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

It make sense that the contact should be drawn in the "safe" or de-energized state.
Contradictory arguments when used on E-stops as you do yourself.
Most safe is contacts open.
De-enegized state is closed (i.e. no-one is pushing the button).

This is how I have been drawing them for years.
I dont know about USA, but over here there is absolutely no discussion for E-stop buttons with contact that must be opened when activating the button. These must be drawn as N.C. They always comes with "N.C." written on the terminals, and use the N.C. standard terminal numbers (11-12, 21-22, etc.).
 
I don't think you read my OP.


As I stated, the signal is on when the door is closed I am talking about what to CALL the contacts of door switch. I think it makes more sense to call them normally open, rather than normally closed.

I guess you're right in that i didn't specify what the devices is called, i was saying way it is the way it is. Sorry for the confusion.

In my experience, devices are "called", or named or labeled or drawn in their de-energized on the shelf state regardless of how the circuit is held in its "normal" operational state. In this instance i would imagine the switch would be NO as its normally open when not acted on. If you wanted to get more specific it would be NO/HC as others have said, because its normally open when de-energized but held closed in its normal operational state.
 
Is my understanding correct here:

"Normally Open Held Open --->Un-actuated/ De-energized State"
"Normally Open Held Closed ---> Actuated/Energized State"
"Normally Closed Held Open ---> Actuated / De-energized state"
"Normally Closed Held Closed ---> Un-acuated / Energized State"

But this is the old fashioned way, so what is the new standard?
 
Last edited:
Originally Posted by testsubject View Post
It make sense that the contact should be drawn in the "safe" or de-energized state.

Contradictory arguments when used on E-stops as you do yourself.
Most safe is contacts open.
De-enegized state is closed (i.e. no-one is pushing the button).

Its safe when the machine cannot run. The machine cannot run because the contacts on the switch are open and power is not being fed back to the safety relay.
I think we're all saying the same thing. just looking at it from different angles.

Your saying machine is safe to run is safe. We're saying machine unable to run is safe.

Unable to Run is better. Its the Standard. Thus it shall be. This is also why I try not to use European parts in our facility.


The N.C. contacts on safety relays are feedback or secondary safety device circuits. I recently put a machine together that has 3 different safety levels. E-STOP/gate limit stops all (Master). Operator area limit switch disables immediate area but not entire machine (Secondary) and limit switch for a third area.(2nd secondary) (Thirdary?)
Safety circuits are intimidating at first but once you get through it they make sense.
 
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.
You're right, that's why I put my disclaimer that you wouldn't normally use a mechanical limit switch in that way. If you're using a mechanical limit switch in the way you describe - where it's mechanically actuated when the guard is opened - then absolutely, you need a normally closed switch. Switch physically actuated by the opening of the guard - switch open, safety circuit broken. Guard closed and switch released to be de-actuated - switch closed, safety circuit complete. Contacts closed when de-actuated >> normally closed.

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.
Tongue and groove switches *are* a little more fuzzy because as you quite rightly point out, the tumbler is actuated in both directions by the tongue. When the tongue is inserted it forcibly pushes the cam one way; when the tongue is removed it forcibly pushes it the other. So you *could* argue that there's no "normal" state for this switch seeing as it doesn't inherently return to either position without the intervention of the tongue. But, I would counter-argue, if that's the case surely it makes the most sense to consider a switch "actuated" when the actuating piece has been physically inserted? As manufactured, the switch contacts are open. If you want to close them, you have to actuate them with the switches actuator (tongue). Therefore, normally open. I know they're labelled normally closed, and I 100% disagree. I also know that (as you mention in another post), out-of-the-box the contacts are closed, because they ship with the tongue installed. That's why in my first rant on the topic, I said "off, de-energised, unactuated, (usually) out-of-the-box state". Some devices are shipped in the actuated state, so "out of the box" state is not *always* true. A tongue and groove switch is manufactured with the contacts open, then actuated by inserting the tongue, then put in the box and shipped out.

