Introduction
Every photovoltaic (PV) power application must use fuses that are properly sized to its system. When you use the incorrect fuse size (or a standard non-PV fuse), you put the system’s reliability and safety in jeopardy. Due to the explosive growth in PV system power, understanding how to properly size a fuse for a PV application is essential.
Why PV Installations Require a Specific Type of Fuse
Fuses can only provide proper protection against overcurrents (which can cause personnel injury, power outages, equipment failure, and property damage) if they are the correct size for the system. The maximum circuit current and system voltage are important factors to determine which fuses can protect these systems.
PV applications require fuses that can protect against overcurrents, which standard fuses would not provide adequate protection against. Therefore, you must use fuses that are specifically designed for PV installations. Please see Figure 1 below.
Why PV Applications Specifically Require PV Fuses
Due to the unique conditions of PV installations, PV fuses have three characteristics that are unnecessary in non-PV applications:
- A higher dc voltage rating
- The ability to withstand harsh, fluctuating temperature changes
- Improved current cycling
A Higher Dc Voltage Rating
Before PV power became a significant source of utility energy generation, most low-voltage dc systems used 300 V dc and below. The physical size of most PV installations has grown immensely since then, and in turn, so has the amount of power these systems generate.
PV fuses are currently required to have voltage levels between 450 V dc and 1500 V dc, which enables them to protect higher power modules. This increase in system voltages is intended to minimize any power loss associated with long conductor runs.
The Ability to Withstand Harsh, Fluctuating Temperature Changes
PV fuses can operate in a wide temperature range, from an extreme cold of -40 °C to an extreme heat of +90 °C. In some regions, PV power systems need to function where extreme temperature variations occur in repeated daytime or nighttime cycles. Therefore, the thermal cycling capability of PV fuses is important to these systems’ safety and long-term reliability.
Improved Current Cycling
PV panels and circuits are subject to inconsistent current levels when sunrise, sunset, clouds, and stormy weather cause fluctuations in power generation. Under these weather conditions, the inconsistent current levels create current cycling, which non-PV fuses are neither designed for nor tested to protect against. Using non-PV fuses under these weather conditions would therefore create nuisance tripping.
Standard fuses also do not provide sufficient protection against periods of low-overload currents, which are more typical in PV applications than in non-PV applications. PV fuses can eliminate overcurrents that would otherwise lead to overheating during low-overload fault conditions.
Therefore, only fuses designed for PV systems may be used in these applications. The result is better longevity, reliability, and safety for the PV power equipment.
Fault Currents Affect PV Panels
A fault current is one of the primary causes of PV panel failure. A PV panel if not properly protected could be subject to melting, arcing, fire, and heat-damaged equipment and property.
Fuse Sizing
The correct fuse size should be calculated according to the National Electric Code®.
- NEC® 690.8(A) provides the maximum current for the PV circuit.
- NEC® 690.8(B) stipulates that the fuse amperage rating shall not be less than 125% of the maximum current identified in 690.8(A) unless listed for 100% operation.
- NEC® 240.4(B) allows the user to round up to the next highest fuse size.
Product Certifications
For PV fuses & fuseholders the following two standards are used.
- For PV fuses: UL 248-19, CSA C22.2 No. 248.19, NMX-J-009/248/19-ANCE
- For PV fuseholders: UL 4248-19, CSAC22.2 No.4248.19:21, NMX-J-009/4248/19-ANCE