Photovoltaic DC Fuses: Principles, Selection, Classification, and Applications

1. What is a DC fuse and how does it work?

A DC fuse is an overcurrent protection device connected in series within a circuit. Its core principle is based on the thermal melting characteristics of metal. When an overcurrent exceeding a specified value flows for a sufficient duration, the fuse element (typically made of materials such as pure silver) heats up and melts, thereby interrupting the circuit and preventing fault escalation.

The working process can be summarized as follows: Under normal operation, the fuse element conducts current as part of the circuit. When a short circuit or severe overload occurs, the high current generates heat, rapidly melting the element and producing an arc inside the fuse tube. Extinguishing a DC arc is much more difficult than an AC arc because DC current has a constant direction and lacks the natural zero-crossing points (occurring hundreds of times per second in AC) that allow for natural arc extinction.

For this reason, DC fuse tubes are filled with high-purity quartz sand. The sand acts to forcibly cool the arc and absorb its energy, forcing the arc to extinguish within an extremely short time. Factors such as the tube length, the purity and composition of the quartz sand, and the design of the fuse element collectively determine the arc extinguishing performance.

2. How to select the right DC fuse for a solar system?

Selecting a PV DC fuse cannot be done by simply adapting AC fuse parameters. The following key factors must be considered comprehensively:

1. Rated Voltage (Ue): Must exceed the system's maximum voltage

The open-circuit voltage of PV modules increases as the temperature drops. According to the IEC 60269-6 standard for PV fuses, the rated voltage of the fuse must be greater than or equal to the maximum open-circuit voltage of the PV string at the expected lowest ambient temperature at the installation site. For example, if the maximum string voltage at low temperature reaches 1000V DC, you must select a fuse rated at 1000V DC or higher. Never use a 600V DC fuse to protect a 1000V DC system.

2. Rated Current (In): Follow the 1.25 safety factor rule

According to international standards such as NEC 690.9 and IEC 62548, the rated current of the fuse should satisfy:

In ≥ 1.25 × Isc (where Isc is the short-circuit current of the PV string).

For example, if a string has an Isc of 10A, the minimum fuse rating required is 12.5A, and you should select the next higher standard rating, such as 15A.

Additionally, coordination principles must be observed: PV modules themselves have a certain reverse overcurrent withstand capability (IEC 61730-2 requires modules to withstand 1.35 times the maximum fuse rating). The selected fuse must blow within the specified time at 1.35 times its rated current to effectively protect the modules.

3. Breaking Capacity (I1): Must cover the maximum fault current

When multiple strings are connected in parallel in a PV array, if a short circuit occurs in one string, all other parallel strings will feed current into the fault point. The rated breaking capacity of the fuse must be greater than the maximum prospective short-circuit current that may occur at that installation point. In large-scale ground-mounted power plants or battery energy storage systems, short-circuit currents can be very high, requiring fuses with breaking capacities of 20kA, 50kA, or even higher.

4. Must use dedicated PV DC fuses

Never replace a DC fuse with an AC fuse. DC arcs do not extinguish on their own. If an AC fuse is used in a DC circuit, it must be significantly derated (for example, a fuse rated at 1000V AC might only be usable in a 440V DC circuit). Therefore, you must select fuses explicitly marked as DC-compatible, conforming to IEC 60269-6 and featuring the gPV characteristic for PV applications.

3. Common types of DC fuses and application comparisons

Based on their installation location and the objects they protect in a PV system, DC fuses can be divided into the following common categories:

1. Cylindrical fuses for PV string protection

These fuses are compact (common sizes: 10×38mm, 10×85mm cylindrical) and are typically installed in DC combiner boxes or at the inputs of string inverters, connected directly in series with each PV string circuit. Their main purpose is to prevent reverse current faults between strings and to protect PV modules and cables from reverse overloads. If a short circuit occurs in one branch, the fuse blows quickly, isolating the faulty branch without affecting other normally operating strings. Their breaking capacity is generally in the range of 10kA–20kA, with common current ratings from 2A to 30A.

2. Square-body blade fuses for combiner and conversion circuits

These fuses are larger in size, featuring an extended square tube design with blade contacts bolted into fuse bases, allowing them to carry and interrupt very high currents. They are primarily used at the output of combiner boxes in PV power plants, on the DC input side of inverters, and for protecting large battery energy storage packs. They serve not only as overload protection but also to handle extremely high fault currents (up to 50kA or higher) generated during busbar short circuits, with interruption times within milliseconds. Typical current ratings range from 35A to 500A. They serve as the main protection devices on DC distribution cabinets and the DC side of large inverters.

3. Very fast-acting fuses for battery systems

In PV energy storage systems, the battery packs and charging circuits require DC-specific fuses with very fast response characteristics. These fuses use ultra-fast blowing characteristics, with interruption times as short as 15–100 microseconds. They are primarily used to protect batteries, charge controllers, and the DC bus of inverters against surge currents or severe short circuits that could instantly damage semiconductor devices. They are typically installed near the battery bank or in dedicated fuse switch disconnectors.

Summary

From the small string protection fuses in PV combiner boxes, to the large-capacity square-body blade fuses in converter cabinets, and the very fast-acting fuses in battery systems, DC fuses form multiple layers of protection on the DC side of a PV power plant. By selecting the correct type and parameters of fuses, and by following the core principles of "DC-rated, voltage-matched, current-derated (with safety margin), and sufficient breaking capacity," you ensure that fuses fulfill their role as the final line of defense, safeguarding the safe operation of the PV power station.