Definitely out of all the possible scenarios this one is the one most able to be argued from either side, but I've chosen sides and this is the hill I will die on* :whistle:


*(hopefully not due to being caught in a machine with a poorly designed safety circuit)
 
Last edited:
My one cent. For the longest time I thought that normally open sounded better for allowing the current to pass threw like walking threw an open door. And normally closed sounded better as cutting off the current kinda like closing a door so it can't allow anything to flow but I know better not Lol.
 
LOL... Exactly as this 'long toothed' member indicated in post #17 !

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.

But as someone else mentioned, I believe most engineers know how to use these devices but the contradiction is in the description of contacts.

Signal Descriptions
Regarding my mention of, for example "Start Hydraulics" push-button, where I said the description (of function) should describe the actuated state, I have found other confusing issues when 'info only' signals from E.Stop or guard circuits are documented in PLC input listings and code.

I believe any PLC input signal should describe the meaning of the 'Logic 1' state, but have come across many descriptions of E.Stop input signals poorly described, some just 'symbolized' or described as "Emergency Stop" when in most cases, if derived from a safety switch N/O contact, should be described as "Not E.Stop" or "E.Stop OK" or similar.

Of course, if from a N/C contact from the safety relay, then it should be described something like... "Emergency Stop Activated" or "E.Stop State" or "E.Stop NOK" or similar.

This poorly documented description problem often doesn't end there...
With inputs often documented as...

"Safety Gate"
"Coolant Tank Level Switch"
"Hyd Pressure Switch"
"Over-travel Switch"

Describing the connected device instead of the function.

All having different meanings depending on the logic state of the input, and how the device is wired.

I have often found this to be a problem in PLC code created by someone whose first language may not be English, and the necessary adverbs or adjectives are omitted or used incorrectly. (But sadly, sometimes found in code created by engineers whose first language is English!)

Perhaps missing...
"... Closed"
"... Open"
"... High"
"... Low"
"... OK"
"... NOK"
"... In"
"... Out"
"... On"
"... Off"

<Rant over! :)>
Hope this hasn't gone 'off topic' too much.
 
My one cent. For the longest time I thought that normally open sounded better for allowing the current to pass threw like walking threw an open door. And normally closed sounded better as cutting off the current kinda like closing a door so it can't allow anything to flow but I know better not Lol.

Yes Greg, can be confusing, if describing Pneumatic or Hydraulic valves then it can change.

When a valve is ON it may well be 'open' and passing fluid.
When a valve is OFF it may well be 'closed' and NOT passing fluid.

When a switch is ON it normally means the contacts are 'closed' and passing current.
When a switch is OFF it normally means the contacts are 'open' and NOT passing current.

o_O
 
LOL... Exactly as this 'long toothed' member indicated in post #17 !



But as someone else mentioned, I believe most engineers know how to use these devices but the contradiction is in the description of contacts.

Signal Descriptions
Regarding my mention of, for example "Start Hydraulics" push-button, where I said the description (of function) should describe the actuated state, I have found other confusing issues when 'info only' signals from E.Stop or guard circuits are documented in PLC input listings and code.

I believe any PLC input signal should describe the meaning of the 'Logic 1' state, but have come across many descriptions of E.Stop input signals poorly described, some just 'symbolized' or described as "Emergency Stop" when in most cases, if derived from a safety switch N/O contact, should be described as "Not E.Stop" or "E.Stop OK" or similar.

Of course, if from a N/C contact from the safety relay, then it should be described something like... "Emergency Stop Activated" or "E.Stop State" or "E.Stop NOK" or similar.

This poorly documented description problem often doesn't end there...
With inputs often documented as...

"Safety Gate"
"Coolant Tank Level Switch"
"Hyd Pressure Switch"
"Over-travel Switch"

Describing the connected device instead of the function.

All having different meanings depending on the logic state of the input, and how the device is wired.

I have often found this to be a problem in PLC code created by someone whose first language may not be English, and the necessary adverbs or adjectives are omitted or used incorrectly. (But sadly, sometimes found in code created by engineers whose first language is English!)

