How do we wire emergency stop buttons? Watch this video to know the answer in just 2 minutes... Then read the other steps!
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Step 1: Correct Wiring Vs Wrong Wiring
Emergency stop buttons and switches are ALWAYS wired using "NORMALLY CLOSED" contacts not normally open ones.
Normally closed makes a continuous electrical signal to the control system (PLC/DCS) like +24v. So when anything happens this signal will disappear and PLC will recognize this as an emergency.
Events that trigger it:
1- User pushing the button: the obvious way.
2- a loose connection: this will make the signal come and go but as soon as it is gone the PLC will stop the circuit, so you could notice the error and fix it.
3- An open circuit: like cutting the wire on the road to PLC. this is the most dangerous one and we'll know why normally open fails at this in the next step.
Step 2: Why "N.O" Is Wrong?
let's consider a wire being cut... here if the user pushes the button, signal will never reach the PLC and thus won't recognize any difference than the "OK" status.... this is the reason why this fails as a safety feature!
So "N.C" is a must for safety emergency stop buttons and switches, while N.O can be used to give indication about the machine working or not.
Step 3: Where Does Emergency Button Is Connected Within Control System
It should never be connected via normal means, like directly to digital input card of a PLC. However, it MUST be connected to a safety relay.
A safety relay is the standard of connecting these types of equipment because of the high level of security that it offers.