Standard reference for estop NC/NO switches

[rroze002]

Just as the title says, I’m looking for a reference as to why estops are normally setup with two separate contacts, one NC and another NO. Any pointers would be appreciated.

Thanks

 

[arpus4KM]

Just as the title says, I’m looking for a reference as to why estops are normally setup with two separate contacts, one NC and another NO. Any pointers would be appreciated.

Thanks

These days the standards are a little different than they used to be.

Most people previous to the last 5-6 years or so would use the NC for the safety circuit, and the NO for an input to identify which E-stop was pushed in.

Nowadays you're more likely to see 2NC and 1 NO. the 2 NC for dual channel safety, and NO for the PLC input for feedback.

 

[Rob...]

Now Safety PLCs are becoming increasingly popular. Usually no need for the NO contact for PLC feedback.

Just 2 NC contacts for duel channel support.

 

[Bit_Bucket_07]

These days the standards are a little different than they used to be.

Most people previous to the last 5-6 years or so would use the NC for the safety circuit, and the NO for an input to identify which E-stop was pushed in.

Nowadays you're more likely to see 2NC and 1 NO. the 2 NC for dual channel safety, and NO for the PLC input for feedback.

Even back in the old days, I always used a 2NC PB contact for E-Stop buttons. There's no good reason for the PLC input to be on a NO PB circuit.

 

[arpus4KM]

Even back in the old days, I always used a 2NC PB contact for E-Stop buttons. There's no good reason for the PLC input to be on a NO PB circuit.

Sure there is. On large systems with 30 doors for access and 10 Estops, it's very useful to know which door is estop is keeping the machine from restarting. Some systems I work on span multiple buildings and floors.

 

[Bit_Bucket_07]

Sure there is. On large systems with 30 doors for access and 10 Estops, it's very useful to know which door is estop is keeping the machine from restarting. Some systems I work on span multiple buildings and floors.

A PLC can read an input's off state as easily as it can read it's on state. Wiring the circuit through a NC contact is more reliable. If you lose control power on the PB you'll get an alarm.

 

[GaryS]

The answer to your question is simple
E-Stop require dual channel monitored circuits to prevent tampering with the controls
I have seen more than a few that were tamper with to prevent the button from working properly
If one channel is bypassed the safety monitor relay will not allow the relay to be reset so the it a failsafe control they can be 2 channel with both set of contacts N.O held closed by the button or as some designs are 2 channel with one channel N.O. held closed the other channel in N.C. held open and they have to change state within a few ms of each other or the safety relay will not reset
Also keep in mind E-Stops are not to be used as a normal stop button they are for emergency stop only
It is important that all safety controls are evaluated carefully on every job your life may depend on it.

 

[joseph_e2]

Also look at so-called "self monitoring" contact blocks. I've seen a dangerous situation in an aluminum casting operation where the standard NC contact vibrated loose which kept the system from shutting down correctly when the button was pushed. There was a backup where the PLC's NO contact worked and the furnace went down but it took longer than the e-stop should have taken and could have been very bad. It was a very near miss. The self monitoring blocks open when the contact block is detached from the button OR when the button is pushed and would have prevented that issue. .


If you have multiple e-stops (or door switches) wired in series (suitable for up to category 3 architecture, which may be sufficient based on the risk assessment), then definitely wire in the aux contact. If they're individually wired back to a safety controller (far preferred), the aux isn't necessary.

 

[roxusa]

I wire every E-Stop, Gate, Pull-Cord, etc. 2 wire N.C. to its own Safety Monitoring Relay set up for auto reset and then do a dual loop off each Safety relay to the Main Safety relay Monitored Manual Reset. This way I use the Safety relay from each item for my PLC
to monitor Ch-1, Ch2. This will tell me which Item is Set and also if it has a single contact failure. Kind of elaborate but Safety is an easy sell.

 

[rroze002]

Also look at so-called "self monitoring" contact blocks. I've seen a dangerous situation in an aluminum casting operation where the standard NC contact vibrated loose which kept the system from shutting down correctly when the button was pushed.

This scenario is what I was thinking about (I probably read something like this online). Great example. Thank you.