DC Fuses and how to size them
What is a Fuse?
A fuse is an electrical safety device. It protects an electrical circuit against high currents. The fuse is placed in the supply cable to an electrical device. As soon as current flows through the fuse that is higher than its current rating, for a certain amount of time, the fuse will blow. Once the fuse has blown, no more current will flow into the circuit.
Higher than expected current situations can occur when an electrical device develops a fault or when there is short-circuit in the electrical circuit.
How does a fuse work?
Traditionally, a fuse contains a wire or a strip of metal that melts as soon as an unacceptable high current passes through the fuse. When the wire in the fuse has melted, the electrical circuit has been broken and no more current will flow in the circuit. Once the fuse has blown” it will need to be replaced by a new fuse to make the circuit operational again. These fuses are one-time use fuses. Once they have blown, they can’t be reset. They need to be replaced by a new one.
Another type of fuse is the automatic fuse, often called circuit breaker mainly used in AC circuits. They do not need to be replaced like the traditional fuses.
Location of the fuses
Each consumer that connects to a battery needs to be fused. The fuse is placed in the positive cable. Each individual consumer needs to have an individual fuse. No matter how big or small the power rating of the equipment is. Batteries can potentially produce very high currents that can cause a fire. If the consumer develops a fault and internally short circuits, a very large current will flow, potentially causing a fire hazard. A DC circuit usually contains a main battery fuse, after which it branches of to the individual consumers. Each consumer has an individual fuse.
Location of the PV array circuit breakers
A fuse needs to be located between a PV array and the solar charger. Please check with the local authorities, regulations per application and country will vary.
Fuses need to be placed in fuse holders. The fuse holder securely holds the fuse in place. And in some cases, they also provide electric insulation. Circuit breakers are usually mounted on DIN rail. Fuses and circuit breakers are usually located in a switch board, preferably inside an enclosure.
How to select the correct fuse
It is important to choose the correct fuse that will match the circuit and match the power consumption of the equipment in that circuit. The rating of the fuse is displayed on the fuse or can be found in the fuse’s data sheet or its specifications.
If there is only one consumer in a circuit, the fuse will need to match the current rating of that consumer or the current rating of the cable, whichever is the lowest of the two. If there are multiple consumers in a circuit, then the fuse will need to match the current rating of the cabling in the circuit.
The fuse voltage rating needs to be equal or bigger than the expected maximum voltage in the system. The fuse needs to be specifically rated for the required type, DC and/or AC. Most DC fuses are suitable for 12 and 24 V, but they are not necessarily suitable for 48 V and higher. Please note that not all fuses or circuit breakers can be used in both AC, and DC circuits. If the fuse is able to be used for both AC and DC, the voltage for AC is often rated higher than the DC voltage rating. Also take care that circuit breakers might not be unidirectional, so for DC it matters which way they are wired into the circuit.
The speed of a fuse is the time it takes for the fuse to open when a fault current occurs. This is dictated by the fuse material, its mechanism, the current and the temperature. There are slow and fast blow fuses. Slow blow fuses are commonly used in DC applications that can be found in automotive and marine.