Why Normally Closed Contact is used for Emergency or Stop Push Button

Why do we use a normally closed push button as a stop PB?

• Usually, this stop PB stays in closed condition; it should be operated  to make the machine stop or halt. 
• If it is in NO state, then the machine may run in the event of open circuit in the wire due to lose connections or breakage.  .
• Usually, we use two kinds of Digital Inputs to design an electrical circuit using a PLC or any relay such as 

a. Normally Open (NO) contact,

b. Normally Closed (NC) contact. 

• But the most important thing is everyone must know about the meaning and exact difference between these two switches.
• The Stop Push Button is a Normal Push Button to stop a process, it can be used as an Emergency      Push Button.

What do you mean by Normally-Open Contact?

• Normally-Open Contact is a type of contact when relay gets powered up it closes      the respective circuit and permits the flow of electricity through it.
• This switch opens itself and restricts the flow of electricity through it when relay gets de-energized.
• Observe the circuit shown below, 

• In an open condition, when the relay gets energized, the contact gets closed to permit the current from the source to the lamp making the lamp glow. 
• The lamp gets turned off when the contacts get opened, & the relay      is now de-energized. 

What do you mean by Normally-Closed contact?

• Normally-Closed Contact is a type of contact when relay gets powered      up; it opens the respective circuit and blocks the flow of electricity through it.
• This switch closes itself and permits the flow of electricity through it when relay gets de-energized.
• Observe the circuit shown below

• In a closed condition, when the relay gets energized, the contact gets opened to restrict the current flow and doesn’t allow the lamp to glow. 
• The lamp gets turned on due to the flow of current when the contacts get closed, & relay is de-energized.  

In the above section we have studied the basic concepts. Now let us see.

Why we must use Normally-Closed Contacts for Stop Push Buttons?

In this section, we will consider three essential elements.

1. Start Push Button 
2. Stop Push Button 
3. Electrical Motor

Consider an ideal circuit shown in the image below.

In this circuit, 

1. The Start Push Button (PB) is normally open.
2. The stop push button (PB) is normally closed.
3. An imaginary motor is triggered through the relay device

• An electrical power supply is allowed to flow from start PB to Stop PB and then to relay to trigger when this Start Push Button is pressed. 
• The relay contact gets closed & the circuit is latched.  
• Here the power continues to flow from the relay contact even after releasing the Start Push Button.
• Now, we need to press the Stop Push Button to terminate or restrict the power flow.
• On pressing the Stop Push Button de-latch relay gets opened to stop the flow of the power supply as both relay contacts, & the start push button are in the open state.
• The stop push buttons are mechanical type switches, these switches behave like a closed switch.
• When this switch is pressed the circuit gets opened and the power flow is restricted 

Using a stop Push Button as a Normally Closed 

Here the switch provides a better advantage.

• If the pushbutton malfunctions,the contact behaves as open so that there is no power flow. 
• This makes the operator troubleshoot the system. This makes the motor remain safe in off condition due to improper stop push button      functionality.

Why is the emergency switch in electrical circuits wired with NC contacts? 

Normally Closed (NC) contacts are commonly used for emergency or stop push buttons in industrial and safety applications for several reasons:

Safety

• The main reason for using NC contacts is safety. When the emergency or stop push button is at rest (not pressed), the NC contacts are closed. 
• In this state, the circuit is complete, allowing current to flow through the control system and keep the machinery operational. 
• If there is any fault or emergency situation, a person can simply press the button, which will break the NC contacts, and the circuit will open, immediately cutting off power to the machinery and stopping it. 
• This ensures a quick and reliable response to emergencies, preventing potential accidents or injuries.

Fail-safe design

• NC contacts have a fail-safe design. 
• If there is any open circuit due to wire breakage or cut when using NC contact in a circuit, the circuit naturally opens and prevents the passage of current in the circuit and addresses the failure scenario and keeps machinery in safe condition.

Standardization 

• The use of NC contacts for emergency or stop functions has become an industry standard and is often required by safety regulations and standards. 
• Standardization helps ensure consistency and makes it easier for operators and maintenance personnel to understand the functionality of the push button.

Logical operation

• The use of NC contacts is intuitive for operators. Pushing the button causes it to break the circuit, which is a natural response in an emergency situation. 
• This simplicity reduces the chance of human error during critical moments.

Troubleshooting 

• If there is a problem with the control system, NC contacts that are stuck open will prevent the machinery from starting, alerting maintenance personnel to investigate the issue before the equipment is operated again.

Why the Emergency Button is in Normally Closed Condition?

• During electrical control system design normally closed contact is selected and considered as best for emergency stop.
• The machine stops working if and only if there is a failure of the emergency stop part of the line with minimum loss.

What are NO and NC in the push button?

• Basically, a switch is provided with single input and a single output. 
• The switch may be Normally Open (NO) or Normally Closed (NC) type.
• NC/NO switch consists of single input and double outputs with two consequent actions.

What are a Stop Button and an Emergency Button?

• A general-purpose switch is a spring-return type switch used to start and stop the respective machine from a functional point of view.
• But an emergency stop button is considered a safety device to stop harmful activities to safeguard the plant equipment, & personnel safety.

What is the difference between NO and NC contacts?

• NO contact is Normally Open, this closes when the coil is energized. 
• NC contact is Normally Closed, this opens when the coil is energized. 

Why Emergency Stop Button is in red color?

• Basically, we all know that an emergency stop push button breaks the power supply in case of any hazardous moment.
• These emergency      stop push buttons are coated in red color so that it can to be identified immediately during any emergency.