Normally closed (NC) contact in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Normally Closed (NC) Contact Behavior

In a PLC ladder logic diagram, what happens to the output coil when a Normally Closed (NC) contact is activated (i.e., the input device connected to the NC contact is triggered)?

A The output coil is de-energized because the NC contact opens when triggered, breaking the circuit.

B The output coil is energized because the NC contact opens when triggered, breaking the circuit.

C The output coil remains energized because the NC contact stays closed when triggered.

D The output coil toggles between energized and de-energized rapidly.

💡 Hint

Think about what 'normally closed' means and what happens when the input device activates the contact.

A Normally Closed (NC) contact is closed when the input device is not triggered, allowing current to flow. When the input device triggers, the NC contact opens, breaking the circuit and de-energizing the output coil.

💻 Command Output

intermediate

Output of Ladder Logic with NC Contact

Given the following ladder logic snippet, what is the state of the output coil Q0 when the input I0 is ON? - NC contact connected to input I0 - Output coil Q0 connected after the NC contact

PLC Programming

I0 = ON
NC_contact = Normally Closed contact linked to I0
Q0 = Output coil after NC_contact

A Q0 is ON

B Q0 is OFF

C Q0 toggles ON and OFF

D Q0 state is unknown

💡 Hint

Remember that NC contact opens when input is ON.

When I0 is ON, the NC contact opens, breaking the circuit and turning Q0 OFF.

🔧 Debug

advanced

Identify the Error in NC Contact Usage

A programmer wrote this ladder logic snippet: - NC contact connected to input I1 - Output coil Q1 connected after the NC contact The output Q1 never energizes even when I1 is OFF. What is the most likely cause?

A The NC contact should be replaced with a Normally Open (NO) contact.

B The output coil Q1 is faulty and cannot energize.

C The input I1 is stuck ON, so the NC contact never closes.

D The NC contact is wired incorrectly and is always open.

💡 Hint

Think about how NC contacts behave and wiring errors.

If the NC contact is wired incorrectly, it may always be open, preventing current flow and keeping Q1 off regardless of I1 state.

🚀 Application

advanced

Using NC Contact for Safety Stop

You want to design a safety stop circuit in a PLC using a Normally Closed (NC) emergency stop button connected to input I2. Which ladder logic behavior correctly represents this safety stop?

A When the emergency stop button is pressed (I2 ON), the NC contact opens and the output coil controlling the machine stops energizing.

B When the emergency stop button is pressed (I2 ON), the NC contact opens and the output coil controlling the machine stops de-energizing.

C When the emergency stop button is pressed (I2 ON), the NC contact closes and the output coil controlling the machine energizes.

D When the emergency stop button is pressed (I2 ON), the NC contact closes and the output coil controlling the machine de-energizes.

💡 Hint

Emergency stop buttons usually break the circuit when pressed.

The NC contact is closed when the emergency stop button is not pressed, allowing the machine to run. Pressing the button opens the NC contact, breaking the circuit and stopping the machine.

🧠 Conceptual

expert

Effect of Multiple NC Contacts in Series

In a PLC ladder logic rung, three Normally Closed (NC) contacts connected to inputs I3, I4, and I5 are wired in series before an output coil Q2. If I3 and I5 are OFF, but I4 is ON, what is the state of Q2?

A Q2 toggles ON and OFF due to conflicting input states.

B Q2 is ON because two NC contacts are closed, allowing current flow.

C Q2 is OFF because the NC contact linked to I4 opens when I4 is ON, breaking the circuit.

D Q2 is ON because NC contacts ignore the ON state of inputs.

💡 Hint

Remember that NC contacts open when their input is ON.

Since I4 is ON, its NC contact opens, breaking the series circuit and turning Q2 OFF regardless of I3 and I5 states.

Practice

1. What does a Normally Closed (NC) contact do in a PLC circuit?

easy

A. It allows current to flow when the input is OFF.

B. It allows current to flow only when the input is ON.

C. It always blocks current regardless of input.

D. It acts like a fuse that breaks the circuit permanently.

Solution

1. Step 1: Understand NC contact behavior

   A Normally Closed contact is closed (conducts electricity) when the input is OFF.

2. Step 2: Apply to PLC circuit

   This means current flows through the NC contact when the input is not activated.

