How Surge Protectors Work

The aim of insulation coordination is to reduce the chances of dielectric failure of any equipment, thereby preventing injury to people and damage to equipment and ensuring the availability of power supply.

We presented the different types, causes, and impacts of the power surges in our separate article. You can choose from several components and use them to eliminate or limit these surges. These components are used in surge protective devices or included in certain electronic equipment itself. Let is first understand the functioning of these individual components to fully understand how surge protectors work.

What is Surge Protection Devices

Also named a surge protector, surge diverter, surge suppressor, surge protection device (SPD), transient voltage surge suppressor (TVSS), and spike suppressor, this device is primarily intended to protect electrical devices and systems in AC circuits from voltage spikes or surges.

The voltage spikes in LV systems may exceed 1000V in magnitude and last for about 1 to 30 microseconds causing damage to sensitive electronic equipment and devices like computers, TV, modems, and battery chargers.

The ability of any electrical device to withstand a surge is dictated by two main electrical parameters:

Clearance in Air: The clearance is defined as the shortest distance between two conductors in the air. The possibility of arcing for a given air gap depends on the voltage applied and the degree of pollution.
The creepage distance on the insulators: It is the shortest distance between any two live parts along the surface of the insulator.

Any surge protective device can be classified into

Primary Protection Device: These are designed to deal with direct lightning strikes.
Secondary Protection Device: They are designed to protect against indirect lightning, switching, and power frequency surges.

Main Concepts in Surge Protective Devices

The various methods by which any surge protection device can limit the transient overvoltage are described below. They all either block or short the currents to reduce the voltage.

Use of Inductors: They provide a high impedance against a sudden change in currents. As di/dt increases, there is a high voltage drop across the inductors, thereby reducing the voltage seen by the electrical installation.
Shorting of Currents: Implemented by use of spark gap, gas discharge tube, metal oxide varistor or MOV, and zener diode type semiconductors.
Use of Capacitors: The main characteristic of any capacitor is that it does not allow a sudden change in voltage.

The surge protection devices may be a combination of the above basic elements.

Some of the surge protection devices divert the electric current to the ground, while others absorb the transients and release the energy as heat.

Let us familiarize ourselves with some important concepts and terminologies, which will allow us to understand the discussions in the later sections.

Let Through or The Clamping Voltage

It is the voltage at which the protective components of surge protective devices are designed to short the currents. While a lower value of the clamping voltage will provide more protection, but shortens the life of the protective device as it will be acting more frequently.

UL 1449 specifies the three lowest protection levels as 330V, 400V, and 500V. For a 120V device, 330V clamping voltage is standard.

The two common related terms that you may come across are voltage protection rating (VPR) and Suppressed Voltage Ratings (SVR). SVR was used in the 2nd edition of the UL standard, while VPR got introduced in the 3rd edition and is defined using the measured limiting voltage test performed at six times higher current than the 2nd edition.

Hence these two terms are not comparable. You must compare the VPR rating of one device to the VPR rating of another, not with the SPR. A surge protection device manufactured according to the 3rd edition or later is much safer and has a much higher life expectancy.

Needless to say, the device with a higher let-through will allow a higher voltage to the protected devices.

Joule rating

Devices that absorb the surge energy, like a MOV, have a joule rating to specify the amount of energy they can absorb and dissipate as heat.

For example, consider a spike of 10 KA, lasting for 10 microseconds, with the incoming power supply line resistance of 1 ohm. The energy absorbed by the MOV is

10 x 10³ x 10 x 10³ x 10 x 10^-6 x 1 = 1000 Joules.

MOV-based good surge protection devices have joule ratings above 1000 J and can handle currents over 40 KA. The lower durations of the spikes result in lower values of absorbed energies. If the let-through energy is higher than the joule rating may result in fusing, melting, or short circuit and needs to be cleared by other protective devices like circuit breakers and fuses.

An electrical supply line with lower resistance will have a higher current. This will require a MOV with a higher Joule rating. Inside your house, the smaller wire sizes increase the resistance. So smaller rated MOVs will suffice.

Hence the whole house protection MOV in the main distribution board will have a higher rating. The power strips typically have 20W single or multiple MOVs in parallel, while a battery charger may have just a 1 W MOV. As per the IEEE/ANSI assumptions, the maximum surge inside the building can reach 6 kV and 3000 amps with 90 Joules of energy. This includes the surges from external sources but excludes the effect of a lightning strike.

The minimum impact of lightning surge inside the building is assumed to be 10 KA. This is with the assumption of a 20 kA direct strike on a power line and current flowing equally in both directions on the power line. The actual surge current from a lightning strike may exceed 200 KA.

Such currents are best dealt with by the utility in a pole-mounted lightning arrester. Or you may put a whole house surge protection for the same.

The MOV deteriorates with each spike it encounters, and gradually the threshold voltage to trigger the current flow reduces. The MOVs should have fuses or circuit breakers to prevent the adverse consequences of their eventual meltdown or failures.

Certain manufacturers deploy the MOVs in parallel to increase the Joule rating. It is better to derate the complete set by 20% to account for the weakest MOV and the gradual degradation of the set.

Response time

Different transient voltage suppressor device types have different response times to surge voltage waves. For example, a gas discharge tube (GDT) has a much slower response than a MOV. MOVs typically respond in a few nanoseconds, while the surges require a duration of a few microseconds to the peak.

Hence while a MOV will protect the equipment connected from the damaging portion of the spike, a GDT may not if the system impedance is particularly low. Better designs combine the best of both worlds by using slow but otherwise beneficial technologies with fast ones.

The Main Surge Protection Components & How Surge Protectors Work

While you would have a fair idea about many surge protection devices by now, let us systematically discuss the salient points of the most important ones in the upcoming section.

