The most basic overcurrent protective element on the system is a fuse. Fuses are relatively inexpensive and maintenance-free. For those reasons, they are generally used in large numbers on most utility distribution systems to protect individual transformers and feeder branches
(sometimes called laterals or lateral branches).
Figure shows a typical overhead line fused cutout. The fundamental purpose of fuses is to operate on permanent faults and isolate (sectionalize) the faulted section from the sound portion of the feeder. They are positioned so that the smallest practical section of the feeder is disturbed. Fuses detect overcurrent by melting the fuse element, which generally is made of a metal such as tin or silver. This initiates some sort of arcing action that will lead to the interruption of the current. There are two basic kinds of fuse technologies used in power systems:
1. Expulsion fuses
2. Current-limiting fuses
The essential difference between the two is the way the arc is quenched. This also gives the fuses different power quality characteristics. An explusion fuse creates an arc inside a tube with an ablative coating. This creates high-pressure gases that expel the arc plasma and fuse remnants out the bottom of the cutout, often with a loud report similar to a firearm. This cools the arc such that it will not reignite after the alternating current naturally goes through zero. This can be
as short as one-half cycle for high currents to several cycles for low fault currents. This determines the duration of the voltage sag observed at loads. An expulsion fuse is considerably less expensive than a currentlimiting fuse.
A current-limiting fuse dissipates the energy in the arc in a closed environment, typically by melting a special sand within an insulating tube. This process actually quenches the arc very quickly, forcing the
current to zero before that would naturally occur. This can have somebeneficial impacts on the voltage sag characteristics.
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(sometimes called laterals or lateral branches).
Figure shows a typical overhead line fused cutout. The fundamental purpose of fuses is to operate on permanent faults and isolate (sectionalize) the faulted section from the sound portion of the feeder. They are positioned so that the smallest practical section of the feeder is disturbed. Fuses detect overcurrent by melting the fuse element, which generally is made of a metal such as tin or silver. This initiates some sort of arcing action that will lead to the interruption of the current. There are two basic kinds of fuse technologies used in power systems:
1. Expulsion fuses
2. Current-limiting fuses
The essential difference between the two is the way the arc is quenched. This also gives the fuses different power quality characteristics. An explusion fuse creates an arc inside a tube with an ablative coating. This creates high-pressure gases that expel the arc plasma and fuse remnants out the bottom of the cutout, often with a loud report similar to a firearm. This cools the arc such that it will not reignite after the alternating current naturally goes through zero. This can be
as short as one-half cycle for high currents to several cycles for low fault currents. This determines the duration of the voltage sag observed at loads. An expulsion fuse is considerably less expensive than a currentlimiting fuse.
A current-limiting fuse dissipates the energy in the arc in a closed environment, typically by melting a special sand within an insulating tube. This process actually quenches the arc very quickly, forcing the
current to zero before that would naturally occur. This can have somebeneficial impacts on the voltage sag characteristics.
THE CHIEF REQUIREMENT OF GOOD EARTHING IS LOW SOIL RESISTIVITY.
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