Objective Questions on Electrical Protection | 1

  1. The positive sequence reactance will be equal to negative sequence if the equipment is

    Transformer and transmission lines have no rotational part so positive and negative sequence reactance will be equal.

  2. The sequence components of the fault electric current are as follows : Ipositive = j1.5 p.u., Inegative = − j0.5 p.u., I0 = − j1 p.u. Which fault is this?

    Only LLG fault electric current has a unique property that the sum of all sequence components of the fault electric current is zero.

  3. On a power system network the load is increasing represents the value of impedance

    Normally 70 % of loads are inductive in nature. If at all load is increasing then inductor in parallel is increasing. Hence inductive reactance will decrease.
    As we know that, Z = R + j( XL - XC ). Hence impedance will decease.

  4. If a fault occurs on any feeder, the fault electric current is

    The fault electric current is much lower than with unsectionalised busbar because a feeder fault is fed from one section.

  5. A loss less transmission line having surge impedance loading (SIL) of 2280 MW is provided with a uniformly distributed series capacitive compensation of 30 %. Then SIL of compensated transmission line will be

    There will not be any effect on active power capacitive compensation.
    ∴ SIL = 2280 MW.

  6. When a neutral point of 3 - Φ system is directly connected to earth then it has what type of grounding?

    A neutral point of a 3 phase system is directly connected to earth through a wire of negligible resistance and reactance. It is said to be solid / effective grounding.

  7. In a three-phase system, when the loads are perfectly balanced, the neutral electric current is

    Under perfectly balanced condition electric current through the 3 different phases are equal and no electric current flows through the neutral.

  8. The line electric current IR flowing into balanced delta connected load is -j5√3 A. If the phase sequence is RYB the phase electric current IYB is

    Line electric current is - j5√3 A, phase electric current is -j5.

  9. A fuse wire of circular cross-section has a radius of 0.8 mm. The wire blows off at a electric current of 8 Amp. Calculate the radius of wire that will blow off at a electric current of 1 amp?

    We know that, I2 proportional to R3, (I2 / I1 )2 = ( R2 / R1 )3, R2 = R1 × ( I2 / I1 )(2 / 3) = 0.8 × (1 / 8)(2 /3 ) = 0.2 mm

  10. Which of the following material is not used as fuse element?

    The melting point of carbon is 3500°C. It is very difficult reach that high temperature when short circuit electric current flows through the fuse. Hence for short circuit or over electric current condition the fuse will not melt to disconnect the circuit if the fuse is made off carbon.

  11. Rating of fuse wire is expressed in

    A fuse in electrical circuit is used to protect the circuit from over electric current and short circuit current. Actually fuse elements melt to disconnect the continuity when electric current beyond the predetermined value flows through them. So it is most appropriate to express the rating of fuse in Ampere.

  12. Fusing factor =

    Fusing factor = minimum fusing electric current / normal electric current rating of fuse element.

  13. Third pin of a 3 pin plug is thicker and longer due to

    Third pin is for earth. In case of any earth fault heavy electric current flows through the thicker pin without over heating or burning the contacts. Thicker earth pins provide more surface area.

  14. Switchgear equipment include

    Switchgear covers a wide range of equipment concerned with switchching and interrupting currents under both normal and abnormal conditions. Switchgear equipment include : switch, fuse, circuit breaker and relay.

  15. The rating of C.B is in terms of

    A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity to immediately discontinue electrical flow. Generally the rating of C.B is in terms of voltage.

  16. MCB stands for

    MCB is miniature circuit breaker. Miniature circuit breaker construction is very simple, robust and maintenance free. Generally an MCB is not repaired or maintained, it just replaced by new one when required.

  17. Recovery voltage in CB is equal to

    Recovery voltage in Circuit breaker is equal to rms voltage at 50 HZ frequency.

  18. Dielectric constant for vacuum is

    Dielectric constant of vacuum is unity i.e 1.

  19. A 3-phase breaker is rated at 2000 MVA, 33 KV, its making electric current will be

    Symmetrical making electric current = ( 2000 × 106) / ( √3 × 33 × 103 ) = 34.99 KA, making electric current = 2.55 × 34.99 = 89.22 KA

  20. A three phase, 33 KV oil circuit breaker is rated 1200 A, 2000 MVA, 3s. The symmetrical breaking electric current is

    Breaking capacity = 1.732VLIFL
    ⇒ 2000 × 106 = 1.732 × 33 × 103IFL
    ⇒ IFL = 34.999 KA = 35 KA.