The E-stop pushbutton on a PLC's master control relay circuit causes power to be removed from all the PLC's outputs. What contacts are used for E-stop pushbuttons?

The E-stop pushbutton is designed to ensure a rapid and reliable method of stopping all operations within a system by interrupting power to the PLC outputs. For this purpose, normally closed contacts are used because they allow current to flow through the circuit when the button is not engaged. When the E-stop is pressed, these normally closed contacts open, breaking the circuit and thus removing power from all outputs.

Using normally closed contacts in the E-stop function is critical for safety, as it provides a fail-safe mechanism. If the E-stop button fails or is pulled out, the contacts will remain open, preventing the machine from operating inadvertently. This design ensures that, in an emergency situation, the system can be stopped immediately by pushing the E-stop button, effectively cutting power and preventing further operation.

In contrast, normally open contacts would allow current to flow only when the button is pressed, which doesn't align with the safety objectives of an emergency stop feature. Relay and auxiliary contacts serve different purposes in a PLC system and are not specifically focused on the act of stopping machinery in an emergency. Therefore, normally closed contacts are the most appropriate choice for E-stop pushbuttons in a PLC master control relay circuit.