Building a ladder diagram

Introduction

Learning to read a ladder diagram (schematic diagram) is a required skill for every HVAC technician.  Learning to read ladder diagrams is like learning another language, with its own alphabet (symbols) and structure.  The best way to learn to read a new language to first learn the alphabet.  This web quest is the first in a series for building a ladder diagram and will focus on understanding the symbols.

Task

The student will investigate the definitions of a manual switch and an automatic switch, focusing on the differences between the two.

The student will investigate the two standards symbols originate from: NEMA and IEC.

The student will investigate methods of representing a connection point in a schematic diagram.

The student will consider a symbol or switch from the perspective of what type of switch it is compared to what position that switch is in.

Process
  1. First you will do an internet search on “Manual Switch” and “Automatic Switch”.
  2. You will then construct a brief definition of each of the switches.
  3. In the context of manual/automatic switches, give a definition of what the “operator” is.
  4. In terms of the “operator”, what is the main difference between a manual and an automatic switch?
  5. Next you will research different methods for indicating how a connection point is represented in a schematic diagram.
  6. Indicate two main methods of indicating a connection point, together with their respective representation of “unconnected”.
  7. You will now conduct an internet search on IEC symbol charts.
  8. You will now conduct an internet search on NEMA symbol charts.
  9. Define what is meant by the term "normally open" and "normally closed"
  10. Consider the following:  A normally open switch may be shown in the closed position and a normally closed switch may be shown in the open position.
  11. Find the NEMA symbol for a normally open pressure switch and draw that symbol in the closed position.
  12. Find the Nema symbol for a normally closed temperature switch and draw that symbol in the open position.
  13. Whenever you see a symbol you have to ask yourself two questions: first, what type of switch is it (N.O or N.C) and second, what position is it in?
  14. Using NEMA symbols you will complete the symbol quiz sheet.  Indicate what type of switch it is and also, what position it is in.                                                                                                               
  15. Using the given schematic diagram indicate which line in the schematic contains the listed symbol.
Evaluation

Marking Rubric

Listed

Process

 

Beginning

1 Mark

Developed

2 Marks

Accomplished

3 Marks

Exemplary

4 Marks

 

#2 - Manual Switch Definition

 

 

 

 

 

#2 - Automatic Switch Definition

 

 

 

 

 

#3 - Operator Definition

 

 

 

 

 

#4 - Difference between Manual/Automatic Switch

 

 

 

 

 

#6 - Type 1 Connection

 

 

 

 

 

#6 - Type 2 Connection

 

 

 

 

 

#9 - Normally Open Definition

 

 

 

 

 

#9 - Normally Closed Definition

 

 

 

 

 

#14 - Symbol Quiz

 

 

 

 

½ mark for the correct graphic and ½ mark for the correct position

Total 20

#15 - Schematic Drawing

 

 

 

 

1 mark for each correct answer

Total 10

Total /62

 

 

 

 

 
Conclusion

Conclusion

Congratulations on taking the first step towards understanding ladder (schematic) diagrams.  One of the hardest concepts to grasp in this unit is the difference between “the type of switch” verses “the position it is in”.  Please follow up this unit with a supplementary lecture video regarding this topic.  (video coming soon)

The next unit will discuss the basic structure of the ladder diagram.