Fuses vs. breakers

Eventually, every plant electrical system will experience overcurrents. Moderate magnitudes of overcurrent can overheat system components and damage insulation, conductors and equipment – unless they are removed expediently. Large overcurrents can destroy insulation and melt conductors, while very high currents can produce magnetic force capable of bending and twisting bus bars...

09/28/2007


Eventually, every plant electrical system will experience overcurrents. Moderate magnitudes of overcurrent can overheat system components and damage insulation, conductors and equipment %%MDASSML%% unless they are removed expediently. Large overcurrents can destroy insulation and melt conductors, while very high currents can produce magnetic force capable of bending and twisting bus bars, pulling cables from terminals and cracking spacers and insulators.

Damaging fault current and short circuits can produce fires, explosions, arc flash and arc blast, which could cause injury or death to plant personnel. Plant managers must be proactive by designing electrical systems and providing programs and training that ensure the safety of their workers.

Typical plant electrical distribution

Electrical power enters a typical manufacturing facility through medium voltage switchgear. From there, power is distributed to medium voltage equipment %%MDASSML%% typically motors %%MDASSML%% and to secondary substation transformers, which step medium voltage down to low voltage.

Low voltage switchgear further distributes the electrical power through feeders to branch circuits that consist of motor control centers and drives, load centers and associated support equipment such as metering modules, capacitors, harmonic filtering and uninterruptible power supplies. Some larger facilities also have paralleling switchgear used with onsite power generation or backup power generators, which operate through an automatic transfer switch.

In most cases, power from the load centers flows through distribution transformers, some of which step down the voltage further for lighting and control panels, and other panelboards that require voltages lower than 480 Vac %%MDASSML%% typically 240 V, 208 V and 120 Vac.

Electrical protection

No plant electrical distribution system would be complete without devices that protect circuits and equipment from overcurrent situations. Overcurrent is current that exceeds the ampere rating of conductors, equipment or devices under conditions of use. Overcurrent 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 %%MDASSML%% or ac resistance %%MDASSML%% determines short circuit or fault current magnitude, which can range from fractions of an Amp to 200 kA or more.

An overload is an overcurrent condition within normal current paths %%MDASSML%% there is no insulation breakdown. However, if an overload is allowed to persist, it will cause 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, or from starting large motors or other inductive loads. Temporary overloads occur frequently, are typically harmless and should be allowed to subside. Overcurrent protective devices should not open the circuit, allowing motors to start and loads to stabilize.

Sustained overloads can be caused by continually overloading mechanical equipment, failed bearings or other equipment malfunctions. They are also 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 will eventually overheat circuit components and cause thermal damage to insulation and equipment.

The National Electric Code has established basic power system overcurrent protection requirements and recognizes fuses and circuit breakers as the two basic types of overcurrent protective devices. 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 their plant's electrical system.

Fuses

The fuse is the oldest, simplest and least expensive type of electrical protection device. Its operation is simple: excessive current creates thermal energy, which causes a fuse element to melt, interrupting the path of electrical current flowing through it. The concept of how fuses operate has changed very little. However technological advances have made fuses more predictable, faster and safer.

A common fuse myth is that a fuse will blow as soon as the current flowing through it exceeds its rated value. A typical fuse has an inverse time-current characteristic: the higher the current, the faster the fuse will blow. However, a single fuse class has only a single time-current characteristic, which cannot be adjusted. Fuses cannot be given an external command to trip.

By nature, fuses offer very reliable current limiting features. They do not require an overload relay with instrument transformers to tell them when to blow.

Fuses are single-pole devices %%MDASSML%% 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 V. They are available with short-circuit interrupting ratings of 200 kA or more.

Some fuse types can be 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 very quickly %%MDASSML%% within 1/2 cycle %%MDASSML%% when subjected to a high-level fault.

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

Circuit breakers differ in construction, operation and maintenance requirements depending on how and where they are used. Breakers can be low, medium or high voltage. High-voltage circuit breakers are found in electrical transmission and distribution system substations and are beyond the scope of this article.

Low voltage electrical systems operate at electrical potentials less than 1,000 Vac, whereas medium voltage systems operate between 1 kV and 100 kV. Low-voltage circuit breakers are rated at electrical potentials up to 1,000 Vac. However, most industrial applications are 600 V or less; therefore most low-voltage breakers are referred to as 600 V. Available as 1, 2, 3 or 4-pole devices, circuit breakers are rated from 10 A 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), low-voltage power circuit breaker (LVPCB) and insulated-case circuit breaker (ICCB). 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, 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 rating in their enclosures, which is not the case with all types of low-voltage circuit breakers (when used in an enclosure). LVPCBs have short time and interrupting ratings, allowing them to be used for selectivity and coordination with downstream devices.

Essentially, ICCBs have characteristics of both MCCBs and LVPCBs. They are tested according to MCCB specifications, but share some characteristics with LVPCBs such as a two-step stored energy mechanism, drawout switchgear availability and partial field maintainability.

