UNIT-V
ELECTRICAL INSTALLATIONS
Introduction:
Switchgear: The apparatus used for switching, controlling and protecting the electrical circuits and
equipment is known as switchgear. The term ‘switchgear’ is a generic term encompassing a wide range of
products like circuit breakers, switches, switch fuse units, off-load isolators, HRC fuses, contactors, earth
leakage circuit breaker, etc...
Classification of Switchgear: Switchgear can be classified on the basis of voltage level into the
following:
1. Low voltage (LV) Switchgear: up to 1KV
2. Medium voltage (MV) Switchgear: 3 KV to 33 KV
3. High voltage (HV) Switchgear: Above 33 KV
Components of LT Switchgear:
The term LT Switchgear includes low voltage Circuit Breakers, Switches, off load electrical
isolators, HRC fuses, Earth Leakage Circuit Breaker, Miniature Circuit Breakers (MCB) and Molded Case
Circuit Breakers (MCCB) etc i.e. all the accessories required to protect the LV system. The most common
use of LV switchgear is in LV distribution board.
FUSE: Fuse is perhaps the simplest and cheapest device used for interrupting an electrical circuit
under short circuit, or excessive overload, current magnitudes. The action of a fuse is based upon the heating
effect of the electric circuit. The fuse has inverse time-current characteristics as shown below.
Rated Current: The current that an electrical device can carry, under specified conditions, without resulting
in overheating or mechanical overstress
,Fusing current: It is the minimum current at which the fuse element melts and thus disconnects the circuit
protected by it. Obviously, its value will be more than the current rating of the fuse element
The part which actually melts and opens the circuit is known as the fuse element. Fuses have following
advantages and disadvantages:
Advantages:
1. It is cheapest form of protection available.
2. It needs no maintenance.
3. Its operation is inherently completely automatic unlike a circuit breaker which requires an elaborate
equipment for automatic action.
4. It interrupts enormous short circuit currents without noise, flame, gas or smoke.
Disadvantages:
1. Considerable time is lost in rewiring or replacing a fuse after operation.
2. On heavy short circuits, discrimination between fuses in series cannot be obtained unless there is
considerable differences in the relative sizes of the fuse concerned.
3. The current-time characteristics of a fuse cannot always be correlated with that of the protected device.
* Fuse is provided only in phase or live pole, never on neutral pole.
FUSE UNITS: The various types of fuse units, most commonly available are:
1. Round type fuse unit.
2. Kit-kat type fuse unit.
3. Cartridge type fuse unit.
4. HRC (High Rupturing Capacity) fuse units.
5. Semiconductor fuse units.
1. Round type fuse unit: This type of fuse unit consists of a porcelain or Bakelite box and two separated
wire terminals for holding the fuse wire between them. This type of fuse is not common use on account of
its following disadvantages:
1. One of the terminals remain always energized and, therefore, for replacement of fuse either the worker
will have to touch the live mains or open the main switch.
2. Appreciable arching takes place at the instant of blowing off fuse and thus damage the terminals. After
two or three arcing the fuse unit becomes unusable.
, 2. Rewirable or Kit-kat Type Fuses: The most commonly used fuse in “house wiring’ and small current
circuits is the semi-enclosed or rewirable fuse (also sometimes known as Kit-kat type fuse). It consists of a
porcelain base carrying the fixed contacts to which the incoming and outgoing live or phase wires are
connected and a porcelain fuse carrier holding the fuse element, consisting of one or more strands of fuse
wire, stretched between its terminals.
The fuse wire may be of lead, tinned copper, aluminum or an alloy of tin-lead. The actual fusing
current will be about twice the rated current.
The specifications for rewirable fuses are covered by IS: 2086-1963. Standard ratings are 6, 16, 32,
63 and 100A. A fuse wire of any rating not exceeding the rating of the fuse may be used in it i.e. a 80A fuse
wire can be used in a 100A fuse, but not in the 63A fuse.
Disadvantages of Rewirable or Kitkat Type Fuses:
• Unreliable operation.
• Lack of discrimination.
• Small time lag.
• Low rupturing capacity (the maximum power a circuit breaker can interrupt under a fault).
• No current limiting feature.
• Slow speed of operation.
3. Cartridge Type Fuses: This is a totally enclosed type fuse unit. It essentially consists of an insulating
container of bulb or tube shape and sealed at its ends with metallic cap known as cartridge enclosing the
fuse element and filled with powder or granular material known as filler. There are various types of materials
used as filler like sand, calcium carbonate, quartz etc. This type of fuse is available up to 660V and the
current rating up to 800 A.
