NC & NO contacts

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Designer69

Senior Member
I thought I understood what is meant by NC or NO contacts until I really thought about it.

Does NC mean that a contact is closed if the coil side of the relay is de-energized? then the contact opens when coil side of the relay is energized?

or is it the other way around?

thanks
 
augie47

augie47

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In terms of standard realy contacts, that is correct. No voltage on the coil, a set of Normally Closed contacts is "made". Apply volthge to the coil, that set opens.
 
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dicklaxt

Senior Member
NC and NO is the shelf state of the contacts of a relay and when laying on a shelf the coil is certainly denergized when a control device functions and energizes the coil then the contacts change state.A relay can be put into a circuit in the operating mode with coil energized in this case when the coil is denergized then the contacts change back to the shelf state.

There are many many scenarios of coils and contact arrangements.Electronic relays with iniating contacts and on-delay/off delay timers is another that will force a hat change every now and then:)

I always liked to work with hard wired relay logic in lieu of DCS systems and Data Hiway PLC's controlling nearly everything.They are basicly falling by the wayside with the advent of the computer world.They are oft times used in a backup mode but in a triple reduntdancy role so they don't get tested to often.

Hard wired relays started fading out in the 70's/80's and has been downhill ever since.

dick
 
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zgozvrm

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Northern California
In general, contacts are in their normal condition when ALL energy is released. So, for a limit switch for instance, the contact are in their normal state when the lever is not being pressed (that is, the internal spring is storing as little kinetic energy as possible). When the limit switch is activated, the spring is being compressed (storing more energy) as it tries to return the lever to the relaxed position.

The same holds true for pressure switches: no pressure differential (the pressure inside the switch is the same as atmospheric pressure outside the switch) means that the contacts are in their normal position. Positive pressure or negative pressure (vacuum) causes the internal mechanism (usually a spring) to store energy. When this occurs, the switch is not in it's "normal" state and the contacts change states.

That being said, there ARE exceptions. The most prominent exception is with safety switches (door switches). The NO/NC nomenclature tends to be backward when dealing with these kind of switches. The thought being that since the switch has to be activated in order for the safety circuit to allow a machine to run, it is then in it's "normal" position. Although I understand this, but I don't like it.
 
kwired

kwired

Electron manager
Location
NE Nebraska
zgozvrm said:
In general, contacts are in their normal condition when ALL energy is released. So, for a limit switch for instance, the contact are in their normal state when the lever is not being pressed (that is, the internal spring is storing as little kinetic energy as possible). When the limit switch is activated, the spring is being compressed (storing more energy) as it tries to return the lever to the relaxed position.

The same holds true for pressure switches: no pressure differential (the pressure inside the switch is the same as atmospheric pressure outside the switch) means that the contacts are in their normal position. Positive pressure or negative pressure (vacuum) causes the internal mechanism (usually a spring) to store energy. When this occurs, the switch is not in it's "normal" state and the contacts change states.

That being said, there ARE exceptions. The most prominent exception is with safety switches (door switches). The NO/NC nomenclature tends to be backward when dealing with these kind of switches. The thought being that since the switch has to be activated in order for the safety circuit to allow a machine to run, it is then in it's "normal" position. Although I understand this, but I don't like it.
Click to expand...

Those are called normally open - held closed and normally closed held open. The basic switch is still NO or NC but the way it is logically applied adds the held open or held closed designation.
 
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wireguy8169

Senior Member
Location
Southern Maine
Shelf state

Shelf state

As sparky stated whatever the shelf state, is what they are. So if you have a normally closed contact on a relay you energize the coil and they become open and the opens close. On a safety switch when its in the made position its closed when its released or hit it opens. I do kinda see what your saying in the realm of safety switches, due to the fact they are in different states like a door switch that covers say a drive belt is open in its shelf state but when the door puts pressure on the actuator it closes it. And then like you said pressure switches may open at a high limit or a low limit as long as its in the specified range its closed. When in doubt I ring it out...
 
