Solar Surge Protection: Do You Need SPD for Your Rooftop Solar System?

Most rooftop solar systems in India need properly designed surge protection. An SPD for solar, also known as a Surge Protection Device, limits sudden voltage spikes that could damage the inverter and other electrical components. Depending on the system design and lightning risk, protection may be required on the DC (direct current) side, AC (alternating current) side, or both.

A nearby lightning strike does not have to hit your panels directly to affect your entire rooftop solar system. It can induce a voltage surge in the cables running between the panels and inverter. Surges can also enter through the utility grid.

This blog explains how an SPD for solar panels and inverters works, where it is installed, and the most common SPD mistakes that can damage your solar installation.

What Is an SPD in a Rooftop Solar System?

An SPD is an electrical protection device that limits short, sudden increases in voltage. During a surge, it diverts excess current toward the earthing system and reduces the voltage reaching the inverter and other sensitive components.

• An SPD for solar normally remains inactive while the system operates within its designed voltage range.
• When the voltage suddenly crosses its operating threshold, the device creates a low-impedance path for surge current to flow toward earth.

Once the surge passes, the surge protection device returns to its normal state. If the event damages or exhausts the device, its status indicator may show that the cartridge needs replacement.

An SPD does not regulate ordinary voltage fluctuations or correct a sustained overvoltage. These conditions require other forms of electrical protection.

The Two Surge Pathways: Direct Strike vs Induced Transient

A rooftop solar system can be affected in two ways.

• Lightning may strike the building or solar installation directly.
• A nearby lightning strike may induce a sudden voltage spike in the system’s cables.

A direct strike carries far more energy, while an induced transient is more common.

Let’s understand both closely.

Direct Lightning Strike

A direct lightning strike may hit the building, its external lightning protection system, or, in severe cases, part of the rooftop solar installation.

Because a direct strike carries an extremely high amount of energy, an SPD alone cannot guarantee complete protection. The building may require a coordinated safety system that includes external lightning protection, proper earthing, bonding, and appropriately designed surge protection.

Induced Transient Surge (The More Common Failure Mode)

An induced transient is the more common surge-related threat to rooftop solar equipment. When lightning strikes nearby, its electromagnetic field can create a sudden voltage spike in solar cables without physically striking the system.

Transient surges can also originate from:

• Grid switching operations
• Restoration of electricity after an outage
• Electrical faults in the distribution network

These events carry less energy than a direct lightning strike but can still damage the inverter and other sensitive electronics. Because these two surge pathways create different risk levels, SPDs are divided into different types based on the surge energy they are designed to handle.

Type 1, Type 2, and Type 3 SPDs: Which One Does Your System Need?

Type 1 SPDs handle partial lightning current, Type 2 SPDs limit induced and switching surges, and Type 3 SPDs provide additional protection near sensitive equipment. The appropriate type depends on the lightning protection design and installation conditions.

Here’s a breakdown of all types of SPDs:

• Type 1 SPD: Used where partial lightning current may enter the electrical installation. It is commonly considered when a building has external lightning protection, and the required separation distance cannot be maintained.
• Type 2 SPD: Protects against induced lightning surges and switching surges. It is the type most commonly encountered on the AC and DC sides of residential rooftop solar systems.
• Type 3 SPD: Provides protection close to particularly sensitive equipment. It has a lower discharge capacity and works downstream of Type 1 or Type 2 protection.
• Combined Type 1+2 SPD: Performs both lightning-current and surge-protection functions in one device. It may be used where the system design requires both levels of protection.

Which Solar Components Can a Surge Damage?

A surge can enter through the PV (photovoltaic) cables or the building’s AC network and affect several components, including:

• Solar inverter
• Optimisers or microinverters
• Charge controller in an off-grid or hybrid solar system
• Battery-management and power-conversion electronics
• Monitoring and communication equipment
• Protection and metering equipment
• Module junction boxes or bypass diodes during severe events
• Household appliances connected to the AC network

The inverter is particularly vulnerable because it contains sensitive power electronics and connects the outdoor solar array to the building’s electrical system.

Do You Need an SPD for Your Rooftop Solar System?

Most professionally designed rooftop solar systems require suitable surge protection on the AC side, DC side, or both. However, the exact number, type, and placement of SPDs must be determined through a site-specific assessment rather than by applying one configuration to every installation.

Surge protection requires particular attention when:

• The property is in a lightning-prone area.
• The building has an external lightning protection system.
• The required separation from lightning conductors cannot be maintained.
• Long cables connect the solar array, inverter, and distribution board.
• Solar panels are installed across different roof sections.
• The system includes optimizers, batteries, or monitoring equipment.
• The utility supply experiences frequent switching disturbances.
• The inverter manufacturer specifies external surge protection.

Installing solar panels does not automatically increase the likelihood of a lightning strike. However, the panels, outdoor cables, inverter, and grid connection can create multiple pathways through which a surge may reach sensitive electrical equipment.

The installer should use these conditions to determine the required SPD type, rating, number, and installation points.

Where Are SPDs Installed in a Rooftop Solar System?

SPDs are installed near vulnerable equipment and at points where cables enter or leave a protected area. Depending on the design, they may be placed in the DC Distribution Box (DCDB), AC Distribution Box (ACDB), string combiner box, inverter, or main distribution board.

The required location depends on whether the SPD protects the photovoltaic DC circuit or the building’s AC circuit.

