relays and fuses

7/29/2019 Relays and Fuses

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Protection of

power system


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objectives

Physical appearance

Definition

Working and construction How it protect power system?

Applications

Advantages and disadvantages Questioning about my topic


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Physical appearance


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What is fuse A fuse is a short piece of metal

inserted in a circuit, which meltswhen excessive current flowsthrough it and thus break thecircuit.

The fuse element is generally

made of materials having lowmelting point, high conductivityand least deterioration due to lowoxidation e.g, silver, copper etc

Under normal conditions the fuseelement is at a temperature belowits melting point.

When a short-circuit or overloadoccurs, the current through fuseincrease beyond its rating thismelt the fuse


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Protection of power system

Power system Protection-Basic Components:

Fuses: Self-destructing to save the equipment being protected.

Relays

earth screen

Bus-bars Circuit Breakers: These are used to make circuits carrying

enormous current, and also to break the circuit carrying the faultcurrents for a few cycle based on feedback from the relays.

DC batteries: These give uninterrupted power source to the relays andbreakers that is independent of the main power source being protected.

Lightning Arresters

Surge Absorber

Over head ground wires


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Working And ConstructionFunction:

A fuse is generally inserted into anelectrical circuit for 1 of 2 reasons,either to protect the power sourcewhich includes the wire that connectsthe power supply to the electricaldevice, or to protect the electronicequipment. The electronic equipment

manufacturers specify a fuse rated toopen the electrical circuit beforedamage can be done to the device oropen the circuit if the electronicdevice fails in some way (electronicdevices may pull excessive currentwhen they fail). If a fuse larger thanthe specified fuse is used, a smallmistake when installing theequipment may cause catastrophicfailure of the equipment.


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In the diagram below, you seethat there is a fuse betweenthe battery and the amplifier. Inthis configuration, the fuse canbe used to protect the wire andthe amplifier. If the fuse is the

proper one for the amplifier, allyou have to do is make surethat the wire segments 'A' and'B' are rated to pass morecurrent than the fuse and you'llbe OK. Wire segment 'A' mustbe as short as possiblebecause it is NOT protected bythe fuse.


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In the this diagram, thingsget a little morecomplicated. As you can

see, wire 'A' is used todeliver power to thedistribution block. Wire 'A' isof a large enough gauge topower both amplifiers. 4gauge wire is commonlyused as a main power wire.Fuse 'A' must be rated toprotect wire 'A'. Again, fuse'B' protects wire 'B' and fuse'C' protects wire 'C'. Wire

segments 'X' and 'Y' MUSTbe as short as possiblebecause, unless they are ofthe same gauge as wire 'A'(or larger), they could be afire hazard.


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In most cases, the wire size isreduced at the point of distribution.ANY time that the wire size isreduced, you must add a fuse in theline (at the point of distribution) toprotect the smaller wire. Look at the

following for more detailed infoabout changing wire sizes.

Wire "A" is unprotected which iswhy you want the main (125 amp)fuse as close to the battery aspossible. If this length of wire getsshorted, it WILL burn.

Wire "B" is protected by the 125amp fuse.


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Fuse Opening Time


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Protection of power systemAdvantages

It is the cheapest form of protection available. It requires no maintenance.

Operation is completely automatic.

It can break heavy short-circuit currentswithout noise or smoke.

The inverse time-current characteristic of a

fuse make it suitable for overcurrent protection.

Time of operation shorter than circuit breakers


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Disadvantages

Time is lost in rewiring or replacing a fuse

after operation.

On heavy short-circuits discrimination

between fuses in series can not be

obtained.

The current-time characteristic of fuse can

not always be co-related with that of

protected apparatus.


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Important terms

Fusing currentIt depends upon the following factors

material of the fusing element

Length, smaller the length greater the current

diameter

Previously history

Size and location of terminal used

Fusing factor

fusing factor = min. fusing current

current rating of fuse


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Important terms

Prospective current.

Cut-off current.

Pre-arcing time.

