Should The Emergency Stop Button in PLC Circuits Be Wired To Normally Open (NO) Or Normally Closed (NC) Contacts?

Emergency stop buttons are typically installed in areas where equipment operation is potentially hazardous. When danger is imminent, pressing the emergency button will break the control circuit, halting the equipment to ensure safety.

Emergency Stop Button

When the emergency stop button is pressed, it mechanically locks and does not reset automatically; it can be manually rotated to reset.

NC, NO Contacts

Emergency stop buttons usually have two sets of contacts, one set of normally open (NO) contacts (which are typically open) and one set of normally closed (NC) contacts (which are typically closed). Depending on the circuit requirements, you can also choose an emergency stop button with only one set of normally closed contacts or one set of normally open contacts.

In contactor and relay circuits, the emergency stop button is generally wired using the normally closed (NC) contacts in series with the control circuit.

Motor Start and Stop with Energy Consumption Braking

As shown in the diagram, the normally closed contacts of the emergency stop button SB2 are in series with the motor's start and stop control circuit, and normally open contacts are also used for energy consumption braking.

When the emergency stop button is pressed, the normally closed contacts open, the motor control circuit KM1 is broken, and the motor stops. The emergency stop button's normally open contacts close, KM2 is energized, and the motor winding is connected to a resistor, causing the motor to stop quickly.

Emergency Stop in PLC

Motor Start and Stop Ladder Diagram

When writing PLC ladder diagram programs, it is customary to refer to the circuit diagram. If normally open contacts are used in the circuit diagram, the same is written into the PLC ladder diagram program. If normally closed contacts are used in the circuit diagram, the same is written into the PLC program. When wiring externally for PLC, start, stop, overload, and emergency stop are all wired to normally open contacts connected to terminals X0-X3.

The advantage of this is that it is easy to understand, logical, and allows one to read both the circuit diagram and the PLC ladder diagram. There are no major issues with wiring start, stop, and overload externally with normally open contacts, but the emergency stop button is an infrequently used button, used only in emergencies. If wired with normally open contacts, if the contact terminal disconnection or the wire is broken, the machine will continue to operate, which is not easily detected. When the emergency stop button is needed, it may have already failed, which can easily lead to accidents.

When the emergency stop button is wired with normally closed contacts, if the contact terminal disconnection or the wire is broken, the machine cannot operate normally, and this fault is easily detected. Therefore, wiring the emergency stop button with normally closed contacts is safer than with normally open contacts.

Motor Start and Stop Ladder Diagram

Wiring the emergency stop button with normally closed contacts is simple; just change the emergency stop from normally closed to normally open in the PLC ladder diagram.

When the PLC ladder diagram program is changed to normally open for emergency stop, and the external wiring for emergency stop is wired with normally closed contacts, the external wiring for emergency stop in the closed state will send a signal to the PLC program, and the emergency stop signal in the program will close, allowing the program to run normally.

Wiring the emergency stop button with normally closed contacts is safer than with normally open contacts. In use, the emergency stop button should not serve as both an emergency stop and a stop button. The emergency stop button and the stop button should be used separately in the circuit.