Door Safety Switch, Should it be called NO or NC?

I find all of this hilariously vindicating. I mentioned this to my fellow controls engineer and he just look at my like I was an idiot and said "no it makes perfect sense to call it normally closed". He makes me want to :sick: myself sometimes.
 
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My comments, for what they are worth...

I have always believed that “normally open” should describe the status of a contact when its associated ‘actuator’ is not physically ‘forced’

But the text description/comment should always describe the result of actuating it. For example... a n/o push button to start the hydraulics will be drawn ‘open’ but described with what happens in its ‘on state’ eg “Hydraulics On” or “Start Hydraulics”

Safety switches have become a minefield of issues regarding application, connection diagrams and descriptions.

This could be debated and debated but I believe there will always be disagreement among engineers. Very divisive topic.

If I had my way, a safety gate type contact should be drawn in its open state (ie when the ‘gate is open’ and switch not actuated) but its description (relating to its signal) should be “Safety’s Gate Closed”)

As mentioned earlier, device manufacturers appear to have hijacked the description but have not taking into account the historic usage of the n/o and n/c terms.
 
If the contacts are mechanically actuated, then N.C.
Argument: For mechanical contact is a safety circuit, they must be forcibly actuated when changing to the safe state (opening of the door).
You usually have this forcible actuation only in one direction, so it is always chosen in the safe direction.
It is practically impossible that the contacts cannot be forced open when opening the door, even if they are welded, so the safety is maintained in this case. If in theory they could get stuck in the closed position that they cannot be opened, then the door also cannot be opened.

If activated magnetically, as is typical with many safety switches with magnet actuation, then N.O.
The contact symbol in the documentation must indicate that the actuation is magnetic.
 
*(if you really want to see me get worked up about normally closed and safety, ask me why safety contactors come with so many normally closed auxiliaries)

Why do safety contactors come with so many normally closed auxiliaries?
 
Why do safety contactors come with so many normally closed auxiliaries?

Some n/c contacts can be used in a feedback loop to test that the contactor(s) have physically ‘dropped out’ when de-energised. This would be monitored by the master safety relay.
 
This is one of my biggest pet hates*. When an electrical component is described as "Normally x", this should always describe the off, de-energised, unactuated, (usually) out-of-the-box state of the contact. That's how electrical devices have always been. It's how they're (supposed to be) drawn. If it's a pushbutton, you draw it and describe the state when the pushbutton is not pressed. If it's a limit switch, you draw it and describe the state when the limti switch is free to spring to whichever is its "neutral" position and it's not actuated by any other object. If it's a pressure switch, you describe the state when no pressure is applied to the switch (or more accurately, when atmospheric pressure is applied to the switch). If it's a tongue-and-groove door switch, it's the state when the switch is not actuated by the tongue. If it's a coded magnet switch, it's the state when the switch not actuated by its coded partner.

.......

When I draw something, I draw it in the non-actuated, de-energised, out-of-the-box state. Always. I will fight anyone who says that my guard switch has normally closed contacts. It ******* does not.

........

100% agree.

"Normal" in the context of contact status has NOTHING to do with the status of the contact during so-called normal operation.
 
I 100% agree with the standard (old fashioned?) definition of normal. This reminds me of the old joke about Microsoft or IBM who were so big that they could invent their own standard. I forget the most of it but the punchline is "……..they just changed the standard to dark."


I think the same thing applies here.
 
Gomez_ said:
I'm with you on on everything except this. If you have safety doors on your machine, do you normally run with them open?
"Normally" has nothing to do with the normal running state of the machine. The sensor has no concept of your machine. It doesn't know or care what your machine is doing. It doesn't know if it's on a $2b NASA spacecraft or a cat flap in my back yard. "Normally" is with reference to the device itself. What is the normal state of the contacts on a non-contact guard switch, before you actuate it by placing an actuator next to it? Open. Normally open

retirednow said:
I have always believed that “normally open” should describe the status of a contact when its associated ‘actuator’ is not physically ‘forced’

