Current limitation is a function of how quickly a fuse can react to a short-circuit condition and clear it before the fault current can build up to destructive magnitudes. In order to understand current limitation, it is first important to understand what is a short-circuit. A short-circuit is defined in the 2023 NEC as “An abnormal condition (including an arc) of relatively low impedance, whether made accidentally or intentionally, between two or more points of different potential.” When a short-circuit occurs the current in the circuit will dramatically increase due to the alternate low impedance path that has been introduced into the circuit. That is a key thing to understand, the path of the current being taken is outside of the normal current path. The current that flows during a short-circuit is called fault current.
The NEC defines fault current as “The current delivered at a point on the system during a short-circuit condition.” It also defines available fault current as “The largest amount of current capable of being delivered at a point on the systems during a short-circuit condition.” The available fault current magnitude will vary based upon the size of the electrical service and the location in the system. For instance, the available fault current in a one-family dwelling, with an electrical service of 200A, may only be around 10-22kA. Whereas the available fault current for a large commercial or industrial facility, with an electrical service of 3000A, may be around 65kA. Even higher available fault currents can be encountered when connected to a utility electrical grid, some in excess of 200,000A.
Protecting electrical system components from fault currents is critical when selecting fuses. Where higher levels of available fault current are present, fuses provide an increased level of protection for components and electrical equipment when compared to non-current-limiting fuses. When protected by a non-current-limiting fuse, the prospective available short-fault current permits fault current to build up to its full value and lets through an immense amount of destructive current, heat and magnetic energy before opening as shown in the figure below. The figure shows an operation time of 1 cycle, but some non-current-limiting fuses can allow the fault current to flow for 1 cycle or more.
Non-current limiting fuse opens short-circuit for 1 cycle or more
The figure below demonstrates the current-limiting ability of a current-limiting fuse when the available fault current is within its current-limiting range.
As shown in the figure, the current-limiting fuse opens and clears the fault within the first 1/2 cycle and limits the let-through energy to a fraction of the system’s available fault current. The shaded area under the curve (in blue) represents energy being dissipated in the circuit with both magnetic forces and thermal energy being directly proportional to the square of the current, making it important to limit the short-circuit current to as small a value as possible. The maximum magnetic forces vary as the square of the “peak” current and thermal energy varies as the square of the “RMS” current.
The degree of current limitation depends upon the fuse class, ampere rating and available fault current. For instance, Class J fuses are much more current limiting than Class R fuses. In addition, as the ampere rating of the fuse increases, so does the current-limiting threshold (when the fuse becomes current-limiting) as well as the let-through current. Similarly, higher levels of available fault current will result in increased current let-through of the fuse. To determine the let-through of fuses, manufacturers publish current-limiting let-through curves for the specific manufacturer’s fuse.
The use of current-limiting fuses provides increased protection of downstream components and equipment as well as numerous other benefits as shown below.
- Increase component and assembly short-circuit current ratings in equipment such as HVAC control panels, industrial control panels, industrial machinery, and elevator control panels.
- Meet the requirements of selective coordination as required for some critical systems when properly selected.
- Enhance workplace safety by reducing the incident energy personnel are exposed to under fault conditions when the available arcing current is in the current-limiting range of the fuse.