Fuse - Basic Functionality and Properties
How A Fuse Operates
The fuse is a reliable overcurrent protective device, primarily used as a circuit protection device for overcurrents, overloads and short-circuits. A "fusible" link or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse. Electrical resistance of the link is so low that it simply acts as a conductor. However, when destructive currents occur, the link very quickly melts and opens the circuit to protect conductors and other circuit components and loads. Fuse characteristics are stable. A fuse does not require periodic maintenance or testing. The fuse has three unique performance characteristics
- They are safe. The modern fuse has an extremely "high interrupting" rating-can withstand very high fault currents without rupturing.
- Properly applied, a fuse will prevent "blackouts." Only a fuse nearest a fault opens without upstream fuses (feeders or mains) being affected, thus, they provide "selective coordination." (These terms are precisely defined in subsequent pages.)
- A fuse provides optimum component protection by keeping fault currents to a low value... They are said to be "current limiting."
Circuit Protection
A fuse can offer protection of electrical circuits. Electrical distribution systems are often quite complicated. They cannot be absolutely fail-safe. Circuits are subject to destructive overcurrents. Harsh environments, general deterioration, accidental damage or damage from natural causes, excessive expansion or overloading of the electrical distribution system are factors which contribute to the occurrence of such overcurrents. Reliable fuses and other protective devices prevent or minimize costly damage to transformers, conductors, motors, and the other many components and loads that make up the complete distribution system. Reliable fuses and other circuit protection is essential to avoid the severe monetary losses which can result from power blackouts and prolonged downtime of facilities. It is the need for reliable profection, safety, and freedom from fire hazards that has made the fuse a widely used protective device.
Overcurrents
A fuse can provide protection in the event of overcurrents. An overcurrent is either an overload current or a short-circuit current. The overload current is an excessive current relative to normal operating current but one which is confined to the normal conductive paths provided by the conductors and other components and loads of the distribution system. As the name implies, a short-circuit current is one which flows outside the normal conducting paths.
This photograph illustrates the overcurrents on components when protective devices are not sized to the ampere rating of the component. Considerable damage to electrical equipment can result if the interrupting rating of a protective device is inadequate and is exceeded by a short-circuit current.
Overloads
A fuse can provide protection to circuits from overloads. Overloads are most often between one and six times the normal current level. Usually, they are caused by harmless temporary surge currents that occur when motors are started-up or transformers are energized. Such overload currents or transients are normal occurrences. Since they are of brief duration, any temperature rise is trivial and has no harmful effect on the circuit components. (It is important that protective devices do not react to them.)
Continuous overloads can result from defective motors (such as worn motor bearings), overloaded equipment, or too many loads on one circuit. Such sustained overloads are destructive and must be cut-off by protective devices before they damage the distribution system or system loads. However, since they are of relatively low magnitude compared to short-circuit currents, removal of the overload current within a few seconds will generally prevent equipment damage. A sustained overload current results in overheating of conductors arid other components and will cause deterioration of insulation which may eventually result in severe damage and short-circuits if not interrupted.
Short-Circuits
A fuse can offer protection to circuits from short-circuits. Whereas overload currents occur at rather modest levels, the short-circuit or fault current can be many hundreds of times larger than the normal operating current. A high level fault may be 50,000 Amps (or larger). If not cut off within a matter of a few thousands of a second, damage and destruction can become rampant-there can be severe insulation damage, melting of conductors, vaporization of metal, ionization of gases, arcing, and fires. Simultaneously, high level short-circuit currents can develop huge magnetic-field stresses. The magnetic forces between bus bars and other conductors can be many hundreds of pounds per lineal foot; even heavy bracing may not be adequate to keep them from being warped or distorted beyond repair.
Fuse Voltage Rating
Most low voltage power distribution fuses have 250 volt or 600 volt ratings (other ratings are 125 volts and 300 volts). The voltage rating of a fuse must be at least equal the circuit voltage. It can be higher but never lower. For instance, a 600 volt fuse can be used in a 208 volt circuit. The voltage rating of a fuse is a function of or depends upon its capability to open a circuit under an overcurrent condition. Specifically, the voltage rating determines the ability of the fuse to suppress the internal arcing that occurs after a fuse link melts and an arc is produced. If a fuse is used with avoltage rating lower than the circuit voltage, arc suppression will be impaired and, under some fault current conditions, the fuse may not safely clear the overcurrent.