Arcing time

Total operating time

Breaking capacity


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Types of fuses

1. Low voltages fuses

2. High voltages fuses

low voltages fuses1. Semi-enclosed rewireable fuse

2. High rupturing capacity cartridge fuse

3. H.r.c. fuse with trippping device

High voltages fuses1. Cartridge type

2. Liquid type

3. Metal clad fuses


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H.R.C. cartridge fuse

Advantages1. They are capable of clearing high

as well as low fault currents

2. They do not deteriorate with age

3. They have high speed of operation

4. They provide reliable discrimination

5. They require no maintenance

Disadvantages

1. They have to be replaced after

each operation

2. Heat produced by the arc mayaffect the associated switches


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H.R.C. fuse with tripping device

Advantages1. In case of a single phase fault

on a three-phase system, the

plunger operates the tripping

mechanism of circuit breaker to

open all three phases and

prevents single phasing

2. It also deals with very small

fault currents

3. This permits the use of a

relatively inexpensive circuitbreaker


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Liquid type fuse

These are filled withcarbon tetrachloride

Having widest range

of application to h.v.systems

Used for circuitupto100A rated

current on 132 kv Breaking capacities of

6100a


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Relays


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Physical Appearance of

Relays


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What is Relay

A relay is a device that detectsthe fault and initiates theoperation of circuit breaker toisolate the defective elementfrom the rest of the system

A relay is a simpleelectromechanical switchmade up of an electromagnetand a set of contacts. Relaysare found hidden in all sorts ofdevices. In fact, some of the

first computers ever built usedrelays to implement Booleangates.
http://home.howstuffworks.com/electromagnet.htmhttp://home.howstuffworks.com/boolean.htmhttp://home.howstuffworks.com/boolean.htmhttp://home.howstuffworks.com/boolean.htmhttp://home.howstuffworks.com/boolean.htmhttp://home.howstuffworks.com/electromagnet.htm


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Construction and working

A relay is used to isolate one electricalcircuit from another. It allows a lowcurrent control circuit to make orbreak an electrically isolatedhigh currentcircuit path. The basic relay consistsof a coil and a set of contacts. The

most common relay coil is a length ofmagnet wire wrapped around a metalcore. When voltage is applied to thecoil, current passes through the wireand creates a magnetic field. Thismagnetic field pulls the contactstogether and holds them there untilthe current flow in the coil hasstopped. The diagram below showsthe parts of a simple relay.


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The animated picture shows a working relay with its coil and switch contacts. You

can see a lever on the left being attracted by magnetism when the coil is switched

on. This lever moves the switch contacts.


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Single pole single throw single pole double throw

Double pole single throwDouble pole 2 throw


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Relay

specifications

There are twospecificationsthat you mustconsider whenselecting a relayfor use in anautomobile, thecoil voltage andthe currentcarrying

capability ofcontacts.


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Protective relay And

Construction A protective relay is a complex

electromechanical apparatus,often with more than one coil,designed to calculate operatingconditions on an electrical circuitand trip circuit breakers when afault was found. Design and

theory of these protective devicesis an important part of theeducation of an electrical engineerwho specializes in power systems.Today these devices are nearlyentirely replaced (in new designs)with microprocessor-based

instruments (numerical relays) thatemulate their electromechanicalancestors with great precision andconvenience in application
http://en.wikipedia.org/wiki/Protection_and_monitoring_of_the_electrical_energy_transmission_networkshttp://en.wikipedia.org/wiki/Power_engineeringhttp://en.wikipedia.org/wiki/Power_engineeringhttp://en.wikipedia.org/wiki/Protection_and_monitoring_of_the_electrical_energy_transmission_networks


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Operating Principle

Most of the relays in service on

electric power system today are

of electro-mechanical type

They work on the following two

main operating principles

1. Electromagnetic attraction2. Electromagnetic induction

Attracted armature type relay Solenoid type relay Balanced beam type relay


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Induction type Relays

.Over current relay (non-directional) Over current relay (directional)

Directional power relay


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Others types of functional relays

1. Distance relays

2. Differential relays

3. Translay system

Current differential relay

Voltage balance differential relay


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Types of protection

1. Primary protection

2. Back-up protection


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Advantages of relays .

Relays can switch AC and DC, transistors can only switch DC. Relays can switch high voltages, transistors cannot.

Relays are a better choice for switching large currents (> 5A).

Relays can switch many contacts at once.

disadvantages of relays Relays are bulkierthan transistors for switching small currents.

Relays cannot switch rapidly (except reed relays), transistors canswitch many times per second.

Relays use more powerdue to the current flowing through their

coil. Relays require more current than many ICs can provide, so a

low power transistor may be needed to switch the current for therelay's coil.


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Busbar protection


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Techniques

Differential protection Fault bus protection


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References

www.bcae1.com (for protection project)

www.bcot1.com

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