What Is the Difference Between AC and DC SPDs for Solar?

A DC SPD protects the circuit between the solar array and inverter, while an AC SPD protects the inverter output and building-side electrical network. They have different voltage requirements and operating characteristics, so one cannot replace the other.

Here’s a side-by-side comparison of both:

Why Do Some Systems Need More Than One SPD?

One SPD may not adequately protect equipment located at both ends of a long cable. IEC-based manufacturer guidance, where IEC stands for International Electrotechnical Commission, commonly uses approximately 10 meters as the distance at which additional protection at both ends should be assessed.

This principle may apply separately to:

• The DC cable between the solar array and inverter
• The AC cable between the inverter and main distribution board
• Data and communication cables connected to monitoring equipment

A long cable can pick up another induced surge after the first protection point. Therefore, an installation may require one SPD near the cable’s entry point and another close to the equipment being protected. The installer must determine the final arrangement based on cable routing, equipment specifications, and site conditions.

How Is an SPD Selected for a Solar System?

An SPD must match the circuit voltage, surge risk, earthing arrangement, and equipment it protects. Homeowners should understand the main ratings, but the installer should make the final selection.

Important selection parameters include:

• Maximum continuous operating voltage: The highest voltage the SPD can withstand continuously without activating or degrading prematurely.
• Maximum PV voltage: A DC SPD must account for the maximum possible open-circuit voltage of the complete string, including temperature effects.
• Voltage protection level: The voltage that may remain across the protected equipment while the SPD discharges surge current.
• Nominal discharge current: The surge current a device can withstand repeatedly under its specified test conditions.
• Maximum discharge current: The maximum surge current a Type 2 SPD can handle.
• Impulse discharge current: The rating used for Type 1 devices designed to carry partial lightning current.
• Short-circuit capability: The SPD and its backup protection must safely withstand the available fault current.
• AC or PV DC rating: A device must be designed for the circuit in which it is installed.
• Environmental protection: An outdoor enclosure should have an appropriate IP (ingress protection) rating for dust and water protection.
• Replaceable cartridge and status indicator: These features make inspection and replacement easier.
• Applicable certification: The product should comply with relevant BIS (Bureau of Indian Standards) or IEC (International Electrotechnical Commission) standards and project specifications.

Common Solar Surge-Protection Mistakes

Even a high-quality SPD may offer inadequate protection when it is incorrectly selected, positioned, or connected.

Common mistakes include:

• Protecting only the DC side and ignoring grid-origin surges
• Protecting only the AC side and leaving PV cables exposed
• Using an AC SPD in a DC circuit
• Choosing a device only by its kA rating
• Selecting a DC SPD below the maximum possible string voltage
• Installing an SPD too far from the equipment being protected
• Using long or sharply bent earth connections
• Treating an MCB (Miniature Circuit Breaker) as a substitute for an SPD
• Assuming earthing alone provides surge protection
• Assuming a lightning arrester removes the need for SPDs
• Leaving a failed SPD cartridge in service
• Buying unverified components without appropriate certification

Conclusion

Most rooftop home solar panel systems need a properly designed SPD for solar to limit transient voltage reaching the inverter and other electrical equipment. Depending on the installation, protection may be needed on the DC side, AC side, or both.

• An SPD for solar cannot replace earthing, circuit breakers, or an external lightning protection system.
• Its voltage rating, type, location, and connections determine how effectively it performs.

SolarSquare designs rooftop solar as a complete electrical system, including the protection required for the site and system configuration. Book a free consultation with our experts to plan a safe and reliable PV system for your home.

Frequently Asked Questions

Is an SPD for solar the same as a lightning arrestor?

No. A lightning arrester or external lightning protection system intercepts lightning and conducts its current toward earth. An SPD, on the other hand, limits transient voltage within an electrical circuit. The two can form part of the same protection system but do not replace each other.

Can earthing protect solar panels without an SPD?

No. Earthing provides a path for fault and surge current, but it does not independently limit the voltage reaching the inverter. An SPD uses the earthing and bonding system to divert surge current, so both must be designed and installed correctly.

Is the inverter’s built-in surge protection enough for a rooftop solar system?

Not necessarily. Some inverters contain internal surge-protection components, but their protection level and replaceability vary. The installer must check the inverter specifications, cable lengths, site risk, and manufacturer requirements before deciding whether external AC or DC SPDs are also necessary.

How much does an SPD for solar cost?

A standalone SPD for solar generally costs between ₹500 and ₹9,000 in India. Basic AC and DC SPDs are available for ₹500-₹2,000, while certified, higher-rated, or premium-brand models can cost ₹3,000-₹9,000.

Disclaimer: The prices listed above are indicative online retail prices based on standard market research. The actual cost may vary significantly from the said range based on the SPD type, AC or DC application, voltage and discharge ratings, number of poles, brand, certification, taxes, location, and seller discounts.

How do I know if my solar SPD is working?

Many SPDs have a status window that changes color when the cartridge reaches the end of its service life. The meaning of each color must be checked on the product label or manual. The device should also be examined during scheduled system maintenance.

Can I install or replace a solar SPD myself?

No. SPD selection and replacement involve live electrical circuits, and solar DC cables can remain energized during daylight. Incorrect ratings or connections can cause shock, fire, and equipment damage. Only a qualified solar technician should perform the work.