relay contacts

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alan mcneil

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I questioned the electrical engineer on our job
about the relay and associated open and closed contacts are shown on his diagram.
The relay is shown as energized, however the contacts associated with it are not shown in a changed state.
This seemed confusing to me, this relay would normally be energized because it is a supervised circuit.
Please excuse my ignorance on this subject but the engineer got very huffy about it and I did not press the question further.
Am I that wrong? I usually see things as de-energized but in this case the relay is required to be energized as its normal state.
 
Re: relay contacts

All plans I have seen show relay contacts in the de-energized state whether or not the coil is energized under normal conditions. This can be confusing however, as you say.
 
Re: relay contacts

I have often found that engineers are often like doctors, that they are the experts and are never wrong. I agree with you in that devices are shown in their normal state. If a relay is automatically energized as soon a power is applied to the system then the relay is shown with the coil energized and the contacts shown in the position when the coil is energized.
I've often found 3-ph and 1-ph voltages listed wrong which also can be confusing. The difference between 240/120 and 120/240 can cost a lot of money when 120/240 is intended to be a 3ph, 4w, 240v delta with a 120 lighting tap and not 120/240v 1ph, 3w.
The issues are to be able to speak the same language and to not have to be a detective and to guess what they are trying to say.
 
Re: relay contacts

I don't understand the question.

On a wiring diagram the relay contacts should always be shown un-changed even if the coil is shown wired in a way that it is always on.

It can be confusing when people use terms like "normally" open or closed. What is normal?
 
Re: relay contacts

You are probably correct that designers and engineers tend to get defensive about their work. This doesn't mean that we are impervious to change. This is probably true for anyone. We wouldn't do it the way we did if we thought we were wrong, would you?

Getting to your question about the contact state. When you reference a wiring diagram and notice a contact being used from a relay, contactor, motor starter, etc., do you automatically refer to the coil circuit to determine what state the coil is in normally? What is a normal state? Should we have to decide first what the normal state of a coil is? What if it is a toss-up between energized and de-energized? What about when there is a controller such as a PLC operating the coil? Will the PLC energize the coil "normally"?

I believe it is best to try and maintain a certain consistancy and referring to all contacts of a drawing in a de-energized state helps prevent accidental mis-interpretation of a circuit and possible injury as a result.

This is consistent with our pneumatic drawings as well. All of our solenoid valve drawings are illustrated in a de-energized state.

Sound reasonable?

Bob
 
Re: relay contacts

Originally posted by templdl: I have often found that engineers are often like doctors, that they are the experts and are never wrong.
Click to expand...
I resemble that remark! :D
I agree with you in that devices are shown in their normal state.
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I do not agree. Rather, I agree with those who have questioned the use of the word ?normal.? Suppose for example that you have two identical pumps, and run one of them at a time. Suppose further that you alternate running them every day. Do you really want me, the engineer, to state on the drawings that ?pump1 running? is ?Normal? on Monday, and that?pump2 running? is ?Normal? on Tuesday, and that the condition of being ?Normal? changes day by day in this pattern, starting on Monday June 29, 2004? ;)

The easiest way for the mind to be prepared to read such a drawing is to expect all contacts to be shown in the ?shelf condition,? and then to discover as you go through the prints which coil will be energized under what operating conditions.
 
Re: relay contacts

I got to thinking about this a bit more. Alan said the relay coil was shown energized. Or does the wiring diagram show that the coil is directly wired such that when power is applied to the circuit the contacts would change state. Anyway, the question is how does one show a coil energized or de-energized? I've looked at all of my schematic books and have found no note or marking to that effect.
I've also done a lot of "take-offs" and had to interpret control wiring diagrams and have also drawn them for enclosed control and MCCs. I have always considered any wiring diagram in a de-energized state so a relay that would be energized when power is applied to the circuit would change state from that of which is shown in a wiring diagram.
 
