Answer / rakesh bharadwaj

I think its DC guys..
coz AC is sinusiodal it will return to zero at a point so
that one can stay away frm the shock during that instant
but DC dont have return-to-zero value hence cud be more
lethal

Answer / mandar

I think DC is more dangerous than AC only in cases where
the DC voltage is high,as they are the direct current &
does not return to zero.

Answer / shivdayal vishwakarma

AC is more dangerous, since its nature is attractive. at every sinusoidal reversal your body potential is reversed, thats why it is attractive. in case of DC source, its creates a capicitive effect, once your body is charged with DC potential then its repell the body from source.
also for same potential difference, lethality current range for DC is higher. it means you required higher current than AC to get shock in case of DC.generally 60 mA of AC (rms, 60 Hz) and 300–500 mA of DC can cause shock to human.I THINK THIS DISCUSSION IS SUFFICIENT

Answer / amar 9009936456

i think DC is more danderous than AC becoz AC is
sinusiodal it will return to zero at a point so
that one can stay away frm the shock during that instant
but DC dont have return-to-zero value hence cud be more
lethal & dengerous.

Answer / thomas mathew

Their is no dought the most dangerous current is in DC form
because in AC current in each half cycle their is a null
piont or zero volt for a short time depends upon the
applied voltage but in DC current their is no break in the
applied voltage .

Answer / sahil

I think nobody has done 12th here....
AC increases in its half cycle from 0 to (say) I●..... The value we see is the virtual value of AC but peak value I● is √2 times of the value shown by AC voltmeter....

So,we can say that from AC we will get (approx.) 1.5 times more voltage shock as shown in its virtual value(that we often think is the maximum value)
I think this is higher level answer for those who didn't have done +2 with physics.
Thankyou

Answer / shashidhar

DC is more dangerous than AC because, AC signal is
sinusoidal therefore when it comes to zero state the flow of
current is becoming less. but in DC the flow of current is
always same.

Answer / d kiran kumar

Iam giving a simple answer AC having frequency
(representation).But dc contain zero frequency i.e constant
magnitude representation.Once shock no body can save u.

Answer / satender

ac is more dangerous due to its skin effect.and ac is
attractive while dc is repulsive

Answer / sanuj chopra

AC....BCOZ OF HIGH RMS VALUE AND MORE SKIN BURNING EFFECTS.

