EXAMINATION : ELECTRICAL AUTOMATION

1. PT-100 sensors are some times used with 3 or even 4 wires. What is the reason for this?
Higher measuring accuracy.

2. In the diagram below we want to verify the resistance of the PT100 sensor. What is the correct calculation?
R(Pt100) = R1 - R2

3. In some cases we can see that a resistor is connected over an alarm contact (see diagram).  Why is this connection used?
To monitor the cable/wires for break.

4. Please indicate the correct sequence of action to take place in an alarm/monitoring and safety system.
Alarm, st.by start, slowdown. Shutdown

5. What is an A/D and a D/A converter?
A converter that converts an analogue signal to a digital signal and vice versa.

6. Which of the 3 alternatives below is giving an OR-function?  
Figure 2.

7. What is an I/P Transducer?
A transducer that converts a known electric current to a  pressure proportional to the current.

8. Which of the 3 alternatives given is correct for a  NOT-gate?
Figure 1.

9. Which of the 3 alternatives given is correct for an AND-gate?
Figure 3.

10. When calibrating an pressure transducer we have to adjust both SPAN and ZERO. Please indicate in which order these adjustments should be done.
First Zero adjustment and the Span adjustment. Then Zero  should be rechecked.

11. In a PID controller we have possibility to change the setting of the Proportional band (P), the Reset time (1) and the Rate time (D). Please indicate which curve shows the typical response to change in demand if the setting of the proportional band is too wide.
Figure 1.

12. In a PID controller we have possibility to change the setting of the Proportional band (P), the Reset time (I) and the Rate time (D). Please indicate which curve shows the typical response to change in demand if the setting of the reset time   is too fast.
Figure 3.

13. n a PID controller we have possibility to change the setting of the Proportional band (P), the Reset time (I) and the  Rate time (D).  Please indicate which curve shows the typical response to change in demand if the setting of the rate time  is too short.
Figure 2.

14. Earthing of signal cables. Which of the 4 alternatives is correct for a cable for analogue signals?
Figure 1.

15. In a fresh water tank with a height of 5 meters we shall use a pressure transmitter for level measurement. The tank has a vent-pipe leading to deck 8 meters above the top of the tank. What pressure shall we use as a guidance when ordering the pressure transducer?
1,3 Bar.

16. In a fresh water tank we are using a pressure transmitter with a range of 0-0.5 bar / 4-20mA for level measurement. The transmitter is for different reasons installed 30 centimeters from the bottom of the tank, and the tank is 5 meters high. What will the output from the transmitter be when the tank is full?  (Pending no calibration is made)
18,8 mA.  

     

17. In a fresh water tank we are using a pressure transmitter with a range of 0-0.5 bar/4-20mA for level measurement.The transmitter is for different reasons installed  30 centimeters from the bottom of the tank,and the tank is 5 meters high. What will the output from the transmitter be when the tank is empty?
4 mA.

18. What is the signal we measure out from a NiCr/Ni thermocouple?
mV.

19. In connection with exhausts gas measurement with a thermocouple we are normally using an amplifier which gives a mA-signal out. Why is this done?
The signal out from a thermocouple is mV,and can not be transferred over any longer distance without loss of voltage, giving a very bad accuracy.

20. Which of the 4 alternatives shows a PNP transistor?
Figure 2.

21. Which of the 4 alternatives shows a NPN transistor?
Figure 3.

22. Due to the relative low resistance of a Pt 100 sensor we might have to compensate for the cable-resistance to ensure a correct reading. Which of the 4 alternatives given is correct for the connection of this compensation resistance.
Figure 3.

23. When the cable length from the temperature sensor to the place where we want to read the temperature exceeds approx. 10 meters we normally have to, in case of a Pt 100, in some way compensate for the cable resistance. Do we also    have to do this if we choose to use a T802 temperature sensor?
No

24. Some equipment may be marked with the following symbol: What does that mean?
Intrinsically safe.

25. In installations of INTRINSICALLY SAFE equipment it is required that all equipment should be separated from non-intrinsically safe equipment, and to be of a special colour.  What colour is that?
Blue

26. What is this?
A zener barrier. (Positive)

27. What is a ZENER BARRIER?
A zener barrier is a unit made to obtain intrinsically safety in installations for instrumentation in hazardous areas.

28. Can a ZENER BARRIER be installed in an hazardous area?
No, as only the output from the barrier is intrinsically safe this is not allowed.

29. What is the difference between a THERMOSTAT and a TEMPERATURE TRANSMITTER, if any?
The thermostat has one or more contacts (open or closed) depending on the temperature/setting. The temperature transmitter converts a temperature signal to an electric signal.

30. What is the difference between a PRESSURE SWITCH and a PRESSURE TRANSMITTER, if any?
The pressure switch has contact(s) that will change between open and closed position.The pressure transmitter converts a pressure signal into an electric signal.

31. What is the symbol for a THYRISTOR?
Figure 2.

32. What is the definition of DEAD BAND?
The change needed in the input signal to produce a change in the output signal.

33. What is an analogous signal?
A variable electric signal.

34. What is digital signal?
A signal representing 0 = "low" and 1 = "High"

35. What is Hz the abbreviation for?
Frequency in an Alternating Current (AC)

36. P = U x I x  Root of 3)  x cos (fi)    What is the formula for I ?
Formula 1

37. Which alternative is correct for measuring current over the load L?
Figure 3.

38. Temperature sensors may be marked Pt 100. What does it mean?
100 ohm at 0° C

39. Temperature sensors may be marked T 802. What does it mean?
802 ohm at 20° C

40. What is the most common signal from an I/P converter?        
 4-20mA/3-15 PSI

41. Which method is best for determining earth failure on a 24 VDC power supply?
A lamp, (0-50 mA) and a mA-meter in series.

42. Which statement is correct with regards to power supply for equipment supplied through Zener-barriers?
The 0-voltage must always be grounded.