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Protective devices.

Nov 30, 1995 12:00 PM

Included in this category are fuses and fuseholders, LV and MV breakers, relaying equipment, line monitoring equipment, GFCI and GFEP interrupters, and neutral ground resistors.

Minimizing damage to electrical equipment and prevention of injury to personnel are the principal reasons for applying protective devices. As such, the rating of the overcurrent protective device must be adequate for the system which is being protected. These ratings include voltage, frequency, continuous current, fault-interrupting or withstand capacity, and operating or ambient conditions. The development of circuit breakers and fuses of higher ratings has permitted their application on the large systems occurring today.

System protection and coordination

System protection, according to IEEE, is "the detection and prompt isolation of the affected portion of the system whenever a short circuit or other abnormality occurs that might cause damage to, or adversely affect, the operation of any portion of the system or the load it supplies." System coordination, also according to IEEE, is the "selection or setting, or both, of protective devices so as to isolate that portion of the system where the abnormality occurs."

These two functions are the key to a properly designed electrical system and should be considered along with other particular features of the system. The correct way to approach this task is to examine system protection at each stage of design planning, and include into the final system design an integrated protection plan that can be easily coordinated and provide flexibility for future growth.

Circuit breakers

Low-voltage circuit breakers are divided into three main categories: thermal-magnetic or solid-state trip molded case; solid-state trip enclosed (hybrid); and power air circuit breakers. Molded-case CBs range from residential 15A single-pole types to 1200A and larger 3-pole thermal-magnetic trip CBs, with solid-state trips on larger units up to 4000A. Voltage ratings range from 120V to 600V, and interrupting capacities from 5kA to 200kA.

Solid-state trip units provide increased accuracy and protection. Larger enclosed or plastic-cased breakers are hybrids between molded-case and power air breakers, and are available as drawout units with most power switchgear options. Available accessories include shunt trips, remote operation, and auxiliary and alarm switching contacts.

For high fault-current applications, molded-case CBs are available with integral current limiters (special fuses), or as unfused true current-limiting devices. For motor circuits, instantaneous magnetic-only CBs with no thermal overload are available and are used only in a combination motor starter, which has the thermal overload relay for overload protection.

Fuses

Low-voltage fuses can be divided into two categories: 0 to 600A with many types and UL classes available; and 601 to 6000A, available in only one standard UL class. Most are current limiting, with interrupting ratings of 200kA.

Class H fuses are one-time cartridge types with renewable or nonrenewable fusible links, limited in application by their interrupting rating of only 10kA. For higher interrupting ratings, Class K fuses are current limiting and physically interchangeable with Class H fuses. Class R fuses are similar to the Class K except that they have a rejection means to prevent insertion of a non-current-limiting fuse into the fuseholder. Other current-limiting fuses with special dimensions are the Class J, T, and G types.

Medium-voltage fuses are expulsion or current-limiting types. The expulsion type is available in a wider variety of ratings, is slower in operation, and in addition easier to coordinate. The current-limiting type is faster and emits no gases, but has lower maximum continuous and interrupting ratings. There is little standardization in this class of fuse.

For more detailed information on low-voltage overcurrent protective devices, see the July 1991 issue of EC&M, "Special Report: Circuit Protection."

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