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Sunday, April 25, 2010 BSNL TTA EXAM Control Systems BSNL TTA EXAM Control Systems 1. An open loop control system has its (a) control action independent of the output or desired quantity (b) controlling action, depending upon human judgment (c) internal system changes automatically taken care of  (d) both (a) and (b) (e) all (a),(b) and (c) 2. A servo system must have (a) feedback system (b) power amplifier to amplify error (c) capacity to control position or its derivative  (d) all of these (e) none of these 3. The major disadvantage of a feedback system may be  Inaccuracy (b) inefficiency (c) Unreliability (d) instability (e) Insensitivity 4. Properties of a transfer function (a) It is ratio of two polynomials is S and assumes zero initial conditions (b) It depends on system elements and not input and output of the system (c) Coefficients of the powers of S in denominator and numerator are all real constant. The order of denominator is usually greater than or equal to the order of numerator (d) All of these (e) It is a function which transfer one physical system into another physical system. 5. The classical analogous of a simple lever is (a) Capacitor bridge (b) transformer (c) mutual inductor (d) either of these 6. Two blocks G1(s) and G2(s) can be cascaded to get resultant transfer function as (a) G1(s) + G2(s) (b) G1(s) / G2(s) (c) G1(s) G2(s) (d) 1+G1(s) G2(s) (e) 1-G1(s)G2(s) (f) two blocks cannot be cascade 7. The principles of homogeneity and super position can be applied to  linear time invariant system (b) non-linear time invariant system (c) digital control system (d) both (a) and (b) 8. Pick up the nonlinear system (a) automatic voltage regulator (b) d.c. servomotor with high field excitation (c) temperature control of a furnaces using thermistor (d) speed control using SCR (e) all of these 9. Signal flow graph (SFG) is a (a) polar graph (b) semi log graph (c) log log graph  (d) a special type of graph for analyzing modem control system (e) a topological representation of a set of differential equations 10. Disadvantages of magnetic amplifier (a) time lag, less flexible, non-sinusoidal waveform (b) low power consumption and isolation of the active circuit (c) saturation of the core (d) all of these 11. Pick up false statement regarding magnetic amplifiers (a) The gate coil of an ideal magnetic amplifier has either zero or infinite inductance (b) Resistance of control and gate winding is very small (c) Magnetic amplifier gas dropping load characteristics (d) Magnetic amplifiers are not used to control the speed of d.c. shunt motor (e) Magnetic amplifiers can be used in automatic control of electric drivers of higher rating. 12. High power amplification is achieved by using (a) push pull amplifier (b) amplidyne (c) magnetic amplifier (d) DC amplifier (e) D.C. generator 13. Pick up false statement regarding servomotors (a) The d.c. servomotors are lighter than equivalent a.c. servomotors (b) The d.c. servomotors develops higher starting and reversing torque than equivalent a.c. servomotor. (c) A drag cup a.c. servomotor has one windings on stator and other on rotor (d) Output power of servomotors varies from 1/20 W to 100 W 14. To reduce steady state error (a) decrease natural frequency (b) decrease damping (c) increase damped frequency (d) increase time constant (f) increase gain constant of the system 15. A good factor for Mp should be (a) less than 1 (b) lying between 1.1 and 1.5 (c) more than 2.2 (d) zero (e) infinity 16. Pick up false statement. Routh-Hurwitz criterion (a) is used for determining stability of a system (b) is an algebraic procedure (c) gives the exact location of roots of the characteristic equation (d) does not indicate relative degree of stability or instability 17. Which of the following is the time domain method of determining stability of a control system (a) Bode plot (b) Nyquist plot (c) Nicholos chart (d) Routh-Hurwitz array (e) Constant M and (fy) locus (f) Root locus technique 18. The technique which gives transient response quickly as well as stability information is (a) Nyquist plot (b) Routh-Hurwitz criteria (c) Bode plot (d) Root locus plot (e) Nichols plot 19. The bandwidth can be increased by use of (a) phase lag network (b) phase lead network (c) both (a) and (b) in cascade (d) both (a) and (b) in parallel (e) none of these 20. Nyquist plot is drawn on (a) semi log graph paper (b) log log graph paper (c) polar graph paper (d) centimeter graph paper 21. If the gain margin is positive and the phase margin is negative the system is (a) stable (b) unstable (c) indeterminist 22. The Bode plot is applicable to (a) all phase network (b) minimum phase network (c) maximum phase network (d) lag lead network (e) none of these 23. The valid relation between setting time ts and rise time tr is (a) tr>ts (b) ts>tr (c) ts=tr (d) none of these 24. As a root moves further away from imaginary axis the stability (a) increases (b) decreases (c) not affected (d) none of these 25. Flat frequency response means that the magnitude ratio of output to input over the bandwidth is (a) variable (b) zero (c) constant (d) none of above 26. How many octaves are between 200 Hz and 800 Hz (a) Two octave (b) One octave (c) Four octave (d) None of above 27. Human system can be considered as (a) open loop system (b) close loop system with single feedback (c) close loop system with multivariable feedback (d) none of these 28. In a feedback system the transient response (a) Decays at constant rate (b) gets magnified (c) decays slowly (d) decays more quickly 29. Transfer function of a system is used to calculate (a) the steady state gain (b) the main constant (c) the order of system (d) the output for any given input (e) all of the above 30. Transfer function of a system is defined as the ratio of output to input in (a) Laplace transform (b) Z-transform (c) Fourier transform (d) Simple algebraic form 31. Introduction of feedback decreases the effect of (a) disturbances (b) noise signals (c) error signals (d) all the above 32. The system response of a system can be best tested with (a) unit impulse input signal (b) ramp input signal (c) sinusoidal input signal (d) exponentially decaying input signal 33. Which of the following is a closed loop system (a) electric switch (b) car starter (c) de generator (d) auto-pilot for an aircraft 34. Which of the following is used as an error detector (a) potentiometer (b) field controlled ac motor (c) amplidyne (d) armature controlled ac motor 35. The break away point of root loci are (a) open loop poles (b) closed loop poles (c) open loop zeros (d) closed loop zeros 36. Noise in a control system can be kept low by (a) reducing the bandwidth (b) attenuating such frequencies at which external signals get coupled into the system (c) both (a) and (b) (d) none of these 37. Main cause of absolute instability in the control system is (a) parameters of controlling system (b) parameters of controlled system (c) parameters of feedback system (d) error detector where the two signals are compared 38. Basically a controller is (a) a amplifier (b) a clipper (c) a comparator (d) a summer 39. A system with gain margin close to unity or a phase margin close to zero is (a) highly stable (b) highly oscillatory (c) relatively stable (d) none of these 40. Which of following elements is not used in an automatic control system (a) sensor (b) error detector (c) oscillator (d) final control element 41. AC systems are usually preferred to the DC systems in control applications because (a) AC systems are cheaper (b) AC systems are more stable (c) AC systems have better performance characteristics and smaller in size (d) all of these 42. A system has the transfer function (1-s)/(1+s); It is known as (a) low pass system (b) high pass system (c) all pass system (d) none of the above 43. In control systems, excessive bandwidth should be avoided because (a) noise is proportional to bandwidth (b) it leads to low relative stability (c) it leads to slow speed of response (d) none of these 44. In most systems, an increase in gain leads to (a) larger damping ratio (b) smaller damping ratio (c) constant damping ratio (d) none of these 45. A step function is applied to the input of a system and output is of the form y = t, the system is (a) stable (b) unstable (c) not necessarily stable (d) conditionally stable 46. 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What is skin effect?

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The loss of precision in quantity is called A Down time B Delay C Unavoidable delay D None of these

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