Overcurrent Protection: Fuses or Breakers?

When designing non-residential electrical systems, engineers must consider electrical protection from many perspectives. Although the entire electrical distribution system of a facility is important—from switchyard to light bulbs—“the overcurrent protective system is the very heart of the electrical distribution system,” wrote George Farrell and Frank Valvoda, PE, in a 1...

10/01/2007


What has changed since the first publication of the popular article series, “The Art of Protecting Electrical Systems,” is the introduction of more electrically intuitive devices, which have changed electrical design practices significantly.

Overcurrent

Overcurrent is current that exceeds the ampere rating of conductors, equipment or devices under conditions of use, and includes both short circuits and overloads. During a short circuit, current flows outside its normal path. Insulation breakdown or faulty equipment connections can cause short circuits. The load determines circuit current during normal fault-free conditions. However, during a short circuit, electrical current bypasses the load, taking the path of least resistance. System impedance—or AC resistance—determines short circuit or fault current magnitude, which ranges from fractions of an amp to 200 kA or more.

An overload is an overcurrent condition within normal current paths—there is no insulation breakdown. However, if an overload is allowed to persist, it causes equipment or wiring damage. Temporary overloads can be harmless; sustained overloads can cause damage.

Temporary overloads may be caused by momentarily pushing equipment past its limit, such as cutting too deeply with a milling machine or from starting large motors or other inductive loads. Temporary overloads occur frequently, are typically harmless and should be allowed to subside. An overcurrent protective device (OCPD) should not open the circuit, allowing motors to start and loads to stabilize.

Sustained overloads can be caused by continually overloading electrically-driven mechanical equipment, failed bearings or other equipment malfunctions. They also are caused by installing loads such as equipment or additional lighting circuits that increase power demand beyond planned capacity. If sustained overloads are not disconnected within appropriate time limits, they eventually will overheat circuit components and cause thermal damage to insulation and equipment.

When starting up a new facility, after system modification or equipment installation, it's possible to encounter crossed phase wiring, or perhaps even bolted faults. “Bolted faults are characterized by a solidly connected fault path causing high levels of current to flow through this solid connection,” said Todd Lottman, product manager of services at St. Louis-based Cooper Bussmann. “This type of fault is commonly used when testing electrical equipment for short-circuit current ratings and overcurrent protective devices for interrupting ratings.”

Protecting with fuses and breakers

Overcurrent scenarios dictate the type of overcurrent protection that should be used. The National Electrical Code (NEC) has established basic power system overcurrent protection requirements and recognizes fuses and circuit breakers as the two basic types of OCPDs. According to the NEC, a fuse is an overcurrent protective device with a circuit-opening fusible element that is heated and severed by the passage of overcurrent through it. A circuit breaker is a device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.

Fuses and circuit breakers are available in a variety of sizes and ratings. Their similar yet different features and characteristics allow electrical system designers to choose devices appropriate for a facility's electrical system.

Fuses

Fuses are single-pole devices—an individual fuse can open only one phase of a multi-phase circuit. However, multiple individual fuses can be applied together in a disconnect to protect a multi-phase system. Low-voltage fuses are available in sizes from fractions of an amp to thousands of amps at voltage ratings up to 600 volts. They are available with short-circuit interrupting ratings of 200 kA or more.

Some fuse types are classified as current limiting. According to the NEC, current-limiting fuses “...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.” This means that a current-limiting fuse will open quickly—within one-half cycle—when subjected to a high-level fault.

Fuses cannot be given an external command to trip. By nature, fuses offer very reliable current limiting features. Also, they can operate independently—they do not require an overload relay with instrument transformers to tell them when to blow.

When using fuses, a separate disconnect must be used in many situations because they are designed to open under overcurrent conditions only. However, when using circuit breakers, a separate disconnect is not required because breakers are designed to be opened and closed manually, as well as when subjected to an overcurrent condition.

