Design And Construction Of Blown Fuse Indicator

This work is on a blown fuse indicator which comprising of a voltage responsive solid state switching circuit responsive to a predetermined voltage for rendering a solid state switch conducting and thereby energizing a signaling lamp when a fuse is in its open state. The conductors of the blown fuse indicator which are connected at opposite terminals of the fuse are provided with current limiting impedances immediately adjacent the fuse connections to limit current in the conductors as well as limiting the current in the blown fuse indicator circuit.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

1.0      INTRODUCTION

1.1      BACKGROUND OF THE PROJECT

• • PURPOSE OF THE PROJECT

• • AIM OF THE PROJECT

• • OBJECTIVE OF THE PROJECT

• • PURPOSE OF THE PROJECT

• • LIMITATION OF THE PROJECT

• • ADVANTAGES OF THE PROJECT

• • PROBLEM OF THE PROJECT

• • APPLICATION OF THE PROJECT

• • RESEARCH QUESTION

• • PROJECT ORGANISATION

CHAPTER TWO

LITERATURE REVIEW

• • OVERVIEW OF THE STUDY

• • REVIEW OF THE RELATED STUDY

CHAPTER THREE

3.0     CONSTRUCTION METHODOLOGY

3.1      BASIC OF THE SYSTEM

3.2     BLOCK DIAGRAM OF THE SYSTEM

3.3      SYSTEM OPERATION

3.4      CIRCUIT DIAGRAM

3.5      CIRCUIT DESCRIPTION

3.6     DESCRIPTION OF COMPONENTS USED

3.7      POWER SUPPLY UNIT

CHAPTER FOUR

4.0      TESTING AND RESULTS

• • CONSTRUCTION PROCEDURE AND TESTING

• • INSTALLATION OF THE COMPLET DESIGN

• • ASSEMBLING OF SECTIONS

• • TESTING OF SYSTEM OPERATION

• • COST ANALYSIS

CHAPTER FIVE

• • CONCLUSION

• • RECOMMENDATION

• • REFERENCES

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A solid state switching circuit for energizing a lamp (L) is connected around a fuse (12) across an AC supply for detecting a voltage differential at the opposite sides of a blown fuse and applying that differential across the control (g) and switching (c) electrodes of the solid state switch (SCR) to render the same conductive. Current limiting resistors (RL1 and RL2) are placed in the ends of wire conductors which are to be connected at opposite sides of the fuse, thereby to limit current flowing in the conductors over the entire length of the conductor, minimizing electrical shock hazard upon damage to the conductor.

BACKGROUND OF THE INVENTION

This invention relates to signaling apparatus for indicating an open circuit condition of an electric circuit interrupter such as a blown electric fuse. More particularly, this invention relates to signaling apparatus of the aforementioned type which is used on enclosed fusible electrical apparatus and more specifically, on multiphase enclosed fusible electrical apparatus.

It is known in the art to provide a signaling device such as a lamp in series circuit with a semiconductor switch across an electrical source ahead of a fuse and to connect a control circuit for the semiconductor switch across the same source on the load side of the fuse. When the fuse blows, current flow through a control electrode and one switching electrode of the semiconductor switch renders that switch conductive for energizing the lamp to indicate the open condition of the fuse.

It is also known to provide enclosed fusible electrical apparatus with one or more low current neon lamps mounted on or through a wall of the enclosure and to provide each of these lamps with an external, high ohmic resistance for limiting current through the lamp. The lamp and external resistance are connected in parallel with the fuse and a shunting resistor is connected between opposite terminals of fuses in different phases of the protected apparatus. When a fuse blows, a circuit is completed through the shunting resistor and the respective series resistor with the lamp to energize the lamp.

The known circuits described above operate in response to detected current flow. Additionally, none of the known circuits address a potentially hazardous situation created when unprotected conductors of the indicator circuit are connected at the opposite terminals of the fuse, and particularly the conductor connected to the supply side of the fuse. If this conductor should become damaged and come into contact with a conductive portion of the apparatus, or if it is pinched in the enclosure upon closing the doors or cover thereof, the apparatus or the enclosure will be directly shorted to the supply current with no protective means for interrupting that short circuit.

SUMMARY OF THE INVENTION

The present invention provides a blown fuse indicator comprising a voltage responsive solid state switching circuit responsive to a predetermined voltage for rendering a solid state switch conducting and thereby energizing a signaling lamp when a fuse is in its open state. The conductors of the blown fuse indicator which are connected at opposite terminals of the fuse are provided with current limiting impedances immediately adjacent the fuse connections to limit current in the conductors as well as limiting the current in the blown fuse indicator circuit.

A blown fuse indicator for enclosed fusible electrical apparatus comprising, in combination:

a power supply source;

an enclosure containing electrical apparatus and at least one fuse, said fuse being connected in circuit with said power supply source;

indicator lamp means mounted on said enclosure remote from said fuse comprising an individual lamp visible to the exterior of said enclosure for each fuse, and solid state switching means for each lamp responsive to a voltage differential at opposite sides of a blown fuse for effecting energization of a respective lamp; and

means for providing said voltage differential comprising:

pairs of wire conductors for each fuse, said conductors connected at one end to said lamp means and having a second end extending within said enclosure to opposite sides of a respective fuse, and

an impedance connected to each said conductor at said second end thereof and to a respective opposite said of said fuse immediately adjacent said fuse, said impedance limiting current in said conductor to a value safe for human contact.

8. 8. The blown fuse indicator defined in claim 7 wherein said solid state switching means comprises:

a solid state switch having a pair of switching terminals and a control terminal, said switching terminals being connected in circuit with said lamp across said source; and

said means for providing said voltage differential at opposite sides of a blown fuse comprises a voltage divider connected across said source, said voltage divider comprising two parts with a junction therebetween connected to said control terminal, one of said parts comprising two parallel branches, and each branch comprising a respective one of said current limiting impedances.