N.O. vs N.C. switch
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Posts: 46,084
Group,
Is there anyone out there with industrial experience who can explain why
STOP switches are normally closed? I can imagine that if I were making a
machine that "started" by using a relay, I could latch the relay with a
normally closed switch and "stop" it by opening the coil circuit. Other
than that, I can't come up with a good reason to have Start and Stop
switches to my stamp to be anything other than normally-open. I'm sure that
I have overlooked something, so I thought someone out there could shed some
light on this.
Thanks,
Mike
_________________________________
Mike Walsh
walsh@i...
Is there anyone out there with industrial experience who can explain why
STOP switches are normally closed? I can imagine that if I were making a
machine that "started" by using a relay, I could latch the relay with a
normally closed switch and "stop" it by opening the coil circuit. Other
than that, I can't come up with a good reason to have Start and Stop
switches to my stamp to be anything other than normally-open. I'm sure that
I have overlooked something, so I thought someone out there could shed some
light on this.
Thanks,
Mike
_________________________________
Mike Walsh
walsh@i...
Comments
button is in series with the normally open relay contacts that close
to latch the relay. Once the relay is latched, how do you unlatch it?
Put a normally closed switch in series with the relay contacts that
latch the circuit. Doing it this way makes the wiring VERY simple and
reliable
Industrial controls have been aroud a long time before
microcontrollers. Start/stop latching in industtrial controls have
been done like this for a long time.
There also may be other normally closed switches in series with the
stop button, thermal cutout switches, limit switches, etc.
This is digital logic in it's purest, simplest form.
Regards
Rich
--- In basicstamps@y..., MIKE WALSH <walsh@c...> wrote:
> Group,
> Is there anyone out there with industrial experience who can
explain why
> STOP switches are normally closed? I can imagine that if I were
making a
> machine that "started" by using a relay, I could latch the relay
with a
> normally closed switch and "stop" it by opening the coil circuit.
Other
> than that, I can't come up with a good reason to have Start and
Stop
> switches to my stamp to be anything other than normally-open. I'm
sure that
> I have overlooked something, so I thought someone out there could
shed some
> light on this.
>
> Thanks,
> Mike
>
>
>
> _________________________________
> Mike Walsh
> walsh@i...
>Group,
>Is there anyone out there with industrial experience who can explain why
>STOP switches are normally closed? I can imagine that if I were making a
>machine that "started" by using a relay, I could latch the relay with a
>normally closed switch and "stop" it by opening the coil circuit. Other
>than that, I can't come up with a good reason to have Start and Stop
>switches to my stamp to be anything other than normally-open.
Nope - you have hit the nail on the head. Most industrial motors are
switched using contacters and the contacter is held closed by an auxiliary
contact in parallel with the start button. The stop button is directly in
series with the coil so that an emergency stop button (latching in the off
position) ensures that the contacter cannot energize. It is standard
industrial practice based upon the technology of that era and there is
really no good reason to change it.
There are a couple of additional benefits to using NC contacts as stop
and/or emergency stop signalling: (1) breakaway links or switches are
easier to implement (where the control line gets disconnected when a link
is pulled, (2) E-stop conductors are forced to be separate from the start
conductors (less chance of future electricians messing it up in the distant
future), etc.
dwayne
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 17 years of Engineering Innovation (1984 - 2001)
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
Probably also the switch is in series with the relay coil, which would
interrupt the circuit no matter what the controller was doing.
Might look into OSHA specs on this stuff for clarification.
Original Message
> Is there anyone out there with industrial experience who can explain why
> STOP switches are normally closed? I can imagine that if I were making a
> machine that "started" by using a relay, I could latch the relay with a
> normally closed switch and "stop" it by opening the coil circuit. Other
> than that, I can't come up with a good reason to have Start and Stop
> switches to my stamp to be anything other than normally-open. I'm sure
that
> I have overlooked something, so I thought someone out there could shed
some
> light on this.
for a stop switch to work it has to be hooked up. From a safety standpoint
if the switch is normally open how, from the machine point, do I know if the
switch is hooked up or not? With the switch normal closed the machine knows
that the switch is there. If for some reason the switch or connecting wire
fails the system would know and stop.
Hope that helps.