Low-voltage circuit breakers can have a 'toggle' mechanism or a two-step stored energy mechanism circuit breakers. 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.

Selective coordination

Selective coordination minimizes downtime caused by nuisance tripping. According to Joe Schomaker, senior product manager at Cooper Bussmann, 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.”

On the surface it appears that selective coordination and safety from arc flash hazards are opposites. Conventional trip devices in circuit breakers must either be made less sensitive to the current or add time delays to make them selective. However, some believe that coordination and safety can be achieved in the same system. “Selective coordination is not a mutually exclusive goal with arc flash protection,” said Joe Weigel, product manager, Square D Services, Schneider Electric, in a recent PLANT ENGINEERING Webcast on arc flash.

“A big piece of it is the designer's selection of the overcurrent protective device that will best suit the conditions. Sometimes it's a fine line %%MDASSML%% when setting the breaker trip unit %%MDASSML%% (to find) settings that will provide the optimal safety as far as arc flash is concerned and at the same time allow the system to coordinate properly.

“You must choose the device that best suits the conditions,” Weigel continued. “Current-limiting fuses are very quick if they operate within their current-limiting region. They operate withinreduce the incident energy.”

“Circuit breakers can be used in selectively coordinated electrical systems,” said Kenneth Cybart, senior technical sales engineer at Littelfuse, Inc. “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.”

Zone selective interlocking breakers are making their way into low voltage switchgear as well. Already common in high and medium-voltage switchgear, 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.

Changing a plant's electrical system to accommodate selectivity and safety begins with a system study. Electrical documentation %%MDASSML%% especially the one-line diagram %%MDASSML%% must be accurate. It's nearly impossible to analyze an electrical distribution system without up-to-date documentation. “It provides a roadmap for people involved in the work to plan their route, to identify where there may be hazards, identifying lockout-tagout, identifying where to place temporary safety grounds, as well as the information that describes the system,” said H. Landis “Lanny” Floyd, principal consultant, electrical safety and technology at DuPont during the PLANT ENGINEERING Webcast. “Without that document, it's like maneuvering through an unknown city without a roadmap %%MDASSML%% it's very difficult to find your way, and you may find yourself in a dangerous situation and not understand it.”



Webcast: Arc flash awareness must increase

On Aug. 2, Plant Engineering magazine presented “Advancements in Arc Flash Protection,” part of its continuing Webcast series on arc flash and electrical safety. Sponsored by General Electric and Exertherm, the 90-minute program featured a 'dream team' of panelists, which included Gary Fox, specification engineer at General Electric; Joseph Weigel, product manager at Square D Services, Schneider Electric; and H. Landis “Lanny” Floyd, principal consultant, Electrical Safety & Technology at DuPont, and was moderated by managing editor Jack Smith.

The Webcast was presented live as a town hall discussion, with live questions from the audience. Smith directed questions from viewers to the panelists, who provided advice on this important topic. A small sample of the discussions follows:

Smith : What do you see as the most critical issue regarding arc flash today?

Fox : I think the most critical issue is basic knowledge and compliance about arc flash in the industry. For example, I go to job sites and advise them about how their equipment can be modified. In many cases, unless I prompt a discussion, there will be no talk whatsoever about what personal protective equipment might be used when you uncover that equipment.

Weigel : I think it's much better than it was seven or eight years ago when we first started talking about this hazard. Awareness is extremely critical. Although more are aware now than in past years, I still think there are a lot of people who don't understand what is required in the compliance program and what the standards are.

Floyd : A number of organizations and individuals who are currently paying attention to arc flash protection, I feel are primarily looking at personal protective clothing and personal protective equipment, and may not be addressing a full set of control measures that include items that either eliminate or reduce the hazard by design.

Smith : How do engineers address the 2005 NEC requirements for selectivity and the need for safety, reducing incident energy, etc. regarding arc flash? Will the NEC council amend or revise these requirements for selectivity?

Fox : Trying to get selectivity in your system, unfortunately is at opposite goals to trying to achieve arc flash energy (reduction). With conventional trip devices and circuit breakers, you either have to make them less sensitive to the current or add time delay to the current in order to make them selective. There are situations where you can apply advanced protection, particularly in switchgear. There is switchgear available with zone-based protective schemes, or you can apply bus differential or zone interlocking. Hopefully, these will help us get around the selectivity versus arc flash issue.

Weigel : Selective coordination is not a mutually-exclusive goal with arc flash protection. A critical piece of it is the designer's selection of the overcurrent protective device that will best suit the conditions. Our engineers tell me when they are doing arc flash studies; they find that they can significantly reduce the incident energy or the arcing energy with a circuit breaker if it has an instantaneous trip unit function, by turning down the trip unit instantaneous. However, you cannot arbitrarily turn it down to minimum, because you cause nuisance tripping and you lose selective coordination.

Floyd : I think one of the really exciting things that has occurred in this area of arc flash mitigation just in the last five years or so has been the technology innovations that almost all of the major manufacturers of electrical switchgear and motor control protection systems have brought to market.

This Webcast and many others are available at



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