Core difference: Arc-extinguishing methods are completely different.
That is the most fundamental and fatal difference
AC passes through "zero" 100 times (50Hz) or 120 times (60Hz) per second-at which point the current naturally returns to zero, and the arc extinguishes itself like a candle that has gone out. Communications fuses take advantage of this natural advantage and can easily extinguish arcs.
DC has no zeros. The direction and size of the current remain the same. Once a short circuit produces an arc, the arc will continue to burn and hold. DC fuses have to rely on their own "hard power" to force the arc to elongate, cool and absorb-usually by the rapid vaporization the melt and spreading and absorbing quartz sand in the tube. This means that extinguishing the DC arc is much more difficult and requires much more energy than extinguishing the AC arc.
Arc power: DC far outpaces AC
Because DC arcs are not at risk of natural extinction, they last longer and release more energy. At the same voltage level, the DC arc produces more than twice as much energy as the AC arc. This requires DC fuses to have stronger breakability and more internal space to accommodate and absorb this energy, and the design is completely different in scale.
Physical size: DC fuse noticeably longer
In order to provide sufficient space for arc extinguishing, the DC fuse tube body is much longer than the equivalent AC fuse tube. The industry experience is that for every 150V increase in DC voltage, the length of the pipe needs to increase by about 10mm. Ordinary AC fuse tube are about 292-442mm, while 1000V or even 1500V solar DC fuse tubes are much longer to provide sufficient arc channels and quartz sand to fill space.
Internal fillings and materials: Heavy DC fuses
In some cases, AC fuses are not filled with quartz sand inside, whereas solar DC fuses almost have to be filled with high-purity quartz sand --the core medium of forced arc suppression. Quartz sand absorbs heat quickly, cools the arc and limits its spread at high temperatures.
fuse link are also made of different materials. Solar DC fuses usually use silver melt because of its precise melting point and extremely fast vaporization speed, which can complete the melting process in milliseconds, producing enough metal vapor to improve the arc quenching effect. Ordinary alternating current fuses usually use zinc alloys or copper and require less speed and accuracy.
Special Speed Level: gPV is a unique feature of PV DC.
Ordinary AC fuses have their own rated speed (e.g. gG, gL, aM, etc.), while solar DC fuses have a specific rated speed-gPV) (based on IEC 60269-6/UL 2579 standards). This level means that it must be able to deal with two extreme working conditions simultaneously:
Short-circuit current: The current peaks in microseconds and requires extremely rapid melting;
Continuous overload: The current rises slowly and requires to run reliably in seconds to minutes.
The rated speed of ordinary AC fuses is not designed for photovoltaic scenarios that require both speed and stability.
Voltage level: DC side voltage is much higher than household AC voltage
Normal AC fuses are rated between 250V and 690V, while solar DC fuses are rated between 1000V DC and 1500V, and the industry is moving towards 2000V DC. This is not just a quantitative difference, but also means that, on the DC side, the fuse needs to be completed at a higher voltage, and the requirements for insulation, creepage distance and breakability have been improved across the board.
Time constant: unique selection parameter for DC
In AC circuits, the current waveform itself determines the protective characteristics and the selection is relatively simple. In DC, however, the rate of increase in fault current depends on the inductance to resistance ratio of the system (i.e., time constant T=L/R). This parameter directly affects the speed at which the fault current reaches its peak and whether the fuse can cut off the fault current before the peak. The selection of a solar DC fuse must include the time constant in the calculation, which is not a consideration at all for an AC fuse.
Environmental adaptability: Photovoltaic fuses are more "robust";
Whereas conventional AC fuses are commonly used in indoor distribution boxes, solar DC fuses are exposed to the elements for long periods of time and must be able to withstand:
Extreme temperatures (-40 °C to+90 °C)
High humidity, salt fog corrosion
UV ageing
Repeated expansion and contraction caused by diurnal temperature difference;
Therefore, photovoltaic fuses usually use ceramic casings or flame-retardant materials such as PA66, which offer much higher levels of protection and weather resistance than conventional AC fuses.
Can they be replaced?
Never replace a DC fuse with an AC fuse. The arc quenching ability of AC fuses is not enough to deal with DC arc. If a short circuit occurs and the arc continues to burn, the fuse may explode or catch fire. The inverter's The IGBT could immediately damage or even ignite the cable.
On the other hand, while DC fuses can be used in AC circuits (since AC is easier to extinguish), they are overused, more expensive, and their time-current characteristics may not match AC systems.