Circuit breakers

Low voltage circuit breakers differ in construction, operation and maintenance requirements depending on how and where they are used. Circuit breakers are available as 1-, 2-, 3- or 4-pole devices, and rated from 10 amps to thousands of amps. Short-circuit interrupting ratings of circuit breakers are available up to 200 kA.

Low voltage circuit breaker types include molded-case circuit breaker (MCCB) and low-voltage power circuit breaker (LVPCB). The internal parts of an MCCB are enclosed in a molded case of insulating material. This type of breaker is not designed to be opened, which means that it is not field maintainable. MCCBs are used in panelboards, switchboards, motor control centers (MCCs), equipment control panels and as stand-alone disconnects inside separate enclosures.

LVPCBs are used in low-voltage drawout switchgear. They are typically larger and more rugged than MCCBs, and are usually field maintainable. One characteristic that most power circuit breakers have in common is they are rated for continuous operation at 100% of their current ratings in their enclosures, which is not the case with all types of low voltage circuit breakers. LVPCBs have short time and interrupting ratings, allowing them to be used for selectivity and coordination with downstream devices.

Low-voltage circuit breakers can have a toggle mechanism or a two-step stored energy mechanism. The MCCB has a toggle mechanism with a distinct tripped position, which is typically midway between on and off. The LVPCB has a two-step stored energy mechanism, which uses an energy storage device, such as a spring, that is charged and then released, or discharged to close the circuit breaker.

Current limiting OCPDs

Many fuses and some breakers are categorized as current limiting. Within its current-limiting range, a current-limiting device is designed to interrupt all currents; limit the peak current (compared to a solid conductor with the same impedance); and open the circuit in less than one-half cycle (at 60 Hz) after the occurrence of a fault.

A common fuse myth is that it will blow as soon as the current flowing through it exceeds its rated value. Truth is, a typical fuse has an inverse time-current characteristic: the higher the current, the faster the fuse will blow. As the amount of overcurrent increases, the opening time of the fuse decreases exponentially. However, a single fuse class has only a single time-current characteristic, which cannot be adjusted.

Whether fuses or circuit breakers, the current-limiting range of an individual protective device falls between its threshold current and its interrupting rating. Threshold current of an overcurrent device is the specific amount of current that causes it to open the circuit in less than

SCCR and interrupting ratings

It should be noted that there are different ratings involved in overcurrent protection. Short circuit current rating (SCCR) is the same as withstand rating. The rating represents how much short circuit current a device can withstand without self-destructing. “A withstand rating is the maximum RMS symmetrical short-circuit current at which the equipment has been tested under specified conditions,” explained Farrell and Valvoda. “At the end of the test the equipment must be in 'substantially' the same condition as prior to the test.”

SCCR is applicable to non-interrupting equipment including switches; busway or bus duct; switchgear and switchboards; motor starters; contactors; MCCs; and similar equipment. If a short circuit occurs, every component in the system through which the fault current flows must safely withstand the effects of the current, which include heating and magnetic stresses. The protective device must break the current path reliably and safely.

Article 100 of the NEC defines interrupting rating as “the highest current at rated voltage that a device is intended to interrupt under standard test conditions.” The Fine Print Note (FPN) to Article 100 states “Equipment intended to break current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current.”

Section 110-9 of the NEC states: “Equipment intended to break current at fault levels shall have an interrupting rating sufficient for the system voltage and the current which is available at the line terminals of the equipment… Equipment intended to break current at other than fault levels shall have an interrupting rating at system voltage sufficient for the current that must be interrupted.”

Arc flash considerations

Existing facilities are investing intense efforts in complying with the NEC and NFPA 70E. It's more difficult to change the status quo than to engineer in compliance at the beginning. However, opportunities for consulting engineers exist for both existing and new projects. New facilities can be designed correctly before construction begins. Existing facilities must be analyzed, and modified if necessary.

Regardless of whether an electrical system is being designed for a new facility or an existing one, an arc flash hazard analysis must be done to ensure workers are protected from this potentially lethal threat. It is necessary to know the bolted-fault-current value when doing arc flash analysis calculations. It is also necessary to know the available fault current, which should be available from the utility.

