In today’s homes, offices, factories, and digital infrastructures, electrical equipment is more sensitive than ever before. Modern appliances contain delicate circuit boards, microcontrollers, and smart modules that can be easily damaged by sudden voltage spikes. Even a small surge—from lightning, unstable power grids, or switching operations—can shorten equipment lifespan or cause instant failure. This is where the SPD, or Surge Protective Device, plays a critical role.
Many people hear the term SPD but don’t fully understand how it works or why it is essential. Others assume surge protection is only needed during thunderstorms, not realizing that most damaging surges actually come from inside the building itself. In this article, we will break down how an SPD works, where it should be installed, and why it has become one of the most important components for protecting modern electrical appliances—whether in residential buildings, industrial power systems, EV chargers, or solar energy installations.
What Is an SPD (Surge Protective Device)?
An SPD is a device installed in an electrical system to protect equipment against transient voltage surges. These surges can come from lightning strikes, grid fluctuations, switching transients, or internal electrical noise. Instead of allowing the excess energy to enter sensitive appliances, an SPD safely diverts the surge to the ground.
In simple terms:
“An SPD acts like a pressure relief valve for your electrical system. When voltage rises too high, it opens a safe path to discharge the excess energy.”
Think of it as a shield that protects your appliances from sudden electrical shocks.
Why Do Voltage Surges Happen?
Voltage surges occur more often than people realize. There are four major sources:
Lightning Strikes
This is the most powerful type of surge, capable of injecting hundreds of thousands of volts into power lines. Even indirect or distant lightning can create dangerous surges that travel long distances through the grid.
Utility Grid Switching
When power companies connect or disconnect loads, switch capacitors, or reroute energy, the sudden change can create large surges.
Large Motor Equipment
Air conditioners, pumps, refrigerators, elevators, and industrial machinery can create internal surges when switching on and off.
Faulty Wiring or Poor Power Quality
Old distribution boards, loose connections, or unstable voltage supply can create small but frequent surges that gradually damage appliances.
Interestingly, 80% of damaging surges happen inside the building, not from lightning.
This is why SPDs are essential not only for outdoor environments but also for residential and commercial installations.
How Does an SPD Actually Work?
To understand how an SPD works, imagine your electrical system as a water pipe. Under normal conditions, the water (voltage) flows smoothly. When pressure suddenly spikes, the pipe risks bursting. An SPD acts like a pressure-release valve, opening a bypass path for excess pressure before it reaches your appliances.
Technically, an SPD works in three stages:
Detecting a Surge
When a voltage spike occurs, the SPD identifies that the voltage has exceeded the safe operating level.
Diverting Excess Energy
The SPD instantly opens a low-resistance path, redirecting the surge to the ground. This prevents the high voltage from traveling to connected equipment.
Returning to Normal Mode
Once the surge passes, the SPD restores its high-resistance state so normal voltage can flow without interruption.
This “switching” happens in nanoseconds, protecting even the most delicate electronics.
The Main Components Inside an SPD
Although SPDs come in different designs, most rely on at least one of these core components:
| SPD Component | Function |
|---|---|
|
MOV (Metal Oxide Varistor) |
Absorbs and clamps high-voltage surges; most common in AC systems |
|
GDT (Gas Discharge Tube) |
Handles very large surges, often used for lightning protection |
|
TVS Diode |
Ultra-fast protection for sensitive electronics |
|
Thermal Disconnectors |
Safely isolate SPD if overheating occurs |
Most modern SPDs combine MOV + GDT technologies to achieve high energy absorption and low response time.
MOREDAY’s SPDs, for example, use enhanced MOV arrays and integrated safety mechanisms to maintain reliability even under repeated surge events.
Types of SPDs and Where They Are Used
SPDs are divided into three categories based on where they are installed in the electrical system:
Type 1 SPD
- Installed at the building entrance
- Protects against high-energy lightning surges
- Required in areas with high lightning density or for solar PV systems
Type 2 SPD
- Installed in distribution boards
- Protects equipment from switching surges and internal transients
- Ideal for homes and commercial buildings
Type 3 SPD
- Installed close to the appliance
- Protects sensitive devices like TVs, computers, routers
- Used as supplementary protection
In a complete protection strategy, systems often combine Type 1 + Type 2 + Type 3 for full coverage.
What Happens If You Don’t Use an SPD?
Without an SPD, electrical appliances are exposed to constant risks:
- Gradual aging of electronic components
- Reduced lifespan of appliances
- Instant damage from severe surges
- Fire hazards from overloaded circuits
- Unexpected shutdowns in critical systems
- Data loss in computers and servers
Even small surges can accumulate over time, reducing the reliability of equipment and increasing maintenance costs.
For industries like solar energy, EV charging, automation, data centers, and medical equipment, surge protection is absolutely mandatory.
SPD Protection in Solar Systems & EV Charging
Modern installations like solar PV arrays, energy storage systems, and DC fast charging stations face higher surge risks due to long cable runs and outdoor exposure.
SPDs in Solar PV Systems
SPDs protect:
- PV inverters
- DC combiner boxes
- Batteries
- Monitoring systems
Lightning-induced surges are especially common in rooftop systems, making DC-side SPDs essential.
MOREDAY’s solar DC SPDs are designed specifically for PV systems, with high voltage ratings and UV-resistant enclosure materials.
SPDs in EV Charging
EV chargers contain sensitive power electronics that can be damaged by:
- Grid fluctuations
- Lightning
- Switching transients
- High load cycles
This is why EV charger manufacturers—including MOREDAY—always integrate SPDs into AC and DC charging products to protect both the charger and the vehicle.
How to Choose the Right SPD
Choosing the correct SPD involves more than selecting a voltage rating. You must consider:
- System type (AC, DC, solar, hybrid)
- Installation point (Type 1, 2, or 3)
- Voltage protection level (Up)
- Maximum discharge current (Imax)
- Continuous operating voltage (Uc)
- Environmental conditions
- Safety certifications
If you operate a solar plant or EV charging site, it’s recommended to consult professionals. MOREDAY provides engineered SPD solutions optimized for PV, ESS, and charging applications.
SPD Maintenance and Lifespan
SPDs degrade over time because they absorb surge energy. Their lifespan depends on:
- Surge frequency
- Surge intensity
- System voltage stability
- SPD component quality
Most SPDs include visual indicators or remote alarm contacts to notify users when replacement is needed.
A good rule is to check SPDs during yearly electrical inspections or after heavy storms.
Why SPDs Are Essential in Modern Electrical Systems
As electrical appliances become more digital, miniaturized, and sensitive, their tolerance to voltage fluctuations has decreased. Meanwhile, the number of surge events—especially internal switching surges—continues to rise.
Therefore, installing an SPD is not just protection—it is risk prevention.
A single surge can cost:
- thousands in equipment replacement
- downtime in business operations
- potential data loss
- reduced appliance lifespan
This makes SPDs a very low-cost investment with high return.
Conclusion
SPDs are essential for protecting modern electrical appliances from damaging voltage spikes. They work by redirecting excess energy safely to the ground in just nanoseconds, shielding equipment from lightning strikes, grid disturbances, and internal electrical surges. Whether for homes, factories, solar PV systems, or EV charging stations, an SPD ensures stability, safety, and long-term reliability.
MOREDAY provides high-quality AC and DC SPDs designed for renewable energy systems, buildings, and industrial power networks—helping customers achieve durable and fault-resistant electrical installations worldwide.
