Power surges can destroy electrical equipment, causing costly downtime and damage. Without protection, businesses risk losing critical systems. The solution? A surge protective device (SPD), designed to prevent electrical surges from harming your equipment.

A surge protective device (SPD) is an essential electrical safeguard that limits transient voltage and diverts surge currents to protect connected systems from damage.

Surge Pro­tec­tion Devices (SP­D's)

SP­D’s pro­tect elec­trical and elec­tronic equip­ment against tran­si­ents, ori­gin­at­ing from light­ning, switch­ing of trans­formers, light­ing and mo­tors.

These tran­si­ents can cause pre­ma­ture age­ing of equip­ment, down­time, or com­plete de­struc­tion of elec­tronic com­pon­ents and ma­ter­i­als.

A surge protector is a device that protects your home’s electrical system in the event of a power surge, such as from a lightning strike. When there is a variation in the current, the surge protector will redirect the current to the ground or a metal oxide varistor inside the device instead of through the main electrical wiring.

SPDs are critical for industrial, commercial, and residential applications, preventing equipment failure, costly repairs, and system downtime. Without an SPD, sensitive electronics, PV systems, and solar power setups are vulnerable to voltage spikes.

Se­lec­tion Cri­teria

Surge pro­tec­tion devices are clas­si­fied ac­cord­ing to their func­tions:

Type 1
SPD which can dis­charge par­tial light­ning cur­rent with a typ­ical wave­form 10/350 μs. Usu­ally em­ploys spark gap tech­no­logy.This, if re­quired, will be in­stalled in the primary dis­tri­bu­tion board at the ori­gin of the elec­trical in­stall­a­tion. A Type 1 SPD does not in it­self of­fer the re­quired pro­tec­tion level and must be used in con­junc­tion with co­or­din­ated type 2 devices. An in­stall­a­tion with a light­ning pro­tec­tion sys­tem will re­quire a Type 1 SPD.

Type 2
SPD which can pre­vent the spread of over­voltages in the elec­trical in­stall­a­tions and pro­tects equip­ment con­nec­ted to it. It usu­ally em­ploys metal ox­ide varis­tor (MOV) tech­no­logy and is char­ac­ter­ised by an 8/20 μs cur­rent wave.This device would nor­mally be in sub-dis­tri­bu­tion boards and in the primary dis­tri­bu­tion board if there was no re­quire­ment for a type 1 device.

Type 3
These SPDs have a low dis­charge ca­pa­city. They must there­fore only be in­stalled as a sup­ple­ment to Type 2 SPD and in the vi­cin­ity of sens­it­ive loads. Type 3 SP­D’s are char­ac­ter­ised by a com­bin­a­tion of voltage waves (1.2/50 μs) and cur­rent waves (8/20 μs).

How Surge Protectors Work?

SPDs contain a metal oxide varistor (MOV), which acts as a sensor. When the current surges above a certain level, it will lower resistance. If the voltage is too low, it will increase resistance. The MOV keeps the current consistent and protects your devices.

MOVs are made up of metal oxide and two semiconductors. The metal oxide senses the electrical current, while the two semiconductors raise or lower resistance as needed and divert any excess current to the ground.

SPDs function by monitoring voltage levels and responding instantly to surges. They consist of non-linear components that switch between high and low impedance states based on voltage fluctuations.

- Normal Conditions: The SPD remains in a high-impedance state, allowing normal electrical flow without interference.

- Surge Event: When a voltage spike occurs, the SPD quickly shifts to a low-impedance state, redirecting excess current to the ground or back to its source.

- Reset Process: After the surge is diverted, the SPD automatically returns to a high-impedance state, ensuring normal operation resumes.

Ter­min­o­logy

Iimp – Im­pulse cur­rent of 10/350 μs wave­form as­so­ci­ated with Type 1 SP­D’s
In – Surge cur­rent of 8/20 μs wave­form as­so­ci­ated with Type 2 SP­D’s
Up - The re­sid­ual voltage that is meas­ured across the ter­minal of the SPD when In is ap­plied
Uc - The max­imum voltage which may be con­tinu­ously ap­plied to the SPD without it con­duct­ing.

