Safety
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ELECTRICAL PROTECTIVE DEVICES
ELECTRICAL PROTECTIVE DEVICES
ELECTRICAL PROTECTIVE DEVICES
In an electrical power station, when any thing becomes abnormal, it becomes predetermined time delay.
The action may be automatic and selective. The function can be fulfilled by using the protective devices like fuses or relays in the circuit from the damage to be occurred due to short circuit or overloads etc.
FUSE:
Fuse is a device used for protecting the cable in a circuit agiant damage from an excessive current.
FUSE ELEMENT OF FUSE WIRE:
It is the part of the fuse, which melts when an excessive current flows in the circuit , and thus isolates the device from the supply mains.
MAIN FEATURES OF GOOD PROTECTIVE DEVICES:
(i) Sensitivity. The protective system should be sensitive that it should operate for low values of fault current.
(ii) Selectivity. The protective system should select correctly the faulty part of the power system and disconnect the same without disturbing the rest of the system.
(iii) Reliability. The protective system should operate definitely under predetermined condition.
(iv) Quickness. The protective system should be such that it should respond quickly in order to improve quality of service, increase safety & life of equipment and increase stability.
FUSE:
Fuse is a wire of short length or thin strip of material having low melting point and is insertede in an electric circuit as protective device to the flow of an excessive current through the circuit (i.e current greater than the maximum current allowed to flow through the circuit in normal condition).
MATERIAL:
Tin, lead zinc and allow of lead and tin, silver copper, copper, aluminum etc.
material can be used for fuse wire.
WORKING:
Under normal working conditons the current flowing through the circuit is within safe limits but when some faults, such as short circuit occurs or when load more than limiting value the wire gets heated up, melts and breaks the circuit.
It thus protects a machine or apparatus from being damaged due to excessive current.
TYPES OF FUSE:
Following are the types of fuse, such as:
1. Kit-cat type or Rewirable fuse
2. Cartridge type fuse
3. H.R.C (High Ruptuting Capacity) fuse.
KIT-CAT TYPE OR REWIRABLE FUSE:
This is commonly used in house wiring and power wiring up to 440 volts. It is made of porcelain in two parts i.e base and fuse carrier.
The base is fixed with switchboard. The phase or incoming wire is connected to its one terminal. The connection is taken from the other terminal is known as outgoing terminal. The two fuse wire can easily be easily be replaced if fuse blown out.
PARTS OF REWIRABLE FUSE:
1. Fuse carrier
2. Fuse carrier contact
3. Fuse element
4. Asbestos tube
5. Fuse base contact
6. Fuse base
ADVANTAGES OF REWIREABLE FUSE:
1. It is the cheapest from of fuse.
2. It requires minimum time to replace.
3. It require no maintenance
4. It has current limiting effect.
DISADVANTAGES:
Rewireable fuses suffer from the following disadvanatages:
1. Oxidation of fuse wire and consequent thinning of wire section with lapse of time.
2. Loose connection causes the local heating.
3. Heating radiating devices used in the circuit.
4. Single phasing of three phase induction motors when one of the fuses is blown off.
CARTRIDGE TYPE FUSE:
This is usually enclosed type fuse. The fuse element or wire is enclosed in an
insulating container of tube shape.
This is filled with power and sealed at its ends with metallic caps known as cartridge. It is not possible to rewire it.
It is available for voltage up to 660 volts and current rating 800 amperes. The powder provides good insulating path and helps to extinguish the arc at the time of blowing up of fuse. It is not very economical.
PARTS OF CARTDGE TYPE FUSE:
1. Fuse up
2. Cartige
3. Adaptor ring
4. Fuse base
H.R.C (HIGH RUPTURING CAPACITY) FUSE:
It modern power station, extermely heavy would flow due to the fault .
Therefore, HRC fuse units are used for such duties. Theses fuses are specially designed for extremely rapid operation.
PARTS OF H.R.C FUSE:
1. Filling powder
2. Bi-metal thermal control
3. Fuse element
4. Fuse link contact
5. Brass end cap
6. Cartridge
7. Outer element
It has ceramic body with metallic end caps. Fuse element is placed inside and welded to end caps.
Powder is filled in it, which helps in extinguish the fire.
ADVANTAGES OF H.R.C FUSE:
The H.R.C fuses posses the following advantages.
1. Simple and easy installation.
2. No maintenance except periodic tightening of connections.
3. High breaking capacity.
4. Low initial cost compared to circuit breaker.
5. Extreme reliability.
6. Non-deterioration for long periods.
7. Coordination and discrimination to a high degree.
DISADVANTAGES OF H.R.C FUSES:
1. After each operation, its replacement is required.
2. Interlocking is not possible in these types of fuses.
3. The lack relays incomplete discrimination.
SELECTION OF FUSE:
The following considerations may be kept in view while selecting a fuse.
1. There must be security agiant fire risk when the fuse blows.
2. there should be good insulaing path for the fuse.
3. Sufficient clearance between terminals.
4. It should provide easy replacement of fuse wire.
5. It should not operarte under conditons of mementary over loads and wiring switching surges which are experienced when starting a motor and switching on transformers, capacitors etc.
6. It must opearte when sustained overload or short-circuit occurs.
MINIATURE CIRCUIT BREKER:
The miniature circuit breaker, although a somewhat complicated piece of mechanism is gradually occupying the place of the fuse.
IT is being fitted in increasing numbres in distribution boards and consumer control units. Its action is based on the principle as fitted to control large industrial circuits, whereby a predetermined excess of current operates an electromagnet.
This in turn, through a switch mechanism, automatically permits the breaking current to be lower than an equivalently reated rewirable.
1. It can be made to give a close degree of small excess of current protection. This is indirect contrast to the rewireable fuse which may be wrongly wired with a heavier gauge fuse wire, so that a larger current will flow before the wire actually blows.
2. If necessary for certain purposes the mechanism may be fitted with a time delay action.
3. After a fault in the circuit has been cleared, the supply can very easily be restored by the simple toggle switch.
PARTS OF MINITURE CIRCUIT BRESKER:
1. Circuit breaker
2. Auxiliaty switch
3. Current transformers
4. Battery
5. Realay
6. Trip coil
COMPARISION OF H.R.C FUSE AND CIRCUIT BREAKER:
(i) H.R.C fuse posses higher rupturing capacity compared to circut breaker in the medium voltage range. Fuse with rupturing capacity of 75 MVA (breaking current app. 100 KA) are available breakers with corresponding capacity will be too bulky.
(ii) H.R.C fuses are cheaper than the circuit breakers.
(iii) H.R.C fuse take less space and are simpler to install and maintain.
(iv) Circuit breaker are however, more advantageous where by the nature of location, enviroment and duty, the the fuses are likely to blow quite often, as replacement cost of fuses is high.
(v) It takes some time to replace fuses but a circuit breaker can be quickly operated.
TERMS GENERALLY USED:
The following are the definition of a few terms mostly used in the study of fuse.
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