But the text description/comment should always describe the result of actuating it. For example... a n/o push button to start the hydraulics will be drawn ‘open’ but described with what happens in its ‘on state’ eg “Hydraulics On” or “Start Hydraulics”
100% agree. Draw it in the normal (unactuated) state, and describe what it does when actuated. Your mention of hydraulics does raise the other interesting point that hydraulics and pneumatics are (as far as I'm aware) always drawn in the "energised" or "actuated" state. Which is a useful thing to know for anyone playing both sides of the court.
JesperMP said:
If the contacts are mechanically actuated, then N.C.
Argument: For mechanical contact is a safety circuit, they must be forcibly actuated when changing to the safe state (opening of the door).
I see your point, but I still disagree. I would counter that a mechanical guard switch is actuated by the guard closing against it, and that it's exactly the same concept as a magnetic switch being actuated by an magnet closing against it. Think of a simple limit switch on a guard. It's actuated by the guard closing against it, and when the guard is removed, it springs back to the open (safe) position. Of course, you usually wouldn't *actually* use a bog standard limit as a guard switch, but the concept applies just the same. A tongue-and-groove switch is actuated by the tongue. Remove the actuator (tongue) and the contacts open.
Rson said:
Why do safety contactors come with so many normally closed auxiliaries?
:angr:
retuirednow said:
Some n/c contacts can be used in a feedback loop to test that the contactor(s) have physically ‘dropped out’ when de-energised. This would be monitored by the master safety relay.
Quite right. And how many master safety relays do you have, in a typical panel? I know there are certainly instances where you might have several safety relays, but how many master safety relays, that actually drop out (and thus need to monitor) a particular safety contactor do you have? Right, so in 99% of cases we'll need one NC auxiliary. What do we need the other three for?
Let me put it this way. What is the safe state of a safety contactor? De-energised. What is the safe state of a solenoid, pump, fan, heater, or just about anything else? 99% of the time, it's de-energised. So if you want your solenoids and pumps to enter a safe (de-energised) state when your safety contactors are in a safe (de-energised) state, what type of contact do you need on your safety contactor?

Tom Jenkins said:
"Normal" in the context of contact status has NOTHING to do with the status of the contact during so-called normal operation.
🍻
 
JesperMP said:
If the contacts are mechanically actuated, then N.C.
Argument: For mechanical contact is a safety circuit, they must be forcibly actuated when changing to the safe state (opening of the door).
I see your point, but I still disagree. I would counter that a mechanical guard switch is actuated by the guard closing against it, and that it's exactly the same concept as a magnetic switch being actuated by an magnet closing against it.
Nope, using a mechanical limit switch in that way described above would be unsafe (!!), exactly because the opening of the contacts would be by spring force, and not by the force opening the door.
When you use mechanical limit switches for a safety function, there must be a cam that forcibly actuates the limit switch when it shall open the safety circuit.

About tongue-and-groove safety switches:
The tongue actuates a tumbler inside the switch. The tumbler has a cam that actuates the contacts. the tumbler is forcibly actuated in both directions. But the tumbler only forcibly actuates the contacts in one direction.

Here is an example of a tongue-and-grrove switch with variants:
https://cdn.automationdirect.com/static/specs/ideminchsafetyswitch.pdf
On page 6, notice the circle with the arrow inside next to some of the contacts. That symbol indicate which contacts are forcibly acatuated. So only these contacts may be used in the safety circuit. Other conacts can be used for signalling purposes for example.
Also notice that the contact symbol for the forcibly actuated contacts are all N.C.

forcibly_actuated_contacts.png
 
Another NC/NO issue it took me years to get used to was the contacts on a motor overload.

To me a NC contact would open when the overload was set and close when it tripped, and a NO contact would close and let me know everything was OK. I ended up Ohming the contacts to see which one I wanted.

I seem to remember some manufacturers had this same problem, as on one panel I would use the NO contact and on the next it would be the NC contact.
 
Another NC/NO issue it took me years to get used to was the contacts on a motor overload.

To me a NC contact would open when the overload was set and close when it tripped, and a NO contact would close and let me know everything was OK. I ended up Ohming the contacts to see which one I wanted.

I seem to remember some manufacturers had this same problem, as on one panel I would use the NO contact and on the next it would be the NC contact.

Agreed.

An Overload Relay typically has a N/C contact for use in series wit the contactor coil.
However, if using a Motor Circuit Protector (could be described as a combination Circuit Breaker and Overload Relay) has a N/O contact which closes when the MCP is turned to ON.

ASF: When an electrical component is described as "Normally x", this should always describe the off, de-energised, unactuated, (usually) out-of-the-box state of the contact.

Absolutely!

OkiePC: In the olden days, we called that type of switch NOHC (Normally open, held closed) and drew it like:

Yep, just like Timers, NO-TC, NC-TO, etc... or even better, N/O, Delay on Break, N/C Single-Shot, etc. ... The symbols can not always be trusted, like the Single-Shot Timer for example has no JIC symbol that I am aware of.

But, back to the point. De-Energized / Un-Activated, out of the box is my opinion for how they should be drawn, then described as to their function.
 
"Draw it as it comes out of the box".
Not a bad way to define the state it should be drawn, except .... at what state is the tongue-in-grove switch when it comes out of the box ?
It could be with the tongue in place in the lock, which would be N.C.
Or it could be without the tongue in the lock, which would be N.O.
It is not clear at all !

But seriously, use the manufactureres definition. Notice how they all use the same symbols. I wonder why ......
 
I would agree with the "Drawn N.O." camp. It make sense that the contact should be drawn in the "safe" or de-energized state. An Estop is safe in the open position. This is how I have been drawing them for years.
 

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