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Module 1

Basic Terminologies and Characteristicsof Switchgear

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The low voltage switchboardA vital link for electrical distribution

and to provide a continual balance between: • the needs of safety and availability• and the constraints of reliability and maintenability of

installation.

To satisfy the increasing needs of safety of people and equipment and availability of electrical power : its realization is not improvised…

Definitions

Safety

Maintenability

Reliability

Availability

Contents

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Basic Terminologies Switchgear AssemblyA combination of one or more low voltage switching devicestogether with Control, Measuring, Signalling, Protective equipment etc., completely assembled under theResponsibility of manufacturer with all internal electricaland mechanical inter-connections and structural parts.

Role of Switchgear Electrical Protection Safe Isolation from live parts Local or remote switching

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Basic Terminologies

Safety

Availability

Maintenability

Reliability

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Basic Terminologies

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Basic Terminologies Type Tested L.V. Switchgear & Controlgear Assembly (TTA)Assembly conforming to an established type or system without deviations likely to significantly influence the performance from the typical Assembly verified to be in accordance with the standard (IEC 60947)

Partially Type Tested AssemblyAssembly containing both Type Tested and Non type tested arrangements provided that the latter are derived (ex: by calculation) from the Type Tested arrangements.

Dead Front AssemblyAn assembly with a front covering which provides a degree of protectionof at least IP2X from the front. However, live parts may be accessibleFrom other directions.

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Basic Terminologies Enclosed AssemblyAn Assembly which is enclosed on all sides with the possible exception of its Mounting surface in such a manner as to provide a degree of protection of at least IP 2X.Cubicle type AssemblyAn enclosed Assembly of Floor standing type, which may comprise several sections, sub-sections or compartments.

Spare SpacesFree space : An empty space of a section.Unequipped space : A part of a section incorporating busbars only.Partially equipped space : A part of a section fully equipped except for functional units. The functional units which can be installed are defined in number of modules and size.Fully equipped space : A part of a section fully equipped with functional units not assigned to a specific use.

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Basic Terminologies Insulation co-ordination

Clearance : The distance between two conductive parts along a string stretched shortest way between these conductive

parts.

Creepage distance : Shortest distance along the surface of an insulating material between two conductive parts.

- Defines the withstand to surface dielectric ageing; - Defines surge withstand (lightning impulse, switching surge, over

voltage at power frequency.

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Basic Terminologies Altitude

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Basic Terminologies Rated Voltages:Rated Operational Voltage (Ue) : Value of Voltage, which combined with rated current of the circuit, determines its application.

Rated Insulation Voltage (Ui) : Voltage value to which dielectric test voltages and creepage distances are referred.

Rated Impulse withstand voltage (Uimp): Peak value of Impulse voltage of prescribed form & polarity the Assembly is capable of withstanding

without

failure under specified conditions & clearances referred.

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Basic Terminologies Rated Current (In) : Stated by Manufacturer taking into account ratings of components in Assembly, their disposition & application.

Rated short time current (Icw) : Assigned by manufacturer which Assembly can withstand without damage under test conditions. Unless stated by manufacturer, the time is 1 sec.

Rated Peak withstand current (Ipk) : Assigned by manufacturer which Assembly can withstand satisfactorily under test conditions.

Rated Conditional Short circuit current (Icc) : Value of Short circuit prospective current, stated by Manufacturer, with that circuit, protected by a short circuit protective device specified by Manufacturer, can withstand satisfactorily for operating time of the device, under test conditions.

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Basic Terminologies Prospective Short circuit current (Icp) : A current which flows when the supply

conductors to the circuit are short circuited by a conductor of negligible

impedance located as near as possible to the supply terminals of the

assembly.

Cut-off current ; let through current : The maximum instantaneous value

of current attained during the breaking operation of a switching device or fuse.

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Basic Terminologies Relationship between Ipk & Icw :

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Basic Terminologies Conventional free air thermal current (Ith) :

The maximum value of test current to be used for temperature-rise tests of

Unenclosed equipment in free air.

Conventional enclosed thermal current (Ithe):

The value of current stated by manufacturer to be used for temperature-rise tests

Of the equipment when mounted in a specified enclosure.

The above values of ratings shall be at least equal to the maximum value of

The rated operational current in eight-hour duty.

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Basic Terminologies Rated diversity factor : It is the ratio of maximum sum, at any one time, of the

assumed currents of all the main circuits involved, to the sum of the rated

currents of all the main circuits of Assembly.

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Basic Terminologies IEC: International Electro technical Commission

NEC : National Electric Code (counterpart to NFC15-100)

Defines a number of general practical rules to ensure the safety of life and property. Defines cables, protection, installation, earthing etc.

ANSI: American National Standards Institute (counterpart to UTE)

Follows the recommendations of the IEEE (Institute of Electrical and

Electronic Engineers)

NEMA: National Electrical Manufacturers Association

UL: counterpart to Bureau Veritas

Independent organization.- Founded by insurance companies.

Certifies conformity to NEC &ANSI.

Covered only small commercial & Residential {panel boards,MCCBs &

load centers}. Its influence has spread to switchboards and even to MV

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Basic Terminologies Temperature Class:

Class Max.Temp

Y 90

A 105

E 120

B 130

F 155

H 180

200 200

220 220

250 250

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