Bolted fault current is not the same as arcing current. “Arcing faults differ in the fact that the current actually flows through ionized air causing an arc,” Lottman said. “The major difference between these two types of faults is that the energy in a bolted fault condition is dissipated in the faulted equipment while an arcing fault releases energy out into the surrounding environment.”

Coordination issues

Selective coordination minimizes downtime caused by nuisance tripping. Joe Schomaker, senior product manager at Cooper Bussmann, said that selective coordination involves “isolating an overloaded or faulted circuit from the remainder of the electrical system by having only the nearest upstream overcurrent protective device open. Without selective coordination, a single faulted circuit can shut down an entire facility.”

Prior to NFPA 70E and the work done to develop IEEE 1584, most facilities were designed to protect the electrical system and its loads from damage while avoiding nuisance interruptions. However, protecting workers from arcing faults is now a necessary part of the equation. Safety should never be an afterthought.

On the surface it appears that selective coordination and safety from arc flash hazards are opposites. However, some believe that coordination and safety can be achieved in the same system. “Protecting people, while protecting the system and providing continuity of service are not mutually exclusive goals,” said Joe Weigel, product manager, Square D Services, Schneider Electric, in Nashville, Tenn. “But the people-protection aspect is causing electrical designers to reconsider some of the design practices that they traditionally employed—including their choices of overcurrent protective devices.”

Weigel said the primary strategy to reduce the incident energy released during an arcing fault is to detect and clear the fault as quickly as possible. Because of this, there is often a fine line between optimal arc flash energy reduction and system coordination to provide continuity of service. “For example,” Weigel said, “one way to significantly reduce the arcing fault incident energy release is to lower the 'instantaneous' setting on the circuit breaker trip unit (if it has that function). However the instantaneous setting cannot be randomly set at its minimum setting or nuisance tripping is likely to occur as the loads attempt to start. So a time-current coordination study is required, and the coordination study is a critical element of the arc flash hazard analysis for that reason. Once the instantaneous has been properly set based on the coordination study and arc flash analysis, it should never be reset by anyone; unless changes in the arc flash incident energy potential release is considered.”

“Circuit breakers can be used in selectively coordinated electrical systems,” said Kenneth Cybart, senior technical sales engineer at Littelfuse, Des Plaines, Ill. “But specifiers must overlay the time-current curves of all upstream (line side) and downstream (load side) breakers to ensure that the downstream circuit breaker will open under a short-circuit condition before the upstream circuit breaker operates. To ensure that a circuit-breaker-protected system is selectively coordinated, the time-current curves must not overlap at any possible fault current. With fuses, selective coordination is achieved as long as specifiers maintain manufacturer-recommended ratios.”

Common place in high and medium voltage switchgear, zone selective interlocking breakers are making their way into low voltage switchgear as well. Zone selective interlocking uses data network communications between two or more compatible breaker trip units. This technology enables programmed trip unit settings to be altered automatically to respond to different fault conditions and locations. Instantaneous interruption is localized to the specific fault location, while the rest of the electrical system is maintained to provide positive coordination between circuit breakers.

Coordinating electrical systems involves understanding and using time-current curves. “In order to plot the OCPD curves,” said James P. Stroke, PE, a consulting engineer based in Somerset, Mass., “it is necessary to either obtain the OCPD inverse time curves from the manufacturers and plot them out by hand on log paper, or use a software package that plots it all out. I print each OCPD in a different color, which really makes them stand out, and you can more easily spot overlaps and adjust accordingly. In some cases, a 'perfect' coordination is impossible.”

Stroke also said that obtaining electrical design software could pose a problem for some. “These software packages are extremely expensive and not every engineer has these,” he said. “So, the implication is that many of the coordination studies probably just don't get done.” The same applies to short circuit studies, he adds. “What's really needed is an inexpensive software program for short circuit and coordination studies—either on a CD, or available online so anyone can access and use it.”

“There are many choices that take place during electrical system design,” Weigel said, “and these choices should be diligently consistent with optimizing the design in a way that will also optimize arc flash safety.”





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