Metal Oxide Varistor or MOVs

The MOVs are made up of semiconductor material, sintered granular ZnO (Zinc oxide). The let-through voltage is typically 3 to 4 times the nominal circuit voltage. The MOV conducts at any higher voltage and diverts the current away from the protected load.

As stated above, MOVs lose their voltage withstand capability slightly with each passing surge until it lowers to the circuit voltage and eventually fails. They must be protected by a temperature fuse or a circuit breaker to prevent a dramatic meltdown or fire. The thermal fuse blows once the MOV gets hot. Additionally, a LED may be provided to indicate a functioning MOV.

MOVs may be connected in parallel to increase their current ratings and life expectancy. It is essential to match the characteristics of these MOVs to have uniform conduction among them. Their low cost makes them the preferred component in very basic AC systems.

Gas Discharge Tube (GDT) Spark Gap

GDT has a sealed, air-tight glass enclosure filled with a gas mixture. There are two electrodes across which the high voltage spike is applied. The application of high voltages ionizes the gas, conducting the currents across the electrodes. The main characteristics of any GDT are:

They conduct more current than other surge protective devices of similar size.
They can either handle a large number of transients at comparatively lower voltages or a few very high voltage transients.
Exceptionally higher voltages, like in the case of lightning discharges, can cause the device to fail.
It has a much slower response time (in microseconds), and a spike of 500 V or more with a 100 nanosecond duration can easily pass through to the equipment.
The common ratings for a GDT are 400 to 600 V triggering voltage, 5,000 to 10,000 amps current surges with an 8/20 µs waveform as per UL 497.
GDT creates a very effective short for any surge power. It continues to conduct current even when the voltage transient has passed and the incoming voltage has reduced below the triggering value. It conducts till the current dies down and the discharge quenches naturally. The current that continues to flow after the transient is called the follow-on current. When the voltage falls below the triggering voltage, the device is said to operate in the negative resistance zone.
An additional auxiliary circuit may be needed to bring the follow-on current to zero.
GDTs have a low capacitance value, making them suitable for high-frequency lines connected to telecommunications equipment.
They are suitable for power lines due to higher current withstand capabilities, but you must control the follow-on current.

Transient Voltage Suppression Diodes

The TVs diodes are used in data communications as they provide high-speed protection to low-power circuits. It is a type of avalanche diode with a response time in picoseconds. The main features are:

They have a low energy-absorbing capability than other surge protection devices but are extremely fast acting.
The let-through voltage is usually lower than twice the nominal system voltage.
It has a high life expectancy if the current impulses are within the limits of the device.
On exceeding the current limits, the device may fail permanently. Thus they are useful for circuits with low current spikes. The number of such low current spikes does not cause any deterioration in the device.
Reverse-paired series diodes are possible for bipolar operations.
Series paring can reduce the capacitance to the required levels in the communication circuits.

Thyristor Type Surge Protection Devices (TSPD)

These devices have similar characteristics to a GDT or a spark gap but are very fast operating. Their low clamping voltages on triggering allow higher current surges, keeping the heat dissipation low.

Inductors, Capacitors, Chokes, and Line Reactors.

These devices can limit the fault current and prevent or reduce overvoltages. Inductors in the fault current limiting applications are also called line reactors or chokes. They increase the line and reliability of solid-state electronic devices and prevent overvoltage and nuisance trippings.

Surge Currents

The load locations are classified as Category A, B, and C, with category A locations having the lowest currents. The distinction between the three categories is as under.

Category A: The wire lengths of the category A loads exceed 60 ft, as measured from the service entrance. It is estimated that such loads can be exposed to 6KV and 500A surge currents.
Category B: The wire lengths are between 30 to 60 ft from the service entrance to the actual locations of the loads. The design surge parameters are 6KV voltage and 3kA surge currents.
Category C: The wire lengths are less than 30 ft. The surge rating can be 20 KV with 10 KA current levels.

The reduction in the surge parameters is attributable to the increased impedance presented by longer conductors of relatively smaller sizes in the power distribution system within the building.

Types of Surge Protective Devices (SPDs)

The various types of surge protective devices based on their functionality are as under:

Type I SPDs

The Type I SPDs are installed on the line side of the main service entrance, which means on the secondary side of the service transformer, but before the main incoming breaker of your house.

If there is a risk of a direct lightning strike directly to the building or to the overhead supply line feeding power to the building, type I SPD should be used. These SPDs are designed to divert the currents associated with direct lightning strikes with 10/350 µs current waves. They also limit the transient overvoltages, including those from the utility capacitor bank switching, to prevent equipment damage.

Type 2 SPDs

If you have a home in a heavily built area, the probability of a direct lightning strike is much less. In such a scenario, the device at the service entrance can be a Type 2 SPD, suitable to handle indirect lightning strikes with 8/20 µs current waveform, in addition to limiting the surge voltages.

In larger and industrial applications, the Type 2 SPPs are installed on the sub-distribution boards, while the type I SPDs are in the main incoming panels at the service entrances.

Type 3 SPDs

The sensitive equipment is provided with additional protection in their vicinity. Such protection is downstream of the type 2 SPDs and is known as the type 3 SPDs.

For example, the SPDs at the socket outlets belong to this type. Such devices prevent damage to the equipment from the internally generated switching transients. The voltage threshold of these devices is below the withstand threshold of sensitive electronics and other critical devices.

Type 1+2 and Type 1+2+3 can handle direct lightning currents and carries devices to limit overvoltages to other equipment in a single enclosure to save on space and cost. You must limit the length of conductors on the supply side of these devices to as low a value as possible to reduce any inductive voltages.

A type 4 SPD is also known as the surge protection module and is used for servo motors, PLCs, and other devices in industrial applications.