1. Action time
The time from closed to disconnected of normally closed contact is much shorter than the time from natural state to closed of normally open contact, which is particularly important when emergency stop is required in the event of danger, and no matter how short the time, even if it is millisecond or even microsecond, it is very important, and many accidents occur in a very short time.
In such a short period of time, the use of normally open contacts may not stop the machine or disconnect the switch, which can cause major machine damage or personal injury. However, the use of normally closed contacts may stop the machine or electrocute people for a shorter time, avoiding such accidents.
2. a button mechanism
The emergency stop button, regardless of whether it is a normally closed contact or a normally open contact, will spring back up if it is not pressed into place (pressed to the end), thus invalidating the operation. Therefore, there are many occasions that require an emergency stop button with a self-locking function.
When using the normally open contact, if the emergency stop button is not pressed in place, the emergency stop does not play any role (because the normally open contact is not closed); However, when using normally closed contacts, no matter whether the emergency stop button is pressed in place or not, the emergency stop will work only after the contact action.
3. Control the line
The most basic function of the emergency stop button is to make an emergency stop in an emergency to avoid mechanical accidents or personal accidents. However, due to the long-term operation of the machine, the line, especially the part of the line, may cause a fault to break the circuit.
At this time, if the emergency stop button uses the normally open contact, the line fault of the emergency stop part will not be detected, and if the emergency stop button is used, it will be too late. When using normally closed contacts, when the line of the emergency stop part fails, it will only cause the machine to stop at most, and the loss is relatively small.
Therefore, when designing the electrical control system, it is best to use normally closed contacts for the emergency stop button.
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