21. Arc in a circuit behaves as
A. a capacitive reactance
B. an inductive reactance
C. a resistance increasing with voltage rise across the arc
D. a resistance decreasing with voltage rise across the arc
Answer
D. a resistance decreasing with voltage rise across the arc
Explanation:
- During opening of current carrying contacts in a circuit breaker the medium in between opening contacts become highly ionized through which the interrupting current gets low resistive path and continues to flow through this path even the contacts are physically separated.
- During the flowing of current from one contact to other the path becomes so heated that it glows. This is called arc.
- The main designing criteria of a circuit breaker is to provide appropriate technology of arc quenching in circuit breaker to fulfill quick and safe current interruption.
22. A fuse is connected
A. in series with circuit
B. in parallel with circuit
C. either in series or in parallel with circuit
D. none of the above
Answer
A. in series with circuit
Explanation:
Concept:
- Fuse is an electrical safety device used to protect an electrical or electronic device from overcurrents.
- The electric fuse is connected to the live wire.
- When the current above the rated value of the fuse flows through the fuse, the heat generated will cause the fuse wire to melt, thereby breaking the connection.
- An electric fuse is a protective device that protects electrical equipment in the circuit by breaking the circuit when there is a short circuit.
- It contains a fuse wire which has a low melting point, it allows normal current pass through it safely but if there is any short circuit fuse wire can’t withstand the heat produced by that current (infinite current) so it melts and breaks the circuit
- Hence fuse wire used to have a low melting point and low resistance to avoid wastage of electric current.
- Whereas the fuse rating of electrical fuse is the maximum amount of current can pass through it without melting the fuse wire.
- Fuses are always connected in series with the circuit components to be protected from the overcurrent in the circuit, so that when the fuse blows, or opens, it will open the entire circuit and stop current through the circuit components.
- The fuse being a protective device has to perform the duty of separating faulty part of the circuit from the healthy part. This function of the fuse ceases if it is placed in the neutral wire.
Important Points:
The material used for fuse elements must be of:
- Low melting point, low ohmic loss.
- High resistivity.
- Low cost and free from detraction.
- The material used for making fuse element has a low melting point such as tin, lead, or zinc.
- A low melting point is, however, available with a high specific resistance metal.
- The material mainly used for fuse element are tin, lead, silver, copper, zinc, aluminium, and an alloy of lead and tin.
23. The reflection co-efficient at the open circuited end of a transmission line.
A. zero
B. infinity
C. unity
D. none of the above
Answer
C. unity
Explanation:
24. Protection by fuses is generally not used beyond
A. 20 A
B. 50 A
C. 100 A
D. 200 A
Answer
C. 100 A
Explanation:
- Fuses offer circuit protection that is inexpensive, straightforward and fast protection.
- Their faster circuit protection time is perhaps their biggest benefit over circuit breakers.
- This is important when protecting sensitive electronic equipment.
- Protection by fuses is generally not used beyond 100A.
25. The ground wire should not be smaller than No ______ copper.
A. 2
B. 4
C. 6
D. 10
Answer
D. 10
Explanation:
- The ground wire can always be one size smaller than the conductors.
- The ground wire never carries normal circuit current. It only carries current if there is a fault to ground.
- The grounding conductor will only carry a large amount of current for a very short time, allowing for a smaller gauge grounding conductor.
- The ground wire should not be smaller than No 10 copper.
26. Directional relays are based on flow of
A. power
B. current
C. voltage wave
D. all of the above
Answer
A. power
Explanation:
- A relay is an automatic device which senses an abnormal condition of the electrical circuit and closes its contacts
- A relay which is operated on the principle of power flow in both sides of the line called directional relay
- Directional relays are commonly used to preventing excess power flow from a small generating station into a larger system
27. Admittance relay is _______ relay.
A. impedance
B. directional
C. non-directional
D. none of the above
Answer
B. directional
Explanation:
- A Mho relay or admittance relay measures a component of admittance |Y| ∠ θ. But it's characteristic when plotted on the impedance diagram is a circle passing through the origin.
- The relay is called the Admittance relay because its characteristic is a straight line when plotted on an admittance diagram.
- It is inherently a directional relay as it detects the fault only in the forward direction.
- The operation of the Mho relay is most affected due to arc resistance.
Important Points:
- A reactance relay is an overcurrent relay with directional restraint.
- Mho relay is a voltage restrained directional relay.
- A reactance relay is suitable for the protection of a short transmission line because its operation is independent of arc resistance.
- The relay which is selected for a long transmission line should be less affected due to power swings. Hence Mho relay is preferred.
- Impedance relay is suitable for medium transmission lines.
28. The voltage appearing across the contacts after opening of the circuit breaker is called______voltage.
A. recovery
B. surge
C. operating
D. arc
E. none of the above
Answer
A. recovery
Explanation:
Restriking voltage:
- It is the transient voltage that appears across the contacts at or near current zero during the arcing period.
- It also defined as the voltages that appear across the breaking contact at the instant of arc extinction.
- At current zero, a high-frequency transient voltage appears across the contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission lines of the system. This transient voltage is known as restriking voltage.
- The current interruption in the circuit depends upon this voltage.
- If the restriking voltage rises more rapidly than the dielectric strength of the medium between the contacts, the arc will persist for another half-cycle.
- On the other hand, if the dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted.
Arc Voltage:
- It is the voltage that appears across the contacts of the circuit breaker during the arcing period.
- As soon as the contacts of the circuit breaker separate, an arc is formed.
- The voltage that appears across the contacts during the arcing period is called the arc voltage.
- Its value is low except for the period the fault current is at or near the zero current point.
- At current zero, the arc voltage rises rapidly to peak value and this peak voltage tends to maintain the current flow in the form of an arc.
Recovery voltage:
- It is the normal frequency (50 Hz) RMS voltage that appears across the contacts of the circuit breaker after the final arc extinction.
- It is approximately equal to the system voltage.
- Active Recovery Voltage: It may be defined as the instantaneous recovery voltage at the instant of arc extinction.
- The power system contains an appreciable amount of inductance. At the instant of current zero point, the fault current lags behind the arc voltage by 90o. So, the arc voltage must have a peak value at this instant.
29. The material used for fuse must have
A. low melting point and high specific resistance
B. low melting point and -low specific resistance
C. high melting point and low specific resistance
D. low melting point and any specific resistance
Answer
B. low melting point and -low specific resistance
Explanation:
The material used for fuse elements must be of:
- Low melting point, low ohmic loss.
- High resistivity.
- Low cost and free from detraction.
- The material used for making fuse element has a low melting point such as tin, lead, or zinc.
- A low melting point is, however, available with a high specific resistance metal.
- The material mainly used for fuse element are tin, lead, silver, copper, zinc, aluminium, and an alloy of lead and tin.
- The material used for fuse elements must be of low melting point, low ohmic loss, high conductivity (or low resistivity), low cost and free from detraction.
- The material used for making fuse element has a low melting point such as tin, lead, or zinc.
- The conductivity of silver does not deteriorate with oxidation.
30. For cost and safety, the outdoor substations are installed for voltages above
A. 11 kV
B. 33 kV
C. 60 kV
D. 110 kV
Answer
B. 33 kV
Explanation:
- A substation which is used for all voltage levels between 33 KV to 765 KV is called outdoor substation. Such type of substation requires less time for construction but uses more space.
- The outdoor substations are mainly classified into two types, namely pole-mounted substation and foundation-mounted substations.