ELECTRICAL INSTALLATIONS
Introduction:
Switchgear: The apparatus used for switching, controlling and protecting the electrical circuits and
equipment is known as switchgear. The term ‘switchgear’ is a generic term encompassing a wide range of
products like circuit breakers, switches, switch fuse units, off-load isolators, HRC fuses, contactors, earth
leakage circuit breaker, etc...
Classification of Switchgear: Switchgear can be classified on the basis of voltage level into the
following:
1. Low voltage (LV) Switchgear: up to 1KV
2. Medium voltage (MV) Switchgear: 3 KV to 33 KV
3. High voltage (HV) Switchgear: Above 33 KV
Components of LT Switchgear:
The term LT Switchgear includes low voltage Circuit Breakers, Switches, off load electrical
isolators, HRC fuses, Earth Leakage Circuit Breaker, Miniature Circuit Breakers (MCB) and Molded Case
Circuit Breakers (MCCB) etc i.e. all the accessories required to protect the LV system. The most common
use of LV switchgear is in LV distribution board.
FUSE: Fuse is perhaps the simplest and cheapest device used for interrupting an electrical circuit
under short circuit, or excessive overload, current magnitudes. The action of a fuse is based upon the heating
effect of the electric circuit. The fuse has inverse time-current characteristics as shown below.
Rated Current: The current that an electrical device can carry, under specified conditions, without resulting
in overheating or mechanical overstress
,Fusing current: It is the minimum current at which the fuse element melts and thus disconnects the circuit
protected by it. Obviously, its value will be more than the current rating of the fuse element
The part which actually melts and opens the circuit is known as the fuse element. Fuses have following
advantages and disadvantages:
Advantages:
1. It is cheapest form of protection available.
2. It needs no maintenance.
3. Its operation is inherently completely automatic unlike a circuit breaker which requires an elaborate
equipment for automatic action.
4. It interrupts enormous short circuit currents without noise, flame, gas or smoke.
Disadvantages:
1. Considerable time is lost in rewiring or replacing a fuse after operation.
2. On heavy short circuits, discrimination between fuses in series cannot be obtained unless there is
considerable differences in the relative sizes of the fuse concerned.
3. The current-time characteristics of a fuse cannot always be correlated with that of the protected device.
* Fuse is provided only in phase or live pole, never on neutral pole.
FUSE UNITS: The various types of fuse units, most commonly available are:
1. Round type fuse unit.
2. Kit-kat type fuse unit.
3. Cartridge type fuse unit.
4. HRC (High Rupturing Capacity) fuse units.
5. Semiconductor fuse units.
1. Round type fuse unit: This type of fuse unit consists of a porcelain or Bakelite box and two separated
wire terminals for holding the fuse wire between them. This type of fuse is not common use on account of
its following disadvantages:
1. One of the terminals remain always energized and, therefore, for replacement of fuse either the worker
will have to touch the live mains or open the main switch.
2. Appreciable arching takes place at the instant of blowing off fuse and thus damage the terminals. After
two or three arcing the fuse unit becomes unusable.
, 2. Rewirable or Kit-kat Type Fuses: The most commonly used fuse in “house wiring’ and small current
circuits is the semi-enclosed or rewirable fuse (also sometimes known as Kit-kat type fuse). It consists of a
porcelain base carrying the fixed contacts to which the incoming and outgoing live or phase wires are
connected and a porcelain fuse carrier holding the fuse element, consisting of one or more strands of fuse
wire, stretched between its terminals.
The fuse wire may be of lead, tinned copper, aluminum or an alloy of tin-lead. The actual fusing
current will be about twice the rated current.
The specifications for rewirable fuses are covered by IS: 2086-1963. Standard ratings are 6, 16, 32,
63 and 100A. A fuse wire of any rating not exceeding the rating of the fuse may be used in it i.e. a 80A fuse
wire can be used in a 100A fuse, but not in the 63A fuse.
Disadvantages of Rewirable or Kitkat Type Fuses:
• Unreliable operation.
• Lack of discrimination.
• Small time lag.
• Low rupturing capacity (the maximum power a circuit breaker can interrupt under a fault).
• No current limiting feature.
• Slow speed of operation.
3. Cartridge Type Fuses: This is a totally enclosed type fuse unit. It essentially consists of an insulating
container of bulb or tube shape and sealed at its ends with metallic cap known as cartridge enclosing the
fuse element and filled with powder or granular material known as filler. There are various types of materials
used as filler like sand, calcium carbonate, quartz etc. This type of fuse is available up to 660V and the
current rating up to 800 A.