Greg
Original Message
From: MIKE WALSH [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=yy28K8eWHDW4zq-dKqxTk9u5HpJ_lLsoGDx2o2QDuYkkxo7_B7H48ghJntxXqoASoeD_4pT4BWVI]walsh@c...[/url
Sent: Wednesday, May 02, 2001 10:25 AM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] N.O. vs N.C. switch
Group,
Is there anyone out there with industrial experience who can explain why
STOP switches are normally closed? I can imagine that if I were making a
machine that "started" by using a relay, I could latch the relay with a
normally closed switch and "stop" it by opening the coil circuit. Other
than that, I can't come up with a good reason to have Start and Stop
switches to my stamp to be anything other than normally-open. I'm sure that
I have overlooked something, so I thought someone out there could shed some
light on this.
Thanks,
Mike
_________________________________
Mike Walsh
walsh@i...
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
········I'm noexpert but I believe that STOP switches are normally closed for safety asthe primary reason.· Should a failure of the STOP switch wiringhappen (an open) the machine will stop running as the default.· Ifthe switch were normally open and the same failure occur, the STOP switchwould NOT work in an emergency or other situation and you would not knowof the failure until the switch function was needed.· Hope thishelps.· Maybe a few of the list professionals will provide moreexplanation.
Tim
At 10:24 AM 5/2/2001 -0700, you wrote:
Is there anyone out there with industrial experience who can explain why
STOP switches are normally closed? I can imagine that if I were making a
machine that "started" by using a relay, I could latch therelay with a
normally closed switch and "stop" it by opening the coilcircuit. Other
than that, I can't come up with a good reason to have Start and Stop
switches to my stamp to be anything other than normally-open. I'm surethat
I have overlooked something, so I thought someone out there could shedsome
light on this.
Thanks,
Mike
_________________________________
Mike Walsh
walsh@its.caltech.edu
Your use of Yahoo! Groups is subject tohttp://docs.yahoo.com/info/terms/
Timothy Medema
CrystaLite, Inc.
3307 Cedar St.··················(425) 745-6000·· 800-666-6065
Everett, WA 98201···············Fax: (425) 257-0232
www.crystaliteinc.com···········timm@crystaliteinc.com
The information transmitted is intended only for the person or entity to whom it is addressed and may contain confidential and/or privileged material.· Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited.· If you received this in error, please contact the sender and delete the material from any computer.
········I'm noexpert but I believe that STOP switches are normally closed for safety asthe primary reason.· Should a failure of the STOP switch wiringhappen (an open) the machine will stop running as the default.· Ifthe switch were normally open and the same failure occur, the STOP switchwould NOT work in an emergency or other situation and you would not knowof the failure until the switch function was needed.· Hope thishelps.· Maybe a few of the list professionals will provide moreexplanation.
Tim
At 10:24 AM 5/2/2001 -0700, you wrote:
Is there anyone out there with industrial experience who can explain why
STOP switches are normally closed? I can imagine that if I were making a
machine that "started" by using a relay, I could latch therelay with a
normally closed switch and "stop" it by opening the coilcircuit. Other
than that, I can't come up with a good reason to have Start and Stop
switches to my stamp to be anything other than normally-open. I'm surethat
I have overlooked something, so I thought someone out there could shedsome
light on this.
Thanks,
Mike
_________________________________
Mike Walsh
walsh@its.caltech.edu
Your use of Yahoo! Groups is subject tohttp://docs.yahoo.com/info/terms/
Timothy Medema
CrystaLite, Inc.
3307 Cedar St.··················(425) 745-6000·· 800-666-6065
Everett, WA 98201···············Fax: (425) 257-0232
www.crystaliteinc.com···········timm@crystaliteinc.com
The information transmitted is intended only for the person or entity to whom it is addressed and may contain confidential and/or privileged material.· Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited.· If you received this in error, please contact the sender and delete the material from any computer.
Thanks for explaining that! I'm not so sure that this will continue to be
the case, as I have just checked two PLC based packaging machines and find
that neither of them use NC stop switches. The Emergency Stops **ARE** NC
and that makes perfect sense to me. If the Stamp (or PLC) is poling the
switch and then acting on the state of that switch, I can't think of any
reason to chose one type of switch over the other.
You do make a good argument about a plant electrician or mechanic servicing
the switch in the future and assuming that ALL stop switches are NC. Think
I'll see if there are any NEC or OSHA codes that discuss this.
