Sunday, November 8, 2009

Switchgear and control equipment

1. The 3 most imp. characteristic for any relay:

i. Pickup Current valueii. Time Delay iii. Relay Coordination is ultimately determined by the type of Grounding used in Distribution Systems: a. Isolated/ Unearthed Neutral b. Impedance-earthed Neutral c. Solidly (or low impedance) earthed Neutral- In this, the network is generally earthed on the Neutral of an i/coming delta/star tx.former. When this neutral is not accessible, the network is earthed either through and zig-zag tx.former or through a star/delta tx.former connected to the main busbar.

2. When a Fault occurs, Electrodynamic and Thermal stresses built up.

3. For an Induction. motor, a starting current of 6 times the Rated Current(Full Load Current) and a Locked Rotor Resistance , R1 of 3 times the R0 (Running Resist.) causes I2 * r to be 6 X 6 X 3 or 108 times normal.

4. All the elements of universal relay are organized under the application of Feeder Motor Breaker

5. Feeder application group: It provides 3 separate sets of Instantaneous, Definite-Time & Inverse-Time elements.

Instantaneous: Their operating time does not depend upon the current level. This responds to Phase current and provided conventional Phase fault protection.

Definite-Time OR Independent Time relays: Their operating time depends on the current level. This responds to Negative-Sequence Current and provides Phase to Phase fault protection.

Application: Independent Time relays are used when the short-circuit level is very high OR when short circuit current is likely to vary widely at and given point (eg- when and network is supplied from small generators whose short circuit decrement curve falls off rapidly.)

Inverse-Time OR Dependent Time relays: This responds to Residual Current and provides conventional ground fault protection.

Application: Dependent Time relays are used when:

i. The operation of network includes high-level short-time overloads. ii. Magnetizing inrush currents at switch-on may be considerable for several tenths of and second. iii. Relay operation must be coordinated with a large no. of fuses.

6. Motor application group: It shares the Phase, Negative-Sequence and Ground Instantaneous and Definite-Time elements to protect the motor of winding faults or faults in the connecting leads. Another element, responding to both Positive-Sequence & Negative-Sequence current, provides for Overload, Locked rotor, or unbalanced current conditions.

Tuesday, November 3, 2009



1 . Identify the correct Terminal/Connector for the cable in use.This can be done by checking the marking on the connector or from the labels attached to the packages.

2 . Necessary cable preparation to be done using suitable stripper to remove the insulation to the distance equal to the insertion depth of the connector plus suitable extra allowances.(3mm to 5mm)

3 . Brush the surface of the stranded conductor to remove any oxides if present.

4 . Insert the conductor in to the barrel of the connector.Orientation of the palm of the connector should be ensured for the equipment where it is to be connected.

5 . Suitable crimping dies / tools should be selected to ensure that the joints made are mechanically strong.Good joint results into good electrical continuity.

6 . Ensure proper crimping series(as mentioned above)

7 . Clean the surface where the terminal is to be fixed.Necessary nut,bolts,washer as indicated should be used.

Monday, October 26, 2009

Do you know what is an Electrical Network means?

(A simple electric circuit made up of a voltage source and a resistor)

An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources, and switches.

An electrical circuit is a network that has a closed loop, giving a return path for the current. A network is a connection of two or more components, and may not necessarily be a circuit.

Electrical networks that consist only of sources (voltage or current), linear lumped elements (resistors, capacitors, inductors), and linear distributed elements (transmission lines) can be analyzed by algebraic and transform methods to determine DC response, AC response, and transient response.

A network that also contains active electronic components is known as an electronic circuit. Such networks are generally nonlinear and require more complex design and analysis tools.

To design any electrical circuit, either analog or digital, electrical engineers need to be able to predict the voltages and currents at all places within the circuit. Linear circuits, that is, circuits with the same input and output frequency, can be analyzed by hand using complex number theory. Other circuits can only be analyzed with specialized software programs or estimation techniques.

Circuit simulation software, such as VHDL and HSPICE, allows engineers to design circuits without the time, cost and risk of error involved in building circuit prototypes.

Sunday, October 4, 2009

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