I feel I have a pretty good understanding of NO/NC. Simply put NC contact is open "light is on". NO contact is closed "light if off". My problem is trying to recognize and understand when a device is NO/NC held open. Any and all helpful tips are appreciated.
NO/ NC
- Thread starter binney
- Start date
- Status
- Location
- Massachusetts
NO contact = Normally Open = circuit open = light off.
NC contact = Normally Closed = circuit closed = light on.
But before we go further what is the exact device you are thinking of?
hbiss
EC, Westchester, New York NEC: 2014
- Location
- Hawthorne, New York NEC: 2014
- Occupation
- EC
He want's to know what "normally" is. Answer is there is no clear answer because it depends on the device and/or how you install it. Example: a 3 way (SPDT) toggle switch. Which screw is NO and which is NC?
-Hal
-Hal
- Location
- Massachusetts
That is why I asked for what specific device he was asking about.
- Occupation
- Licensed Electrician
An example: I am looking at a drawing which has multiple NC push button switches in it. The difference is that some of them are touching on the top side and some of them are touching from the bottom side. I believe the the one's that are closed from the bottom side are NC, and the ones where the switch is closed from the top side are NO held closed. I can't grasp how a push button switch can be held closed, or even the concept of help open or closed.
goldstar
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
I agree but the reason is that there are some devices, when installed, change the state of the switch for normal operation of the system. For example, when you install a switch on an OS&Y gate valve for a sprinkler system to monitor the valve's position it has an on-off-on switch (or off-on-off depending on how you look at it) and is installed in the device's center position. That way if someone turns the valve it will trigger, if someone removes the switch cover it will trigger and if someone rips the switch off the valve it will trigger. Now, when you're wiring the switch the nomenclature written on the switch may indicate NO_C_NC but you actually have to use a VOM to determine the proper operation for the system.
mgookin
Senior Member
- Location
- Fort Myers, FL
Can you snap a picture of the drawing with your phone and post it? I'm not quite following you.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
I think the OP is probably looking at something like a start/stop switch and does not understand the logic. I am on phone, it would be great if you could post a pic and explain it.
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Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
A couple of people have stated this. jusme123 stated is succinctly so I will quote him. "contacts are shown in the de-energized state (shelf state)"
My question back to you is, "Do you understand this concept for the switch you are questioning?" Especially "shelf state"? The state it is in when sitting on a bench. It may be difficult sometimes figuring out the normal state when something is installed, but you can usually figure it out. Sort of like figuring out the meaning of a word using its context.
K8MHZ
Senior Member
- Occupation
- Electrician
When I was a first year apprentice, my J-man showed me the difference between NO, NC and NO held closed and NC held open. He told me he knew they wouldn't teach this in the apprenticeship.
He was right, they didn't . But, on our 5 year final exam, the extra credit question was about them.
I was the first person in the history of our local to get 101% on our final exam, thanks to knowing what held open / closed switches were.
He was right, they didn't . But, on our 5 year final exam, the extra credit question was about them.
I was the first person in the history of our local to get 101% on our final exam, thanks to knowing what held open / closed switches were.
K8MHZ
Senior Member
- Occupation
- Electrician
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Well, maybe. When I took the manufacturer's training for Siemens, we discovered that you could change the relay state for a single pole double throw (NO-C-NC) HTRI-R by rapping it on the table. You couldn't be sure that NO-C pair was, in fact, open without metering it, or after installation running a RESET cycle on the panel.
sparkyrick
Senior Member
- Location
- Appleton, Wi
I had an alarm guy move my fire alarm EOL resistor on a sprinkler riser and when sprink came back to open the control valves, send the system into trouble. Alarm kid didn't check the positions of the valves (closed) and he thought I wired them wrong, so he changed it. I called the alarm company and they asked if I would fix it for them. No bueno, you can come fix your own screwups.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
But we don't really need to confuse the issue. If you hit a normally closed switch with a big enough hammer it will be open also.
jeremy.zinkofsky
Senior Member
- Location
- nj
"Normally Open" & "Normally Close" are terms that describe electrical contacts, that's all. It is importnat to understand that they don't do anything without some external manipulation or action. For example, a light switch either closes or opens when you use your finger to flip it. All normally closed means is that the two pieces of metal are touching as the default configuration but will separate and stay seperated once an action was performed on them (like flipping a switch). Normally "held" close means that the two pieces of metal are forced together by a spring or something mechanical and will separate when an action was performed on them but get pushed back together once that action is done. Like a piano key, you push it down with your finger and it bounces right back up after you remove your finger.
As for drawings, however they are shown schematically on the drawing indicates whether they are NO or NC. If the are not touching on the drawing they are "Normally Open" contacts and vice versa. The exception to this can lie with the type of symbol used to show limit switches. Just remember that closed always mean touching as the default configuration and open means not touching.
As for drawings, however they are shown schematically on the drawing indicates whether they are NO or NC. If the are not touching on the drawing they are "Normally Open" contacts and vice versa. The exception to this can lie with the type of symbol used to show limit switches. Just remember that closed always mean touching as the default configuration and open means not touching.
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gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
I said "rap," not "smash". The force we used basically simulated brushing it off a table, or dropping it while on a ladder. No permanent damage.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Good call. It's not about being an ornery SOB. Remember, if you touch it you own it. The last thing you need is to be the fall guy for every ill on that panel for the next six months.
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