tube arrester : Apply in the power station's wires
; Plug-in signal arrester : Used on twisted-pair transmission lines to protect communications and computer systems
; High-frequency feeder arrester : Used to protect the microwave, mobile base stations, satellite receiver, etc.
; Receptacle-type surge arrester : Use to Protect the terminal Electronic equipment
; Signal arrester : Apply to modem, DDN line, fax, phone, process control signal circuit, etc.
; Network arrester : Apply in servers, workstations, interfaces, etc.
; Coaxial cable lightning arrester : Used on the coaxial cable to protect the wireless transmission and receiving system
Important specifications
thumb|upright|Single-outlet surge protector, with visible connection and protection lights
These are some of the most prominently featured specifications that define a surge protector for AC mains, as well as for some data communications protection applications.
Clamping voltage
Also known as the let-through voltage, this specifies what spike voltage will cause the protective components inside a surge protector to short or clamp. 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, 400 and 500 V. The standard let-through voltage for 120 VAC devices is 330 volts. A protector with a higher let-through voltage will pass a higher surge voltage to the connected device. The design of the connected device determines whether this pass-through spike will cause damage. Motors and mechanical devices are usually not affected. Some (especially older) electronic parts, like chargers, LED or CFL bulbs and computerized appliances, are sensitive, can be compromised and have their life reduced.
Underwriters Laboratories (UL), a global independent safety science company, defines how a protector may be safely used. UL 1449 compliance is mandatory in jurisdictions that adopted the NEC with the 3rd edition in September 2009 to increase safety compared to products conforming to the 2nd edition. A measured limiting voltage test, using six times higher current (and energy), defines a voltage protection rating (VPR). For a specific protector, this voltage may be higher compared to a Suppressed Voltage Ratings (SVR) in previous editions that measured let-through voltage with less current. Due to non-linear characteristics of protectors, let-through voltages defined by 2nd edition and 3rd edition testing are not comparable. A protector may be larger to obtain the same let-through voltage during 3rd edition testing. Therefore, a 3rd edition or later protector should provide superior safety with increased life expectancy.
Joule rating
alt=|thumb|upright=1.2|A surge protection device mounted on a residential circuit breaker panel
thumb|A varistor inside a consumer-grade surge protector has failed after a close lightning strike.
The Joule rating defines how much energy a MOV-based surge protector can absorb in a single event, without failure. Better protectors exceed 1,000 joules and 40,000 amperes. Since the actual duration of a spike is only about 10 microseconds, the actual dissipated energy is low. If the rating is exceeded, the MOV will either burn open or melt and permanently short circuit, ideally tripping a fuse and disconnecting the protected MOV and the equipment it is protecting from the electrical supply.
Per Ohm's law, the MOV (or other parallel-wired protection device) requires resistance in the supply line in order to limit the voltage. For equipment fed by large, low-resistance power lines, a higher joule-rated MOV is required. Inside a house, with smaller wires that have more resistance, a smaller MOV is acceptable.
Every time an MOV shorts, its internal structure is changed and its threshold voltage is reduced slightly. After many spikes, the threshold voltage can be reduced enough to be near the line voltage, i.e., 120 or 240 VAC. At this point, the MOV will partially conduct and heat up and eventually fail, sometimes in a dramatic meltdown or even a fire. Most modern surge protectors have circuit breakers and temperature fuses to prevent serious consequences. Many also have an LED light to indicate whether the MOVs are still functioning.
The joule rating is commonly quoted for comparing MOV-based surge protectors. An average surge is of short duration, lasting for nanoseconds to microseconds, and experimentally modeled surge energy can be less than 100 joules. Well-designed surge protectors consider the resistance of the lines that supply the power, the chance of lightning or other seriously energetic spikes, and specify the MOVs accordingly.
According to industry testing standards, based on IEEE and ANSI assumptions, power line surges inside a building can be up to 6,000 volts and 3,000 amperes, and deliver up to 90 joules of energy, including surges from external sources, not including lightning strikes.
Some manufacturers design higher joule-rated surge protectors by connecting multiple MOVs in parallel, and this can produce a misleading rating. Since individual MOVs have slightly different voltage thresholds and non-linear responses when exposed to the same voltage curve, any given MOV might be more sensitive than others. This can cause one MOV in a group to conduct more (a phenomenon called current hogging), leading to possible overuse and eventual premature failure of that component. A further problem is that if a single inline fuse is placed in series with a group of paralleled MOVs as a disconnect safety feature, it will open and disconnect all remaining working MOVs. Stating the actual joule rating as the sum of all the individual MOVs does not accurately reflect the total clamping ability.
The effective surge energy absorption capacity of the entire system is dependent on the MOV matching, so derating by 20% or more is usually required. This limitation can be managed by using carefully matched sets of MOVs.
One MOV manufacturer recommends using fewer but bigger MOVs (e.g., 60 mm vs. 40 mm diameter) if they can fit in the device. It is further recommended that multiple smaller MOVs be matched and derated. In some cases, it may take four 40 mm MOVs to be equivalent to one 60 mm MOV.
Thus, response time under standard testing is not a useful measure of a surge protector's ability when comparing MOV devices. All MOVs have response times measured in nanoseconds, while test waveforms usually used to design and calibrate surge protectors are based on modeled waveforms of surges measured in microseconds. Slower-responding technologies (notably, GDTs) may have difficulty protecting against fast spikes. Therefore, good designs incorporating slower but otherwise useful technologies usually combine them with faster-acting components to provide more comprehensive protection. A subsequent revision in 2015 included the addition of low-voltage circuits for USB charging ports and associated batteries.
EN 62305 and ANSI/IEEE C62.xx define what spikes a protector might be expected to divert. EN 61643-11 and 61643-21 specify both the product's performance and safety requirements. In contrast, the IEC only writes standards and does not certify any particular product as meeting those standards. IEC standards are used by members of the CB Scheme of international agreements to test and certify products for safety compliance.
None of those standards guarantees that a protector will provide proper protection in a given application. Each standard defines what a 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 assure sufficient protection, especially in situations of high lightning risk.
The following standards are not standards for standalone surge protectors, but are meant for testing surge immunity in electrical and electronic equipment as a whole. Thus, they're frequently used in the design and test of surge protection circuitry.
- IEC 61000-4-2 Electrostatic discharge immunity test
- IEC 61000-4-4 Electrical fast transient/burst immunity test
- IEC 61000-4-5 Surge immunity test
Primary components
Systems used to reduce or limit high-voltage surges can include one or more of the following types of electronic components. Some surge suppression systems use multiple technologies, since each method has its strong and weak points. The first six methods listed operate primarily by diverting unwanted surge energy away from the protected load, through a protective component connected in a parallel (or shunted) topology. The last two methods also block unwanted energy by using a protective component connected in series with the power feed to the protected load, and additionally may shunt the unwanted energy like the earlier methods
Metal oxide varistor
thumb|[[Metal-oxide varistors]]
A metal-oxide varistor (MOV) consists of a bulk semiconductor material (typically sintered granular zinc oxide) that can conduct large currents when presented with a voltage above its rated voltage. MOVs typically limit voltages to about 3 to 4 times the normal circuit voltage by diverting surge current. Multiple MOVs may be connected in parallel to increase current capability and life expectancy, providing they are matched sets.