Consulting-Specifying Engineer - October 2007 - (Page 50)

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 The type of overcurrent protection used is dictated by the scenarios. The NEC has established the protection requirements. 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 Whether fuses or circuit breakers, the current-limiting range of an individual protective device falls between its threshold current and its interrupting rating. 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 ½ cycle and reduces the peak current. 50 Consulting-Specifying Engineer • OCTOBER, 2007

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Consulting-Specifying Engineer - October 2007 Contents On the Web In the News M/E Roundtable How To Beat the Energy Codes Professional Practices Codes & Standards A Capital Job on Engineered Buiding Systems Going Green in Data Centers Overcurrent Protection: Fuses or Breakers? Third-Party Liability Management Report New Products Product Spotlight Jobs/Classifieds Specifier's Notebook

Consulting-Specifying Engineer - October 2007

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