Solar fuses, also known as photovoltaic (PV) fuses, are specialized electrical safety devices designed to protect solar power systems from overcurrent conditions that can damage equipment. An overcurrent occurs with an abnormal flow of electricity due to various factors, such as short circuits, faulty equipment, or lightning strikes. Solar fuses are essential for ensuring the reliability and safety of solar energy installations.

Like an ordinary fuse, a solar fuse contains a conductor that melts when exposed to excessive current. This action breaks the circuit and interrupts the flow of electricity, preventing further damage or fire of solar components such as solar batteries, inverters and solar panels.

Features of Solar Fuse

• Protection Against Overcurrent: The primary function of a solar fuse is to protect the solar panels and other components of the solar array from overcurrent conditions. The fuse will blow up whenever there is excessive current due to a short circuit or other fault, interrupting the circuit and preventing damage to the solar panels and other connected equipment.
• Preventing Overheating: Overcurrent conditions can generate excessive heat and can lead to a fire risk. Solar fuses help prevent potential fires by interrupting the circuit in the event of an overcurrent, ensuring the safety of the solar installation and the surrounding area.
• Equipment Longevity: Properly sized solar fuses protect solar components such as panels, inverters, charge controllers and batteries from damage caused by excessive current. This protection helps ensure the longevity and reliability of the solar power system, safeguarding the investment made in solar technology.
• Compliance with Safety Standards: Using fuses in a solar array helps ensure compliance with electrical safety standards and codes. This is important for both residential and commercial solar installations to meet regulatory requirements and obtain necessary permits and certifications.
• Facilitating Maintenance and Troubleshooting: A blown fuse can help in identifying the problem area in the solar array, in the event of a fault. This aids maintenance personnel to locate and address the issue, ensuring quicker and more efficient troubleshooting.

When is Solar Fusing Necessary

A solar panel needs to be fused when the total current produced by the solar panel array during a short circuit is greater than the maximum series fuse rating of the solar panels. When the solar panels are wired in series, the voltages add together, whereas the current remains the same. For instance, six 100 W panels that produce 5 A at 20 V which are wired in series will produce a total of 5 A at 120 V (20 V x 6 panels = 120 V). When the solar panels are wired in parallel, the current adds together and the voltage remains the same. Six 100 W panels that produce 5 A at 20 V wired in parallel will provide a total of 30 A at 20 V (5 A x 6 panels = 30 A). Hence, parallel wiring is the only configuration that increases the current output of solar panels. Due to this increase in current, parallel wiring usually requires solar fuses, whereas series wiring does not.

Types of Solar Fuses

Based on its response time, solar fuses are classified into types: the fast-blow fuse and the slow-blow fuse.

• Fast-Blow Fuses: Fast-blow fuses are solar fuses that are designed to blow instantaneously when the current exceeds their rated capacity and thereby provide immediate protection for sensitive electronic components. These fuses operate at lower amperage ratings, usually ranging from a few amps up to 32 A and can handle high voltages, between 600 to 1500 VDC. Fast-blow fuses are used to protect smaller circuits in solar power systems, such as charge controllers, inverters, and other electronic devices that are sensitive to overcurrent conditions. They are commonly installed at critical points in the solar power system, such as between the solar panels and the charge controllers.
• Slow-Blow Fuses: Slow-blow fuses, also known as time-delay fuses, are solar fuses that tolerate brief surges in current without blowing immediately. This characteristic makes them suitable for applications where inrush currents occur, such as during the startup phase of motors or other inductive loads. As slow-blow fuses withstand temporary spikes in current, they allow the circuit to remain operational during normal fluctuations without tripping unnecessarily. This is essential in circuits where brief overcurrents are expected but do not indicate a fault condition. These fuses typically have a thicker or larger filament and are made from materials that can endure higher currents for short periods. These fuses are made using thicker or larger filaments that can withstand higher currents for short periods.

Determining Correct Fuse Size and Rating

The following steps can determine the correct fuse size and rating for a solar fuse.

Step 1: Calculate the Maximum Current: Find the short circuit current (I_sc) of the solar panel typically provided on the solar panel's specification label. If not available, it can be estimated based on the panel's wattage and system voltage using the formula,

For example, for a 400 W solar panel operating at 12 V, 

Step 2: Apply a Safety Margin: Add a safety factor to account for potential current spikes and ensure protection, increase the calculated current by 25% (or 1.25 times the I_sc). 

In the above example, 

In this case, a fuse rated at 45 A can be chosen, which is the next standard size available.

Step 3: Consider Environmental Factors: If the fuse is exposed to high temperatures (above 40°C), apply a temperature de-rating factor. For example, if the ambient temperature is 50°C, a de-rating coefficient (Kf) of 0.90 can be used. Adjust the fuse rating accordingly.

It is to be ensured that the final rating does not exceed the ampacity of the conductors being protected. 

Step 4: Verify Compatibility with System Components: It should be ensured that the fuse rating does not exceed the ratings of other components in the system, such as charge controllers and batteries. For instance, if the charge controller is rated for 15A, use a 15A fuse to prevent damage.

Location of Solar Fuses: The common locations where solar fuses are typically installed are

• Between the Solar Panels and Charge Controller: This fuse protects the charge controller from excessive current generated by the solar panels. If a panel produces a huge current, the fuse will blow thereby preventing potential damage to the charge controller.
• Between the Charge Controller and Battery Bank: This is one of the most common locations for a solar fuse. It protects the battery bank from overcurrent conditions that could arise from the charge controller, ensuring that the batteries are safeguarded from potential damage.
• Between the Battery Bank and Inverter: This fuse protects the inverter from overloads and short circuits that could occur due to faults in the battery bank or inverter itself. This placement is crucial for maintaining the integrity of the entire system.
• At the Panel Cables: For systems with multiple solar panels connected in parallel, fuses can be installed on the individual cables of each panel. This arrangement prevents a fault in one panel from affecting the others by interrupting the current flow in case of an issue.