Re: relay contacts

Originally posted by jim dungar:
It can be confusing when people use terms like "normally" open or closed. What is normal?
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I never had a problem with this. Normal is the way it comes in the box. If you apply an outside influence to get the contact to close, it is normally open.

[ June 30, 2004, 04:25 PM: Message edited by: gent ]
 
Re: relay contacts

Originally posted by templdl: Alan said the relay coil was shown energized.
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I think that this was ?conversational shorthand? for saying that it is evident from a review of the schematic that the coil would be energized while the circuit is being operated in its ?usual manner.?

Note how I cleverly avoided the N? word, by substituting the word ?usual.? When discussing a schematic, I always take care to avoid saying ?normal,? if I can find a clear and unambiguous expression that does not require using that word.
 
Re: relay contacts

From: IEEE Std 100-1996, The IEEE Standard Dictionary of Electrical and Electronics Terms, Sixth Edition

normally open and normally closed: When applied to a magnetically operated switching device, such as a contactor or relay, or to the contacts thereof, these terms signify the position taken when the operating magnet is de-energized. Applicable only to non-latching types of devices.
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Other than the fact that this isn't a Code issue and belongs in "Calcs/engineering" this has become an exceptionally semantic thread.

"Normally open" and "Normally Closed" are clearly defined terms with respect to the original query.

The state of the equipment "out of the box" isn't a bad working definition for other applications either - it just isn't "official," although some ISA standards that I don't have ready access to at the moment come close to that.

Going along with my long standing position that people can't be over-informed (with accurate info at least :D ), if there is any doubt, put a note on the drawing - it's cheap.
 
Re: relay contacts

I get too many requests for 3 position switches with 2 normally open contacts for this not to be a problem. I have had too return way to many products becuse someone could not understand the difference between "normally closed" and "held closed".

The IEEE definition specifically states it is for magnetically operated switches that the are non-latching type. So, their definition is not universal. The "out of the box" or more generally "unmounted" definition is probably the best for all around use.

[ July 01, 2004, 09:29 AM: Message edited by: jim dungar ]
 
Re: relay contacts

Hey Jim,

I've got lots of three position switches with two normally open contacts. DPDT center off. I use them all the time. DC applications mostly. FWD-OFF-REV....they are a must for reversing DC polarity.

Also, speaking from experience as one of those idiot engineers... there's little numbers on the relay bases and other devices. If people would just use them, things would get "wired to print." This way the engineers can show what they want and we don't have to worry about NO or NC. Also makes it easy to poke fun at us engineers when we want all the NOs changed to NCs. And by the way why does everyone think engineers know it all. We're just as stupid as everyone else. It's the one's that THINK they know it all that sour the lot.

And one vote for 'De-energized' please. Who the heck shows anything energized......

Be gentle I'm new to the form.

RSlater,
RSmike
 
Re: relay contacts

And by the way why does everyone think engineers know it all. We're just as stupid as everyone else.
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Who said there weren't any honest engineers? ;)
Just kidding, of course. Without you guys I'd have to be in an office all the time instead of running pipe and wire which is where I want to be. Thanks!
 
Re: relay contacts

rsmike,

I didn't ask for specifically a DPDT, what I wanted was one contact closed in the high speed position and a different one closed in the low speed position and both opened in the off position. Would the item change if it was H-O-L versus O-L-H

My point is you cannot use phrases like normally open and normally closed (only two possible positions) when describing situations that have 3 or more states. That is partly why the IEEE definition is only for 2 position magnetically operated devices.

[ July 02, 2004, 03:13 PM: Message edited by: jim dungar ]
 
Re: relay contacts

Hey Jim,

Whew! I just thought you might be implying that my most favorite type of switch doesn't exist.

I understand what you mean.

RSlater,
RSmike
 
Re: relay contacts

I agree with most the markings of the NC or NO should be shown beside the contacts and if you have a NO or NC beside your contact and the diagram contact picture is drawn the opposite it must be energized.

Ronald
 
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