3. Final Answer:

   It allows current to flow when the input is OFF. -> Option A

4. Quick Check:

   NC contact closed when input OFF = B [OK]

Hint: NC contact is closed when input is OFF, open when ON [OK]

Common Mistakes:

• Confusing NC with Normally Open (NO) contact
• Thinking NC contact conducts when input is ON
• Assuming NC contact permanently blocks current

2. Which of the following is the correct way to represent a Normally Closed (NC) contact in a PLC ladder logic diagram?

easy

A. A coil symbol with a slash

B. A contact symbol without any slash or line

C. A normally open push button symbol

D. A contact symbol with a slash or line through it

Solution

1. Step 1: Identify NC contact symbol

   In ladder logic, NC contacts are shown as contacts with a slash or diagonal line through them.

2. Step 2: Compare options

   A contact symbol with a slash or line through it matches this description, while others represent different elements.

3. Final Answer:

   A contact symbol with a slash or line through it -> Option D

4. Quick Check:

   NC contact symbol has slash = C [OK]

Hint: Look for slash on contact symbol for NC contact [OK]

Common Mistakes:

• Choosing coil symbols instead of contact symbols
• Confusing NC with NO contact symbol
• Selecting symbols without slashes for NC

3. Given the following ladder logic snippet with an NC contact labeled X controlling output Y:

--[ /X ]--( Y )--

If input X is OFF, what will be the state of output Y?

medium

A. Output Y will toggle ON and OFF rapidly

B. Output Y will be OFF

C. Output Y will be ON

D. Output Y will be in an error state

Solution

1. Step 1: Understand NC contact behavior with input OFF

   NC contact is closed when input X is OFF, so current flows through it.

2. Step 2: Determine output state

   Since the NC contact is closed, output Y coil is energized and turns ON.

3. Final Answer:

   Output Y will be ON -> Option C

4. Quick Check:

   NC contact closed when input OFF means output ON = A [OK]

Hint: NC contact closed when input OFF energizes output [OK]

Common Mistakes:

• Assuming output is OFF when input is OFF
• Confusing NC with NO contact behavior
• Thinking output toggles without cause

4. You have a ladder logic rung with an NC contact controlling a motor start coil. The motor never stops even when the stop button is pressed. What is the most likely error?

medium

A. The NC contact is wired as a Normally Open (NO) contact by mistake.

B. The motor coil is wired backwards.

C. The stop button is stuck in the ON position.

D. The NC contact is not connected to the correct input.

Solution

1. Step 1: Analyze motor never stopping

   If motor never stops, the stop control is not breaking the circuit as expected.

2. Step 2: Check NC contact wiring

   If the NC contact is wired as a Normally Open (NO) contact by mistake, pressing the stop button will not open the circuit, and the motor will keep running.

3. Final Answer:

   The NC contact is wired as a Normally Open (NO) contact by mistake. -> Option A

4. Quick Check:

   Motor never stops means NC contact wired as NO = D [OK]

Hint: Check if NC contact is wired as NO if stop fails [OK]

Common Mistakes:

• Blaming coil wiring instead of contact wiring
• Assuming button is stuck without checking wiring
• Ignoring wrong contact type

5. In a safety circuit, a Normally Closed (NC) emergency stop button is used to control a machine. Which of the following best explains why NC contacts are preferred for emergency stops?

hard

A. Because NC contacts allow the machine to run even if the button is pressed.

B. Because if wiring fails or power is lost, the circuit opens and stops the machine safely.

C. Because NC contacts are cheaper and easier to install than NO contacts.

D. Because NC contacts keep the machine running continuously without interruption.

Solution

1. Step 1: Understand safety requirement for emergency stop

   Emergency stop must stop machine immediately and safely if pressed or if wiring fails.

2. Step 2: Explain NC contact advantage

   NC contact is closed normally, so if wiring breaks or power is lost, circuit opens and machine stops safely.

3. Final Answer:

   Because if wiring fails or power is lost, the circuit opens and stops the machine safely. -> Option B

4. Quick Check:

   NC contact fails safe by opening circuit = A [OK]

Hint: NC contacts fail safe by opening on wiring failure [OK]

Common Mistakes:

• Thinking NC contacts keep machine running when pressed
• Choosing cost or installation ease over safety
• Confusing NC with NO contact behavior