Building Electrification : unit - Protection .

 

 Building Electrification

 Unit- Protection

Question:1.What is fue?

✓✓ Fuse: Fuse is a simple device for the protection of an electric circuit from excessive current. The fuse-wire is a thin wire of a material having low melting point, which, when a fault occurs, melts and breaks the circuit.

Question:2.Which type of material is used as a fuse material?

 ✓✓ Fuse material: The materials used for a fuse wire are tin, lead, zinc, silver, antimony, copper, aluminium etc. Lead-tin alloy may be used for current rating upto 10 Amps. Above that tinned copper fuse wires are used.

Note:  fuse is to be provided only in Phase or Live wire and never on the Neutral wire.

Question: 3.Describe a fuse unit.

✓✓ Fuse is a simple device for the protection of an electric circuit from excessive current. The fuse-wire is a thin wire of a material having low melting point, which, when a fault occurs, melts and breaks the circuit.

A fuse generally consists of the following elements:

(a) Fuse Link: It is the fuse element or cartridge,

(b) Fuse carrier: It carries the fuse link. (c) Carrier Contact: It is the removable holder which engages with fixed contacts.

(d) Fuse Base: It carries the fixed contacts. (e) Fixed Contacts: These are connected to fixed terminals.

Question: 4.Classify fuse unit.

✓✓The various types of fuse units commonly used are :

(i)                 Rewireable or kit-kat fuse unit (semi-enclosed)

(ii) Cartridge fuse unit (enclosed)

(iii)              H.R.C. (High Rupturing Capacity) fuse unit (enclosed)

Note: Except above there are other two types fuse unit also available in market but not in use generally.

i)                    Round type fuse unit:

ii)                  Semiconductor fuse unit:

 Kit-Kat Fuse: It consists of a porcelain base carrying the fixed contacts and a porcelain carrier holding the fuse element. These are used in domestic installations and in motor installations of Rating:  voltage rating upto 500 volts and current rating upto 300 Amps. This is also called rewireable fuse.

[[Extra note:

The fuse wire may be of lead, tinned copper, aluminiumor an alloy of tin-lead.

 àThe actual fusing current will be about twice (2 times)  the rated current.

When two or more fuse wires are used, the wires should be kept apart and a derating factor of 07 to 0.8 should be used to arrive at the total fuse rating

The specifications for rewirable fuses are covered by IS 2086-1963-

 Standard ratings are

 6, 16, 32, 63 and 100 A. A fuse wire of any rating not exceeding the rating of the fuse may be used in it ie, a 80 A fuse wire can be used in a 100 A fuse, but not in the 63 A fuse.]]

 Cartridge Fuse: It consists of a ceramic container, sealed at both ends with metallic caps. The fuse-element is enclosed within the container, which

is filled with sand, calcium carbonate etc. Cartridge fuses are to be used where there is risks of fire-hazards. This fuse is more reliable than the rewireable fuse.

Rating: This type of fuse is available up to 660 volt and the current rating up to 800 ampere.

[[Extra note:

There are various types of materials used as filler like sand, calcium carbonate, quartz etc. There is sometimes a blow out device in the side of the tube to indicate when the fuse blown . On overloads or short circuits fusible element is heated to  a high temperature causing it to vaporize. The  powder in the fuse cartridge cools and condenses the vapour and quinches the arc there by interrupting the flow of current. It provides complete security against fire risk as it is a totally enclosed unit.]]

 

 H.R.C. Cartridge Fuse: It consists of a strong glass porcelain tube sealed at both ends with brass cap to which are welded silver fuse element. The space within the container is filled with a filling powder which quenches the arc.

Under normal conditions the fuse elements are at a temperature below their melting point. When the current exceeds a certain limit, the fusible element melts and vapourises. Chemical reaction between the vapour and the filling powder results in the formation of a high resistance substance which becomes an insulator and the current is cut off.

✓ The fusing current of a cartridge fuse is about 1.5 times the rating of the fuse. ✓ Once blown the cartridge fuse is to be replaced by a new one.

Filler material : Carbon tetrachloride solutions, calcium carbonate etc.

✓ Rating: These fuses can be used for rupturing capacity as high as 500 MVA and voltages upto 66 kV and even more.

Advantages and disadvantages:

(i)                 They are cheaper as compared with other types of circuit interrupters of sam breaking capacity

(ii)                No maintenance is required.

(iii)               The operation is quick and sure

(iv)               They do not deteriorate with time.

(v)                 They have inverse time-current characteristic

(vi)              They are capable of clearing high as well as low currents.

(vii)             They are quite reliable and can be selected for proper discrimination

HRC fuses suffer from the following disadvantages:

(i)                 They are required to be replaced after each operation.

(ii)                Interlocking is not possible.

(iii)              They lack relays in complete discrimination

Types: (i) Cartridge type HRC fuse (ii) Tetra Chloride type HRC fuse.

Question 5. : Write down the full form and function of the followings- HRC fuse, MCB, MCCB, RCCB, ELCB, LCCB.

✓✓ HRC- High  Rupturing Capacity fuse. Operates on over current and short current.

✓✓ MCB-Miniature Circuit Breaker. Operates on over current and short current. Thermal trip operates for normal overload ;magnetic trip operates on short circuits.

✓✓RCCB-Residual Current Circuit Breaker

✓✓ELCB-  Earth Leakage Current circuit Breaker

✓✓LCCB-Leakage Current circuit Breaker.

Last three operate for leakage current or for earth leakage current.

Question:6. What are the factors for determination the size of fuse wire in an installation?

✓✓

àThe  factors responsible for determining the size of fuse wire in an installation are.

 

(i)                 Maximum current rating of the circuit and

(ii)                 (ii) Current rating of the smallest cable in the circuit protected by the fuse.

 

Question: 6. On what factors does the fusing current depend

✓✓ The fusing current depends upon ) diameter of wire (ii) length of wire (iii) fuse material (iv) X-sectional area (round or rectangular) (v)size and location of terminals (vi) type of enclosure used (vii) type of surface etc.