Perhaps missing...
"... Closed"
"... Open"
"... High"
"... Low"
"... OK"
"... NOK"
"... In"
"... Out"
"... On"
"... Off"

<Rant over! :)>
Hope this hasn't gone 'off topic' too much.
🍻


OFF TOPIC: I also have a pet peeve where engineers call inputs "extend" or "retract". Instead of "extended" or "retracted". "extend" or "retract" is a verb and describes an output, an input which should be an adjective. Close and Open verse Opened and Closed. Even better put an "Is" prefix on input names.
 
🍻


OFF TOPIC: I also have a pet peeve where engineers call inputs "extend" or "retract". Instead of "extended" or "retracted". "extend" or "retract" is a verb and describes an output, an input which should be an adjective. Close and Open verse Opened and Closed. Even better put an "Is" prefix on input names.

Totally agree! :)
 
🍻


OFF TOPIC: I also have a pet peeve where engineers call inputs "extend" or "retract". Instead of "extended" or "retracted". "extend" or "retract" is a verb and describes an output, an input which should be an adjective. Close and Open verse Opened and Closed. Even better put an "Is" prefix on input names.

I have always used 'AT' and 'TO'

'AT' is for sensors/inputs that tell you where you are ex: AT HOME
'TO' is for motion/outputs that tells you where to go ex: TO WORK
 
I have always believed that “normally open” should describe the status of a contact when its associated ‘actuator’ is not physically ‘forced’
I do not completely agree with this.
If you rephase it to
"the symbol shall describe the status of the contact when it is not physically forced"
then I agree 100%. It is not the button or actuator that is the key, it is the contact.
That and that it is not physically forced. It is only the physically forced direction that counts.

To visually explain it in another way, you take a set of contacts, and then you add arms and levers and cams and whatever to activate the contacts.
The contacts do not change their nature by what has been added to them externally.

Apart from that, I have regretted entering this discussion in the first place. It is too much effort for such a nerdy topic.
Actually, I think everything is OK, whatever symbol you choose, as long as the meaning is clear.
Final word, if someone sees N.O. contacts numbered 11-12, they might get confused. Take that into account.
 
If you rephase it to
"the symbol shall describe the status of the contact when it is not physically forced"
then I agree 100%.
That is exactly what I meant. Thanks for the correction.

Just to clarify, for example, if it is a normally open push button then the ‘graphical symbol’ in the wiring schematic will show ‘open’ contacts, with a text description for the PB describing its operated function (when the contacts have closed) And the ‘PLC description/symbol’ should describe its ‘logic 1’ function.

Also, if for example, it were a ‘normally closed’ PB as an input to “Stop Conveyor” (wired to ‘fail safe’) then...

PB will be labelled “Stop Conveyor”
But within PLC code, the description of the input should elude to the fact that ‘logic 1’ is ‘not Conveyor Stop’ Perhaps “*Conveyor Stop” (using the preceding asterisk to mean NOT)
Or described “NOT Conveyor Stop” Otherwise reading the logic during diagnosis can become very confusing.
 
Last edited:

Similar Topics

Our system has S7 1516 F CPU. We have doors that have contactless safety doorcontact that is cat 4 ple rated. The sensor provides us with two...
Replies
7
Views
2,943
If I have a cell where the doors are monitored with a sufficient level of guarding, is it required to run the locking signal to the door on a...
Replies
4
Views
1,295
Hello gentlemen, I had a discussion with the manufacturer of a pneumatic overhead door open that intrigued me and wondering if the information...
Replies
0
Views
1,385
Hello, We have a Stäubli RX130 robot arm in an enclosed cell with a safety door to be used for educational purposes and we are in the...
Replies
4
Views
4,024
Hello Everbody. We are going to be performing a revamping project on an existing plant. In the mean time we are going to upgrade the "old" Pilz...
Replies
0
Views
2,823
Back
Top Bottom