Normally closed (NC) contact in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a Normally Closed (NC) contact mean in PLC programming?

A Normally Closed (NC) contact means the contact is closed (allows current) when the input or condition is OFF or FALSE. It opens (breaks current) when the input is ON or TRUE.

beginner

How does a Normally Closed (NC) contact behave when the input signal is TRUE?

When the input signal is TRUE, the NC contact opens, stopping the current flow in the circuit.

intermediate

In ladder logic, what symbol usually represents a Normally Closed (NC) contact?

A Normally Closed (NC) contact is usually shown as a contact with a slash or a line crossing it, indicating it is closed when the input is FALSE.

intermediate

Why use a Normally Closed (NC) contact instead of a Normally Open (NO) contact?

NC contacts are used when you want the circuit to be active by default and stop only when a condition is met, like a safety stop or fail-safe design.

beginner

Write a simple ladder logic example using a Normally Closed (NC) contact to control a motor.

Example: If a stop button is NC, the motor runs when the stop button is not pressed (contact closed). Pressing the stop button opens the NC contact and stops the motor.

What happens to a Normally Closed (NC) contact when its input is TRUE?

A It opens and stops current flow

B It closes and allows current flow

C It stays closed regardless

D It toggles rapidly

An NC contact opens when the input is TRUE, stopping current flow.

Which symbol usually represents a Normally Closed (NC) contact in ladder logic?

A A normally open contact symbol

B A contact with a slash or line crossing it

C A coil symbol

D A simple open contact

NC contacts are shown with a slash or line crossing to indicate they are closed when input is FALSE.

Why might you use a Normally Closed (NC) contact in a safety circuit?

A To toggle the circuit randomly

B To keep the circuit always off

C To ensure the circuit is active unless a stop condition occurs

D To increase power consumption

NC contacts keep the circuit active by default and stop it when a condition like pressing a stop button happens.

If a stop button is wired as a Normally Closed (NC) contact, what happens when the button is pressed?

A The contact opens and stops the machine

B The contact closes and starts the machine

C Nothing changes

D The machine speeds up

Pressing the stop button opens the NC contact, stopping the machine.

In PLC ladder logic, what does a Normally Closed (NC) contact do when the input is FALSE?

A It triggers an alarm

B It opens and stops current flow

C It resets the PLC

D It remains closed and allows current flow

When input is FALSE, NC contacts stay closed, allowing current to flow.

Explain how a Normally Closed (NC) contact works in a PLC ladder logic circuit.

Think about what happens when the input changes from off to on.

Describe a real-life example where a Normally Closed (NC) contact is useful in automation.

Consider emergency stop buttons or safety switches.

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