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Why Do Emergency Stops Usually Adopt Normally-Closed Contacts?

by John Ford 13 Oct 2023

An emergency stop button, as the name implies, is an important safety measure that allows an individual to quickly activate it in the event of an emergency. In many industrial environments, you will find prominent red buttons labeled "Emergency Stop" in red text, collectively referred to as E-STOP buttons. Pressing this button directly stops the entire machine or immediately disconnects specific drive components. To restart the unit, simply release the button and rotate it approximately 45° clockwise so that it springs back - essentially "releasing" it.

(1) With regard to response time, the time interval between closing and opening a normally closed contact is considerably shorter than the time interval required for a normally open contact to transition from its natural state to the closed position. You may ask, "How much shorter? In an emergency shutdown, even the smallest duration is important. Even milliseconds or microseconds can be of great significance in such critical situations. Remarkably, accidents happen in the blink of an eye. Failure to use normally open contacts to stop a machine or disconnect a switch in such a brief moment can result in serious machine damage, personal injury or even death. The use of normally closed contacts ensures a faster response time and reduces the likelihood of causing injury.

(2) We know from the pushbutton mechanism that emergency stop buttons, whether they are normally closed or normally open contacts, will bounce back if they are not pressed into place (pressed to the bottom), thus disabling the action. With a normally open contact, if the button is not pressed all the way down, E-STOP will not work (because the normally open contact stays open). However, if a normally closed contact is used, the E-STOP function will function as long as the pressing action starts, regardless of whether the button is fully pressed or not.

(3) With regard to its control circuit, the main purpose of the emergency stop button is to stop the machine quickly in an emergency to prevent mechanical or personal accidents. Over time, the operation of the machine may cause wear and tear, especially on the emergency stop circuit. If a malfunction occurs when a normally open contact is used and the circuit is disconnected, a malfunction in the emergency stop section may go unnoticed until it is too late. Conversely, if normally closed contacts are used, the worst that can happen when the emergency stop circuit fails is that the machine stops with relatively little damage. Therefore, when designing an electrical control system for an emergency stop button, it is recommended that normally closed contacts be used.

In summary, the emergency stop button stands as a critical guardian of safety in industrial settings. Prioritizing swift response and reliability, the use of normally closed contacts is the wise choice. Safety should always come first, as the emergency stop button plays a pivotal role in preventing accidents and protecting lives and machinery.

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