Normally Open, normally closed

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If a contactor states its "normally closed" or "normally open", does this mean without power applied to it?
 
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This is a frequent cause of confusion.

The relay manufacturers define "Normally open" and "normally closed" as the state of the contact when the relay coil is not energised.

But a relay in an item of equipment may be energised in the normal or quiescent state of the system. Often the technical manual for the equipment will describe such a contact on that relay as being normally closed when in fact it is a normally open contact on a relay that is normally operated.
 
yes, no power

I agree

But a relay in an item of equipment may be energised in the normal or quiescent state of the system. Often the technical manual for the equipment will describe such a contact on that relay as being normally closed when in fact it is a normally open contact on a relay that is normally operated.

That's known as normal operating "state" Bernard

Matt
 
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That's known as normal operating "state"
I wish I had a pound for every time there has been confusion between normal and operating states.

An example is an output on an alarm or monitoring system.

When things are normal and safe the relay is energised and the contact closed.

When there is a fault or hazard the alarm system operates and the contact opens.
 
That's known as normal operating "state"
I wish I had a pound for every time there has been confusion between normal and operating states.

An example is an output on an alarm or monitoring system.

When things are normal and safe the relay is energised and the contact closed.

yup it is a relay with its N/O contact -closed, but in it's normal standby operating "state"

When there is a fault or hazard the alarm system operates and the contact opens.

still a operating "state" and still a relay with a normally open contact
 
yup it is a relay with its N/O contact -closed, but in it's normal standby operating "state"
We all know the facts, but I'm with Bernard on this one - I've often seen considerable confusion about the meaning of 'normal' in relation to relay and switch contacts, and similarly with semiconductor gates.

When annotating diagrams involving relays or gates in order to aid understanding of function, I try as hard as I can to avoid using the word 'normal', instead using words like 'usual', energised/unenergised resting/alarm etc. ... and that's not only for others ('the uninitiated'). Even my own mind can get totally tied in knots when looking at one of my own diagrams when there are lots of relay contacts whith are 'normally' (usually) closed, or gates which 'normally' (usually) have a 'high' input or output.

Kind Regards, John
 
yup it is a relay with its N/O contact -closed, but in it's normal standby operating "state"
We all know the facts, but I'm with Bernard on this one - I've often seen considerable confusion about the meaning of 'normal' in relation to relay and switch contacts, and similarly with semiconductor gates.

When annotating diagrams involving relays or gates in order to aid understanding of function, I try as hard as I can to avoid using the word 'normal', instead using words like 'usual', energised/unenergised resting/alarm etc. ... and that's not only for others ('the uninitiated'). Even my own mind can get totally tied in knots when looking at one of my own diagrams when there are lots of relay contacts whith are 'normally' (usually) closed, or gates which 'normally' (usually) have a 'high' input or output.

Kind Regards, John

I look at many as installed circuit diagrams,week in week out and relay contacts are nearly always without exception drawn in their un-energised states

occasionally to make it easier to follow operating conditions you might get a diagram showing the circuit in an "operating state" in which case yes a normally open contact is shown to be "closed"
but the same circuit diagram is showing the whole circuit relay included in a "operating state"

If a contactor states its "normally closed" or "normally open", does this mean without power applied to it?

Tell you what then john
answer mikes question quoted above for me in less than 2 words

Matt
 
I look at many as installed circuit diagrams,week in week out and relay contacts are nearly always without exception drawn in their un-energised states
Exactly - that's what we all look at. Hence my comments about the value of functional annotation. Seeing a diagram which shows (as virtually always is the case) the unenergised state of relay contacts does nothing to tell one whether that is the 'normal' ('usual', 'resting'or whatever) state in the application in question. In the absence of functional annotation, this can be particularly confusing when, as if often the case, the 'normal'/'usual'/whatever state of the relays is for some to be energised and others unenergised.
If a contactor states its "normally closed" or "normally open", does this mean without power applied to it?
Tell you what then john answer mikes question quoted above for me in less than 2 words
As I said, we all know the answer to that, and Mike was told the answer in both the first and second responses to his question. However, the fact that he asked the question should tell you something.

Kind Regards, John
 
Seeing a diagram which shows (as virtually always is the case) the unenergised state of relay contacts does nothing to tell one whether that is the 'normal' ('usual', 'resting'or whatever) state in the application in question.

oh but it does although seemingly some can get their heads around it better than others

If a contactor states its "normally closed" or "normally open", does this mean without power applied to it?
Tell you what then john answer mikes question quoted above for me in less than 2 words

As I said, we all know the answer to that, and Mike was told the answer in both the first and second responses to his question. However, the fact that he asked the question should tell you something.

Kind Regards, John

I knew you wouldn't be able to give a one word answer john! but to answer your question It tells me he wasn't sure and wanted to know the answer so asked the question,your point?

Matt
 
For schematics the following is an informative way to show a relay contact and its function


The black contact is the contact made when the relay / contactor is not energised.

The white contact is the contact made when the relay / contactor is energised.

The wiper / moving contact is drawn to show the normal / quiescent / inactive state of the equipment.
 
Seeing a diagram which shows (as virtually always is the case) the unenergised state of relay contacts does nothing to tell one whether that is the 'normal' ('usual', 'resting'or whatever) state in the application in question.
oh but it does although seemingly some can get their heads around it better than others
Although I'm sure that (as with most things in life) some can get one's head around it better than others, I think you must be joking. With the best will in the world, faced with a diagram showing many relays, all in their unenergised states, some of which are 'normally' energised and some 'normally' unenergised, I don't think there are many people who would not be grateful for some functional annotation such as I described!
I knew you wouldn't be able to give a one word answer john!
No-one actually quite managed 'one word' but, as I said, there was no need for me to repeat it just to please you!
...but to answer your question It tells me he wasn't sure and wanted to know the answer so asked the question,your point?
Exactly. 'My point' was that this indicates that the answer is not quite as obvious to everyone as you seem to be implying.

There are several places in my house where a relay or contactor switches on an 'emergency' light and/or alarm in the event of the failure of a particular circuit. I recall that, when I first wrote the documentation for those, I wrote something like "The (normally closed) relay contacts are normally open". When I realised how potentially confusing a statement that was, I changed the wording!

Kind Regards, John
 

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