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Fuses

Definition Fuses are used to protect circuits against abnormal currents such as overloads or short circuits.

Classification.
1. Low Voltage Fuses. - usually rated less than 600 VAC (voltage class), amperage, interrupting rating and current-limiting. Interrupting rating [IR] is the highest rms symmetrical AC which the fuse can interrupt at rated voltage. These fuses have a minimum IR of about 10,000 amperes and a maximum of 100,000 A DC, or 200,000 A AC.

These fuses are further classified into:

Class	Voltage	AC Amp.
G	300	0 - 60
H	250	0 - 600
K, J	600	0 - 600
L	600	600 - 6000
S	600	0 - 30
CC	600	0 - 20

Types.
a. Cartridge Fuses - composed of a strong fiber casing inside of which is enclosed a fuse wire, generally an alloy of lead. The fuse wire is fastened to copper caps which are fastened to each end of the casing. They are available for circuits up to about 25,000 volts. They are used as a protective device in low-capacity circuits, such as small lighting and power lines, and on the secondary of instrument-potential transformers, when used for metering or relay protection.
b. Knife Blade
c. Bolted
d. Plug

Guide. AC fuses should not be used for DC applications

2. High Voltage Fuses. - used by electric utilities to protect distribution class equipment and by large industrial complexes which have their own electrical distribution systems.

3. Cut-outs also known as transformer fuse-block, which include a porcelain housing, enclosing the fuse and the contact points. One type, the fuse is carried on a plug that is provided with an insulating porcelain knob in order that it maybe removed for re-fusing. A second type consists of a rectangular porcelain receptacle with a removable front door that carries the fuse.
a. Open - rated 20 kA asymmetrical maximum interrupting current at 5 to 35 kV. Maximum continuous current rating is at 200 A.

b. Enclosed - rated 8 to 10 kA asymmetrical maximum interrupting current

c. Open link - rated 1200 A asymmetrical maximum interrupting current with 50 A maximum carrying current rating. Usually used on rural lines and small transformers.

4. Expulsion Types are for higher voltages such as found in power circuits or main feeders. This device consists of a hollow tube, made of some heat-resisting substance such as fiber with a lining of asbestos or some other material, through which is passed a fuse wire. One end of the tube is closed and connected to the line; the other end is opened and allows the fuse wire to project out and connect to the other terminal. When a short circuit or overload occurs, the fuse is blown out and a certain amount of gases form inside the tube. These gases in escaping to the air cause the arc, which is generally produced, to be blown out.

5. Current Limiting Fuses [CLF]. - usually used on distribution systems. These are rated by a) continuous current rating, the maximum current that the fuse is designed to carry continuously, and peak-arc voltage, the maximum voltage generated by the CLF. Guide: Fuses should not be applied to circuits with a voltage less than 50% of the fuse-voltage rating to avoid excessive peak arc voltage.

Classes.
a. General Purpose CLF. - will interrupt currents which will melt the fusible element in one (1) hour.

b. Backup CLF - have a definite minimum interrupting rating usually specified by the manufacturer.

6. Power Fuses. - usually used on power system grids, nominally rated at 20 kA interrupting current.

7. Oil-immersed link. - rated at 12.5 kA interrupting current when used with SF6, and 8 to 10 kA using other liquids.

RULES ON FUSES.

1. High voltage fuses should be mounted in enclosures. Especially the following:
a. Industrial service entrance switchgear
b. Pad-mounted switchgear and transformers for underground circuits.
c. Sub-surface applications.

2. Power fuses should be fitted with mufflers to reduce the intensity of exhaust the ingress of fluid when applied under oil such as in transformers.

3. Fuse cut-outs are not recommended for use in enclosures.

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