protection against overload current

7/28/2019 Protection Against Overload Current

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Load centers use circuit breakers to provide

protection against overcurrent.

The National Electrical Code defines overcurrentas

any current in excess of the rated current of

equipment or the ampacity of a conductor. It may

result from overload, short circuit, or ground fault

(Article 100-definitions).


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Fuse Circuit Breaker


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The purpose of circuit protection is to protect the wirefrom carrying more current than it is able.

If the current becomes excessive the circuit protectionshould interrupt the circuit.

If the current is not interrupted the wire can become hotenough to melt the insulation, and the result is often a

short circuit with adjacent wires causing a meltdown andpossibly a fire.

The principals of circuit protection are the same for ACand DC systems.


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Rewireable fuses are fuse carriers that contain a length of fusewire. The fuse wire is wrapped around the terminals bridgingthe gap between the carrier pins, thus completing the circuit.

The fuse wire is available in different thicknesses, thethickness relates to the rating and current carrying capacity ofthe wire.

Rewireable fuses are obsolete and should be replaced at theearliest opportunity


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Cartridge fuses come in a cylindrical shape

and have contacts points at each end.

They have a fuse link connected to the twoends and are rated for 240-volt circuits and

protect the circuit from over-current


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A HRC fuse is a fuse that is filled with silica sand surrounding thefuse link.

It is used on applications where the fault current needs to besuppressed fast and with no flash over.

On a fault current a tremendous amount of heat is created withinthe fuse. That heat melts the silica sand into glass, and glass beingan insulator, suppresses the arc over and breaks the circuit

instantaneously.


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A re-wireable is exactly that. It is a fuse that you can re-wire in the eventof itoperating.

Early re-wireable fuses had a very low short circuit capacity and were

verydangerous when operating under fault conditions because the fuseelement meltsand splashes the melted copper around and can causefires.

Later re-wireable fuses incorporated asbestos to protect the fuse holderduring thefusing period, thus reducing the risk of fire from scattering hot

metal when rupturing.


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A circuit can break with electromagnetism in a magnetic circuit breaker. The more

electricity flowing through a magnetic circuit breaker, the stronger theelectromagnet gets. When the electricity surpasses the current limit on a magneticcircuit breaker, the electromagnet is strong enough to push the circuit breakerdownward and flip the switch.


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A thermal circuit breaker is a safety device whichbreaks an electrical circuit if the temperature gets toohigh.

These devices are used to prevent fires, damage due

to voltage fluctuations, and other dangerous electricalsituations


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An electromagnetic/heat circuit breaker combines heatand electromagnetism. It features a bimetallic strip thatdefends against extended electrical overheating and anelectromagnet that guards against abrupt electrical surges.


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An electronic circuit breaker includes an input for connection to apower-supply and an output for connection to a load. Connected incircuit between the input and the output are a switch, a relay, and alimitation block, which controls the switch to cause at least partialinhibition thereof in the event of an over-current condition. Alsoprovided is a microprocessor which is connected to the limitation

block to inhibit power supply to the load.


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Mould case circuit breakers perform much thesamefunction as the miniature circuitbreakers we looked at

above but are used ininstallations where higher levels offaultcurrent are present such as in smaller sub-stations.They are usually adjustable to allowfor an amount of over-current. They are ratedup to 1000 amps.


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An Earth Leakage Circuit Breaker (ELCB) is a safety device used inelectrical installations with high earth impedance to prevent shock. It

detects small stray voltages on the metal enclosures of electricalequipment, and interrupts the circuit if a dangerous voltage is detected.Once widely used, more recent installations instead use residual currentcircuit breakers which instead detect leakage current directly.


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is an electrical protection device designed to cut off thepower supply to a circuit, appliance, or machine in the eventof an overload or failure.

These devices typically consist of a manual circuit breaker orcontactor and a current sensing relay which interlocks withit.

Should the appliance or circuit be damaged or overloaded, itwill begin to draw current in excess of its normal operatingparameters

This causes the current sensing relay to trip the circuitbreaker or deactivate the contactor and cut the powersupply.


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Basic Principles

Electric shock - is divided into two parts:

direct contact: contact with parts which result in an electric shock in normalservice

indirect contact: contact with exposed conductive parts which result in anelectric shock in case of a fault.

Thermal Effect - refers to heat generated by the electrical equipment innormal use and under fault conditions. The proper selection of equipmentcomplying with the latest product standards is essential in providingprotection against thermal effects.

Overcurrent - is defined as a current exceeding the rated value of the circuitcomponents. It may be caused by the overloading of a healthy circuit or it maytake the form of a short-circuit current, defined as an "overcurrent resultingfrom a fault of negligible impedance between live conductors having adifference in potential under normal operating conditions". Overcurrentprotection may be provided by using fuses or circuit breakers singly or in

combination.


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Under voltage - refers to the dangers that could be caused by thereduction or loss in voltage and the subsequent restoration, such as theunexpected re-starting of motors or the automatic closing of protectivedevices. The proper selection of control and protective devices must take

the protection against under voltage into consideration.

Isolation - every circuit shall be provided with means of isolation (exceptin certain cases) to prevent or remove hazards associated with theinstallation, equipment and machines. The new standards for circuit

breakers and switch-fuses now take this into account.


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Advantages:

Operate more efficiently.

It can be operated again after damage repair.

Good to know the situation has been in operation or not. No perishable and durable

Disadvantages:

Prices are expensive

If damaged have been refurbished Difficult obtained


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There are several factors that should be taken into consideration in selection of protective

current excess are:

Load type

If the current load is always balanced the proportion of the load current to be

determinants for protective current rating for current excess to be used.

Excess current protective resistance.

Protector current selected excess shall be suitable to short circuit current rating. For

example a fuse that is rated to 15A must be able to withstand a short circuit current

of 15 x 12-fold = 180A. Ability to withstand short circuit current is the current excess

protective immunity in the event of a short circuit is.

Temperature environments

environmental temperature can influence the use of excess current protective fuse

especially to suit the environment temperature


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Special tools

For devices that are sensitive to the current surplus rose sharply should useprotective current surplus of refined class.

Price and danger.

If the current surplus of cheap protectors have the same capacity the betterchosen. But if the danger more than the expensive, the better choose moreexpensive.


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protection against overload current

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