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charlie b

charlie b

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I always cringe when I hear the word "normally" used in this context. I never liked it, for the simple reason that it can easily lead to confusion. I agree that the shelf state of an NC contact is closed, but I still don't like it. I prefer the notation that the shelf state of an "a contact" is open, and that the shelf state of a "b contact" is closed.

The whole "NC/NO" problem gets far worse when you are talking about mechanical valves and instrumentation-related switches. Take for example a, electrically operated valve that requires electric energy to keep it open. Suppose it is open only when you are pumping water. Finally, suppose that the system is running 24/7/365 and is therefore always pumping water. The "shelf state" of that valve would be "closed," but is the "normal" state of that valve "open" or "closed."

So whenever I hear that term used, I like to take a moment to ask the speaker to clarify his or her intent, in using that term.
 
M4gery

M4gery

Senior Member
What about security alarm contacts? A door button is normally open, but it seems like it is called NC (I guess because the alarm system is a closed loop?).

When you are dealing with magnetic contacts, what is their shelf state? Is it when the magnetic is next to the contact or away?
 
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BJ Conner

Senior Member
Location
97006
charlie b said:
I always cringe when I hear the word "normally" used in this context. I never liked it, for the simple reason that it can easily lead to confusion. I agree that the shelf state of an NC contact is closed, but I still don't like it. I prefer the notation that the shelf state of an "a contact" is open, and that the shelf state of a "b contact" is closed.

The whole "NC/NO" problem gets far worse when you are talking about mechanical valves and instrumentation-related switches. Take for example a, electrically operated valve that requires electric energy to keep it open. Suppose it is open only when you are pumping water. Finally, suppose that the system is running 24/7/365 and is therefore always pumping water. The "shelf state" of that valve would be "closed," but is the "normal" state of that valve "open" or "closed." [/SIZE][/FONT]
Click to expand...

When things get complicated it's up to the engineer to put a contact developement table on the drawing. For a valve it might be all the contacts in the valve listed in a row with a columns for open, closed, intermediate positions. Every contact gets an "X" for the positions in which it is closed.

Breaker control switches like the SBM switch can have contacts in different states depending on where they were last. An SBM switch can have a contact that is open if the switch was in the "open" position last or "clsoed if the switch was in the "closed" positon last. There is no way to explain the workings of an SBM switch without a table.
So whenever I hear that term used, I like to take a moment to ask the speaker to clarify his or her intent, in using that term
 
ActionDave

ActionDave

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M4gery said:
What about security alarm contacts? A door button is normally open, but it seems like it is called NC (I guess because the alarm system is a closed loop?).

When you are dealing with magnetic contacts, what is their shelf state? Is it when the magnetic is next to the contact or away?
Click to expand...

It has been my experience that alarm guys refer to their contacts relative to what state they are when the magnet is next to it.
 
iaov

iaov

Senior Member
Location
Rhinelander WI
I like the term shelf state!! Where this stuff can get confusing and get you in trouble is when dealing with PLC's/ladder logic. They use the NC NO symbols but if you think of them as you would relay contacts it doesn't always work!:grin:
 
LEO2854

LEO2854

Esteemed Member
Location
Ma
M4gery said:
What about security alarm contacts? A door button is normally open, but it seems like it is called NC (I guess because the alarm system is a closed loop?).

When you are dealing with magnetic contacts, what is their shelf state? Is it when the magnetic is next to the contact or away?
Click to expand...

Thats right the door button is normally open and when you close the door it is held closed to close the loop ,and when you open the door the button returns to its normal state normally open.
 
M4gery

M4gery

Senior Member
LEO2854 said:
Thats right the door button is normally open and when you close the door it is held closed to close the loop ,and when you open the door the button returns to its normal state normally open.
Click to expand...

But they are commonly called NC.
 
LEO2854

LEO2854

Esteemed Member
Location
Ma
M4gery said:
But they are commonly called NC.
Click to expand...

They are normally open when you get them at the suplly house,there job is to open when the power is cut ..
and when you close the door the button becomes live and now it is closed open the door button is dead and goes back to its normal state 'normally open'
 
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