page_204

< previous page | page_204 | next page >

Page 204

TABLE 11-1 Time-Current Values of Various Fuses

Position

Fault current amperes

Load current amperes

Protecting link

Protected link

D

390

14

15 k

25 k

C

600

23

25 k

40 k

B

875

35

40 k

65 k

Position

Maximum clearing time (protecting link)

Minimum melting time (protected link)

Percent CT/MT

D

0.061 sec.

0.084 sec.

72.6 (.061/.084)

C

0.666 sec.

0.093 sec.

71.0 (.066/.093)

B

0.078 sec.

0.120 sec.

65.0 (0.78/.120)

| under very heavy loads and must interrupt without damage high energy flow of power amounting to thousands of kilowatts, although only for a very short period of time. |

| The interruption of such large currents creates a great amount of heat that must be dissipated, and the blowing of a fuse can be compared to a violent explosion. The heat causes a rapid expansion of the surrounding air, and the gaseous metallic vapors created by the arc, if present for more than a few cycles, may destroy the fuse holders and nearby objects. That is why particular attention is given to coordinating points in the design of fuses and fuse holders. |

| When fuses are installed, they are usually regarded as part of a circuit and, with few exceptions, are given no further attention. The fuse, therefore, must not only be able to withstand extreme weather conditions and all kinds of varying loads but, by its very nature, must open the circuit satisfactorily whenever called upon. |

| There are three principal kinds of power fuses in general use: |

| 1. Open or link fuses |

| 2. Enclosed fuses |

| 3. Expulsion fuses. |

| Although there are many types of each kind, the ones described here are the most typical and illustrate the factors involved in the construction of the several kinds. |

| The fuse element is usually made of tin, lead, zinc, aluminum, or of silver, often alloyed with tin or lead. Its length and cross section generally determine the characteristics of the fuse. |

< previous page | page_204 | next page >