Pros and Cons of Surge Protectors

While surge protectors have many benefits and capabilities, they have limitations. Below are some of the main pros and cons of surge protectors.

Protection for Devices and Appliances:

In some cases, a power surge can ruin the wiring of expensive appliances and devices. Repairing or replacing electrical equipment can be costly and inconvenient. While power surges may not always destroy your equipment, they can put stress on the wiring. It can shorten the device or appliance’s lifespan. A surge protector is a simple way to prevent damage.

Surge Protection Devices Have Voltage Limitations:

Not all surge protectors can handle higher voltages. Basic ones can handle slight fluctuations. Too much exposure to high voltages may cause the surge protector to fail. Higher voltage surge protectors are available but may be more expensive.

SPD Categories or Types

SPDs are classified into two main types: voltage limiting and voltage switching components. Most modern SPDs use a hybrid approach that combines both methods.

1. Voltage Limiting Components

These components adjust their impedance based on voltage levels, clamping surges before they can reach dangerous levels. Examples include:

- Metal Oxide Varistors (MOVs) – Common in solar surge protectors and DC SPDs.

- Transient Voltage Suppression (TVS) Diodes – Offer fast response times but are limited in surge-handling capacity.

2. Voltage Switching Components

These components remain non-conductive until a specific voltage threshold is exceeded, at which point they rapidly turn on and conduct the surge to ground. Examples include:

- Gas Discharge Tubes (GDTs) – Used in high-energy surge protection applications.

- Spark Gaps – Provide reliable DC surge and lightning protection.

Hybrid designs balance the advantages of both component types, offering efficient and reliable surge protection.

Main­ten­ance

Most SPDs have an in­dic­a­tion win­dow that they are op­er­a­tional. If the in­dic­ator is green they are provid­ing pro­tec­tion. If they are red then they have reached ‘end of life’ and will need re­pla­cing. Of­ten there is a re­place­able cart­ridge which can simply be with­drawn and re­placed with a new op­er­a­tional device.

Factors to Consider When Choosing Surge Protection Devices

To choose a surge protection device, first check how frequent power surges occur in your property. If you experience frequent bad weather or use equipment prone to surges, a whole-home SPD can be beneficial.

Next, consider the type of equipment and appliances you’ll be using. If you have a lot of expensive appliances, machinery or technology, investing in a whole-home SPD may be worth it. The cost of the SPD and installation will be much less expensive than repairing or replacing valuable equipment.

How to Compare Different SPDs?

To evaluate different SPDs, consider the following performance factors:

1. Response Time

The speed at which an SPD reacts to a surge is critical.

TVS diodes have the fastest response times.

MOVs offer a good balance of speed and durability.

2. Follow-On Current

Follow-on current occurs when a voltage-switching SPD fails to return to its high-impedance state after a surge.

AC systems handle this naturally due to zero crossings in the AC waveform.

DC systems, such as solar power setups, require specialized DC surge protector devices.

3. Let-Through Voltage

This is the voltage that reaches connected equipment during a surge.

TVS diodes provide the lowest let-through voltage.

MOVs offer moderate protection while balancing other performance factors.

Each type has trade-offs, making hybrid SPDs the preferred choice for most applications.

Surge Protective Device Performance Features

When selecting an SPD, key performance metrics ensure reliability and compatibility with electrical systems:

- Maximum Continuous Operating Voltage (MCOV): The highest voltage the SPD can withstand while functioning normally. Typically 25% higher than the nominal system voltage.

- Voltage Protection Rating (VPR) or Voltage Protection Level (Up): Indicates the let-through voltage during a surge. Lower values indicate better protection.

- Nominal Discharge Current (In) Ratin:  Measures the SPD’s ability to withstand repeated surge events. Higher values indicate greater durability.

- Indication Status: Visual or remote indicators show the SPD’s operational status. Can be mechanical, LED-based, or remotely monitored.

- Surge Current Capacity: Represents the maximum surge energy the SPD can handle. Not standardized, so specifications vary by manufacturer. A properly rated PV SPD or solar surge protector ensures long-term protection for solar power and electrical systems.

Conclusion

Overall, an SPD surge protection device is a worthwhile investment, especially for those who are prone to power surges. If you have sensitive and costly equipment, a whole-home protector will offer the best protection and save you in the long run.