We’re all familiar with thermally activated fuses, where the conducting element self-heats due to current flow, melts at a defined current value, and breaks the flow path. They are simple in concept (although they have their own subtleties, of course), reliable, do one thing, do it well, and provide a first (or last) line of defense against overcurrent damage in a system.
They come in many variations including fast acting, time-delay, and slow-blow, to best-fit the needs of the application. Among the reasons their use is mandated by regulatory codes in so many installations is that they need no initialization, set-up, or software, and can’t be hacked or overridden, all of which adds to their credibility and confidence in their performance.
Current-handling ranges of fuses that most engineers encounter span a fraction of an amp to tens of amps. They come in myriad packages, ranging from the classic 3AG to larger cartridges, as well as blade style used in many cars, Figure 1.
Figure 1 Fuses are available with different current ratings, of course, but also countless packages, including the 3AG glass cylinder, ceramic cartridges of various sizes, and the automotive “blade” style. Sources: RS-Online; Automation Direct; and Harbor Freight Co.
But then I started to wonder: How do they make fuses for hundreds of amps? What’s their packaging? Do the fuses simply get proportionally larger as the current goes to those levels?
My “ignorance” is largely due to lack of exposure to the topic. Higher-power engineering was not a big thing at most engineering schools for many years. That specialty, which encompasses larger-scale power generation, storage, transmission, battery energy storage systems, and solar/wind installations, was considered a backwater niche and not as exciting as designing data networks, devising and coding algorithms, or building faster computers.
But that was then, and times have changed. Today, power engineering is a hot area with all the activity related to electrified vehicles (EVs and HEVs), renewable energy, powering data centers, backup power systems, and more. Look at it this way: an EV draws on the order of 100 A and more, so fusing capabilities must be ramped up to meet appropriate engineering and regulatory requirements. Clearly, this is not a place where electric fuses (e-fuses) alone are suitable.
Would such a fuse be ten times bigger than a standard 10-A fuse? Were there any design shifts of which I should be aware?
I looked into it, and I found there’s a large subclass of thermal fuses dubbed “high rupturing capacity” (HRC) fuses which may be bigger but otherwise look like regular fuses on the outside, yet have an invisible, inside twist: they are filled with sand (silica) or other material, Figure 2.
Figure 2 (left) The HRC fuse features a filler, usually sand; (right) the actual internal construction is more complicated, as shown by this one version (there are others, as well). Sources: Electrical Maker and Swe-Check Pty Ltd
The main design elements that differentiate an HRC fuse from a lower-current conventional fuse—called a low breaking capacity (LBC) fuse—are:
- A heat-resistant, strong outer-fuse body, usually constructed from ceramic or fiberglass; LBC devices instead often have glass enclosures which are more likely to fragment when fusing action is initiated and the overload current is high.
- The cavity inside the fuse body is filled with fine silica sand or quartz to absorb the heat and energy of an over-current. In some cases, other materials such as powdered chalk, plaster of paris, or marble dust are used, but purified sand is most common.
- The metal caps or tags are solidly attached to the fuse body to create an air-tight seal to prevent any energy escaping in the event of an overload.
Why bother to do this? To my simplistic lower-current thinking, it seemed that once the fuse link overheats and opens, there’s not much to worry about.
But in the reality of the high-current world, that sort of simplistic thinking is misguided and even dangerous. The purpose of sand in the fuse is primarily to act as a heat-absorbing medium and to prevent the arc from continuing once the fuse element melts, Figure 3. That allows the fuse to safely interrupt very high fault currents (often several thousand amps) without causing damage to the fuse holder or surrounding equipment.
Figure 3 The current versus time characterization of the HRC fuse has some interesting transitions and jumps. Source: Electrical Maker
The sand or other filler in these fuses plays multiple roles:
- Cooling: When the fuse element melts due to excessive current, the sand absorbs heat, helping to cool the area and prevent fire or damage to surrounding components.
- Arc suppression: If a fuse blows, it can create an electrical arc. The sand helps to extinguish this arc by absorbing energy and providing a medium in which the arc can dissipate safely.
- Isolation: The sand can help to isolate the molten metal of the fuse element, preventing it from causing further short circuits or damage.
- Enhanced safety: By reducing the risk of arcing and overheating, sand contributes to the overall safety and reliability of the fuse.
In short: in an ordinary fuse—a length of exposed wire—the wire will melt and thus break the circuit; so far, so good. However, if a large current is flowing, the wire will also partially vaporize, and permit an arc to be formed. This arc may not be quenched even by the AC zero-volt crossing (and certainly won’t be for a DC circuit) but can continue for many cycles. The sand in the HRC fuse prevents the arc from forming, allowing the circuit to be opened safely and remain so.
There are two points here. First, it is not just a matter of “scaling up”. As with almost every other technical component, when you push the boundaries of capacity or size, things change and important enhancements to existing solutions are needed. While the laws of physics don’t change, their manifestations do. After all, in the electromagnetic spectrum, both gigahertz/terahertz waves and optical waves are defined by Maxwell’s equations, but their realities are very different. This is the case with high-current arcing across the open circuit presented by the blown fuse wire.
The second point nothing is as simple as it seems to be. When someone says, “what’s the big deal? It’s just a fuse” of similar, it really means they don’t know what’s involved. Even a simple function such as a fuse has its own design and fabrication issues that need to be understood and resolved.
Have you ever encountered a component which had unexpected design aspects due to its need to operate under harsh conditions or parameter extremes such as (but not limited to) voltage, current, temperature, or physical stress? Did you come to understand what had been done, and why?
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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