Electrical Fuse

Electrical fuse • On what principle does the fuse operate ? the action of a fuse is based upon the heating

effect of the electrical current. During overloads or short circuits the fuse gets heated up to melting point due to excessive current and breaks the circuit

Electrical Fuse• Fuse : Fuse is a current interrupting device which

breaks or opens the circuit (in which it is inserted) by fusing the element in the circuit when current

exceed a certain value• Current rating : it is defined the r.m.s value of

current which the fuse wire can carry continuously without deterioration and with temperature rise within specified limit

• Fusing current : it is defined as the minimum value of current at which the fuse element or fuse wire melts. its value will be more than the current rating of the element

Electrical Fuse• Fusing factor : The ratio of fusing current and the

current rating of fuse element is known as fusing factor

• The smaller the fusing factor , the greater is the difficulty in avoiding deterioration due to overheating and oxidation at rated carrying current

• For semi enclosed or rewirable fuse which employs copper wire as the fuse element, fusing factor is equal to 1.9 to 2.0

• For the standard duty cartridge fuses fusing factor is equal to 1.45

Electrical Fuse• Fusing current depends upon a various factor such as - Types of material used - The cross sectional area ( round or rectangular section )- Length ( shorter the fuse the greater the current fuse

of smaller length heat is easily conducted away)- Diameter of wire- Types of enclosure employed • Size and location of the Terminals • The approximate value fusing current of a round wire is I=k √d³ Where k= constant depending upon the metal

of the wire d= diameter of the wire

Electrical Fuse• Breaking Capacity : The breaking capacity is the

maximum current that can be safely interrupted by the fuse. Some fuse are designated High Rupture capacity (HRC)

• Voltage Rating : as well as a current rating , fuses also carry voltage rating indicating the maximum circuit voltage in which the fuse can be used

Fuse carrying a 250 volt rating may be safely used in 125 Volt circuit, but reverse is not true as the fuse may not be capable of safely interrupting the arc in a circuit of higher voltage.

Power system Protection (Fuse)

Circuit Breakers V.S. RelaysRelays are like human brain; circuit breakers are like human muscle.Relays ‘make decisions’ based on settings. Relays send signals to circuit breakers. Based the sending signals circuit breakers will open/close.

Power system Protection (Fuse)Fuses V.S. Relays

• Relays have different settings and can be set based on protection requirements.

• Relays can be reset.• Fuses only have one specific characteristic for

a individual type.• Fuses cannot be reset but replaced if they

blow.

HRC Fuse

HRC Fuse Multiple segment Type

HRC Fuse• Fuse Action• Heating/melting (pre arc)• Melting/ vaporising (arcing)• Silica (quartz sand)• Cools the arc• React with molten silver, to form a glass like

material which permanently open circuit.• HRC Fuse ( multiple segment type ) used for high

current rating ,internal element consist of multiple notches or even multiple elements in parallel. For high fault currents , all notches melt and arc simultaneously.

HRC Fuse• Advantages of HRC fuses- Cheap compared with other types of circuit

interrupters of same breaking capacity- simple and easy installation- No maintenance is required - High breaking capacity- Do not deteriorate with time - Consistent in performance Disadvantages of HRC fuses- Time is lost in replacing fuse after each operation- Interlocking is not possible- It causes overheating of adjacent contacts

Fuse time current characteristic

Fuse characteristic• For a fuse to satisfactorily protect a cable, its

characteristic must match, as closely as possible, the heating characteristic of the cable.

• This means that fuses have an inverse time characteristic, i.e. the larger the over current, the faster the blowing time of the fuse.

• Fuse characteristics are drawn on log/log scale as this enables a wide range of currents along with a wide range of time intervals to be charted

Fuse characteristic discrimination

Fuse characteristic discrimination

• In a correctly designed installation, in the event of a fault, the fuse nearest to the fault should interrupt the circuit before any other device has a chance of interrupting it. This is known as discrimination.

Electrical Fuse

Electrical Fuse

Electrical fuse

Electrical Fuse• The total let through energy is proportional to

the energy dissipation during the pre-arcing and arcing intervals and is referred to as the I2t characteristic of the fuse/protective device.

Electrical Fuse • A fuse and its type should be matched to the

device I2t let through – motors, conductors (cables) and semi-conductor devices all have

different I2t Value. e.g Semi-conductor requires a “fast” fuse type

Electrical Fuse Selection Guide• The fuse must carry the normal load current of the circuit without

nuisance openings. However, when an over current occurs the fuse must interrupt the over current, limit the energy let-through, and withstand the voltage across the fuse during arcing. To properly select a fuse the followings must be considered:

• Normal operating current (The current rating of a fuse is typically derated 25% for operation at 25C to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25C ambient.)

• Overload current and time interval in which the fuse must open. • Application voltage (AC or DC Voltage). • Inrush currents, surge currents, pulses, start-up currents

characteristics. • Ambient temperature. • Applicable standards agency required, such as UL, CSA, VDE. • Considerations: Reduce installation cost, ease of removal,

mounting type/form factor, etc.

Basic types of Electrical Fuse • Fuse Type There are three basic types of fuses:

(1) Slow Blow/Time Lag/ Time Delay fuses(2) Fast acting fuses(3) Very fast acting fuses A major type of Time Delay fuse is the dual-element fuse.

This fuse consists of a short circuit strip, soldered joint and spring connection. During overload conditions, the soldered joint gets hot enough to melt and the spring shears the junction loose. Under short circuit conditions, the short circuit element operates to open the circuit.

Electrical Fuse • Slow-blow fuse allows temporary and harmless inrush

currents to pass without opening, but is so designed to open on sustained overloads and short circuits. Slow-blow fuses are ideal for circuits with a transient surge or power-on inrush. These circuits include: motors, transformers, incandescent lamps and capacitate loads.

• This inrush may be many times the circuit's full load amperes. Slow-blow fuses allow close rating of the fuse without nuisance opening. Typically, Slow Blow fuses are rated between 125% to 150% of the circuit's full load amperes.

Electrical Fuse