Comparison of Circuit Breakers and Fuses for Low Voltage Applications
Introduction Recent claims by fuse manufacturers regarding the arc-flash and simplifiedcoordination benefits of fuses do not tell the entire story regarding which type of device is “best” for a given power system. In reality, not only does the wide range of available circuit breaker types allow them to be successfully used on nearly any kind of power system, they can be applied so as to provide selective coordination, arc-flash protection, advanced monitoring and control features, all in a renewable device. This paper gives a feature-by-feature comparison of the merits of circuit breakers vs. fuses, discussing the relative merits of fuses and circuit breakers in each section. While both circuit breakers and fuses are available for application in systems that operate at higher voltage levels, the focus of this guide is on low-voltage systems operating at 600 V or below.
fuse—An overcurrent protective device with a circuit-opening fusible part that is heated and severed by the passage of overcurrent through it. circuit breaker—A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.
The NEC also requires that circuits be provided with a disconnecting
means, defined as “a device, or group of devices, or other means by which
the conductors of a circuit can be disconnected from their source of supply.”
Since fuses are designed to open only when subjected to an overcurrent,
they generally are applied in conjunction with a separate disconnecting
means (NEC 240.40 requires this in many situations), typically some form of
a disconnect switch. Since circuit breakers are designed to open and close
under manual operation as well as in response to an overcurrent, a
separate disconnecting means is not required.
Both fuses and circuit breakers are available in a variety of sizes, ratings,
and with differing features and characteristics that allow the designer of an
electrical system to choose a device that is appropriate for the system under
consideration.A Comparison of Circuit Breakers and Fuses for Low-Voltage Applications 0600DB0601
Low-voltage fuses are available in sizes from fractions of an amp to
thousands of amps, at voltage ratings up to 600 V, and with short-circuit
interrupting ratings of 200 kA or more. Fuses are inherently single-pole
devices (i.e., an individual fuse can only operate to open one phase of a
multi-phase circuit), but two or three individual fuses can be applied together
in a disconnect to protect a multi-phase system. Low-voltage fuses are
tested and rated according to the UL 248 series of standards. Several types
can be classified as current-limiting, which per the NEC definition means
that they “…reduce the current flowing in the faulted circuit to a magnitude
substantially less than that obtainable in the same circuit if the device were
replaced with a solid conductor having comparable impedance.” In other
words, the current-limiting fuses open very quickly (within 1/2 cycle) in the
presence of a high-level fault, allowing them to provide excellent protection
for distribution system components or load equipment. Fuses can be
applied in equipment such as panelboards, switchboards, motor control
centers (MCCs), disconnect switches/safety switches, equipment control
panels, etc.
Circuit breakers are also available with a wide range of ratings—10 A to thousands of amps, also with short-circuit interrupting ratings to 200 kA— and are available as 1, 2, 3, or 4-pole devices. The three basic types of LV circuit breakers are the molded-case circuit breaker (MCCB), low-voltage power circuit breaker (LVPCB), and insulated-case circuit breaker (ICCB). MCCBs are rated per UL 489, have all internal parts completely enclosed in a molded case of insulating material that is not designed to be opened (which means that the circuit breaker is not field maintainable), and can be applied in panelboards, switchboards, MCCs, equipment control panels, and as stand-alone disconnects inside a separate enclosure. LVPCBs, which are rated per ANSI standards and are applied in low-voltage drawout switchgear, are larger, more rugged devices that may be designed to be fully field maintainable. ICCBs can be thought of as a “cross” between MCCBs and LVPCBs—they are tested per UL 489 but may share some characteristics with LVPCBs, including two-step stored energy mechanism availability in drawout construction and partial field maintainability [2]. Both types of OCPDs can meet the basic requirements of the NEC, but are circuit breakers or fuses best suited for a particular application? Unfortunately, there is no simple answer to this question—several other factors must be taken into account, such as the level of protection provided by the OCPD, selective coordination requirements, reliability, renewability, and flexibility. The remainder of this guide will provide a discussion of each of these topics. III. System Protection As discussed above, both circuit breakers and fuses meet the basic NEC requirements for overcurrent protection of electric power distribution systems and equipment. Any type of OCPD must be sized and installed correctly after taking all derating factors and other considerations into account. Particularly for overloads and phase faults, both circuit breakers and fuses provide excellent protection and either is suitable for most applications. A bit more consideration is warranted for some other aspects of system protection, as discussed in the remainder of this section. A. Ground-Fault Protection Conventional wisdom states that the most common type of fault in a power system (by far) is a single-phase-to-ground fault. On solidly-grounded power systems, the available ground-fault current level can be significant. In some situations, ground fault current levels that are even higher than the maximum three-phase fault current level are theoretically possible. However, many ground faults produce only relatively low levels of fault current due to impedance in the fault path (due to arcing or to some othe