Fuse Ampere Rating
Every fuse has a specific ampere rating. In selecting the ampere rating of a fuse, consideration must be given to the type of load and code requirements. The ampere rating of a fuse should normally not exceed current carrying capacity of the circuit. For instance, if a conductor is rated to carry 20 amperes, a 20 ampere fuse is the largest that should be used. However, there are some specific circumstances where the ampere rating is permitted to be greater than the current carrying capacity of the circuit. A typical example is the motor circuit; a dual-element fuse is generally permitted to be sized up to 175% and non-time delay fuses up to 300% of the motor full- load amperes. Generally, the ampere rating of a fuse and switch combination should be selected at 125% of the load current (this usually corresponds to the circuit capacity which is also selected at 125% of the load current). There are exceptions such as when the fuse-switch combination is approved for continuous operation at 100% of its rating.
The fuse is a universal protective device. The fuse is used in power distribution systems, electronic apparatus, vehicles . . . and as illustrated, our space program. The Space Shuttle has over 600 fuses installed in it protecting vital equipment and circuits
Interrupting Rating-Safe Operation
A protective device must be able to withstand the destructive energy of short-circuit currents. If a fault current exceeds a level beyond the capability of the protective device, the device may actually rupture, causing additional damage. Thus it is important in applying a fuse or circuit breaker to use one which can sustain the largest potential short-circuit currents. The rating which defines the capacity of a protective device to maintain its integrity when reacting to fault currents is termed its "interrupting rating."
The interrupting rating of most branch-circuit, molded case, circuit breakers typically used in residential service entrance panels is 10,000 amperes. The rating is usually expressed as "10,000 AIC" (AIC being the abbreviation of "amperes interrupting capacity.") Larger, more expensive circuit breakers may have AIC's of 14,000 amperes or higher. In contrast, most modern, current-limiting fuses have an interrupting capacity of 200,000 amperes and are commonly used to protect the lower rated circuit breakers. The National Electrical Code, Section 110-9, requires equipment intended to break current at fault levels to have an interrupting rating sufficient for the current that must be interrupted.
This diagram shows the minimum ratios of ampere ratings of a HI-CAP and LOW-PEAK fuse that's required to provide "selective coordination" (discrimination) of upstream and downstream fuses.
This burnt out panelboard represents the staggering monetary losses in equipment and facility downtime that can result from inadequate or deteriorated protective devices. It emphasizes the need for reliable protective devices that properly function without progressive deterioration with the passage of years.
Selective Coordination-Prevention of Blackouts
The coordination of protective devices prevents system power outages or blackouts caused by overcurrent conditions. When only the protective device nearest a faulted circuit opens and a larger up-stream fuse will remain closed, the protective devices are "selectively" coordinated (they discriminate). The word "selective" is used to denote total coordination... isolation of a faulted circuit by the opening of only the localized protective device.
Unlike electro-mechanical inertial devices (circuit breakers) it is a simple matter to selectively coordinate a fuse of modern design. By maintaining a minimum ratio of fuse-ampere ratings between an upstream and downstream fuse, selective coordination is assured. Generally, it is not necessary to plot time-current curves. Adherence to the tabulated selectivity ratios normally proves quite adequate.
Current Limitation-Component Protection
If a protective device cuts off a short-circuit current in less than one-half cycle, before it reaches its total available (and highly destructive) value, the device is a "current limiting" device. The most modern fuse is current limiting. A fuse will restrict fault currents to such low values that a high degree of protection is given to circuit components against even very high short-circuit currents. They permit breakers with lower interrupting ratings to be used. They can reduce bracing of bus structures. They minimize the need of other components to have high short-circuit current "withstand" ratings. If not limited, short-circuit currents can reach levels of 30,000 or 40,000 amperes or higher in the first half cycle (.008 seconds, 60 hz) after the start of a short-circuit. The heat that can be produced in circuit components by the immense energy of short-circuit currents can cause severe insulation damage or even explosion. At the same time, huge magnetic forces developed between conductors can crack insulators and distort and destroy bracing structures. Thus, it is important that a protective device limit fault currents before they can reach their full potential level.
A non-current limiting protective device by permitting a short-circuit current to build up to its full value, can let an immense amount of destructive short-circuit heat energy thru before opening the circuit.
A current limiting fuse has such a high speed of response that it cuts off a short-circuit long before it can build up to its full peak value.