Thanks for your input.
Mike
At 12:51 PM 5/2/2001 -0600, you wrote:
>At 10:24 AM 5/2/01 -0700, MIKE WALSH wrote:
> >Group,
> >Is there anyone out there with industrial experience who can explain why
> >STOP switches are normally closed? I can imagine that if I were making a
> >machine that "started" by using a relay, I could latch the relay with a
> >normally closed switch and "stop" it by opening the coil circuit. Other
> >than that, I can't come up with a good reason to have Start and Stop
> >switches to my stamp to be anything other than normally-open.
>
>Nope - you have hit the nail on the head. Most industrial motors are
>switched using contacters and the contacter is held closed by an auxiliary
>contact in parallel with the start button. The stop button is directly in
>series with the coil so that an emergency stop button (latching in the off
>position) ensures that the contacter cannot energize. It is standard
>industrial practice based upon the technology of that era and there is
>really no good reason to change it.
>
>There are a couple of additional benefits to using NC contacts as stop
>and/or emergency stop signalling: (1) breakaway links or switches are
>easier to implement (where the control line gets disconnected when a link
>is pulled, (2) E-stop conductors are forced to be separate from the start
>conductors (less chance of future electricians messing it up in the distant
>future), etc.
>
>dwayne
_________________________________
Mike Walsh
walsh@i...
California Institute of Technology
Biology Electronics Shop
MS 216-76
Pasadena, CA 91125
626-395-6825 , FAX 626-584-1654
237 Beckman Behavioral Biology Bldg.
message to be posted.
Original Message
> So if a wire is shorted, the machine does not suddenly start up?
>
> Probably also the switch is in series with the relay coil, which would
> interrupt the circuit no matter what the controller was doing.
>
> Might look into OSHA specs on this stuff for clarification.
>
>
Original Message
>
> > Is there anyone out there with industrial experience who can explain why
> > STOP switches are normally closed? I can imagine that if I were making a
> > machine that "started" by using a relay, I could latch the relay with a
> > normally closed switch and "stop" it by opening the coil circuit. Other
> > than that, I can't come up with a good reason to have Start and Stop
> > switches to my stamp to be anything other than normally-open. I'm sure
> that
> > I have overlooked something, so I thought someone out there could shed
> some
> > light on this.
system shuts down. It also insures that when power comes back on the system
can't restart without pressing the start button since an auxiliary set of
contacts across the start button is what keeps the relay or contactor
energized.
Larry
At 10:24 AM 5/2/01 -0700, you wrote:
>Group,
>Is there anyone out there with industrial experience who can explain why
>STOP switches are normally closed? I can imagine that if I were making a
>machine that "started" by using a relay, I could latch the relay with a
>normally closed switch and "stop" it by opening the coil circuit. Other
>than that, I can't come up with a good reason to have Start and Stop
>switches to my stamp to be anything other than normally-open. I'm sure that
>I have overlooked something, so I thought someone out there could shed some
>light on this.
>
>Thanks,
>Mike
>
>
>
>_________________________________
>Mike Walsh
>walsh@i...
>
>
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
Larry G. Nelson Sr.
mailto:L.Nelson@i...
http://www.ultranet.com/~nr
1)NC stop switch(momentary)
2)NO start switch(momentary)
3)A coil
4)A NO contact
It is wired like this
L1··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· L2
|··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· |
|··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· |
|·······Stop···········Start·····································································|
|
o||o
|/|
(CR)
|
··················· |··········· |
··················· |----||---|
··· ··· ··· ··· ··· ··· (CR1)
L1
o||o
o[noparse][[/noparse]Motor]o
L2
··················· (CR2)
The way three wire controls work is as follows:
When power is applied initially to the Stop-Start ckt the coil is de-energized, contact CR1 i is open and CR2 is also open, therefore the motor is off.
When the start button is closed CR is energized thereby closing CR1 and CR2 starting the motor.
CR1 is electrically held.· If the stop switch is pushed power is interupted and the motor stops.· There is also a safety factor.· If power is interupted prematurely the motor will not be off when power does return.
Hope this helps
I'm a qualified electrcian with 15 years in the business and 8 years doing control work
·
·Free Web Email & Filter Enhancements.
·http://www.freewebemail.com/filtertools/
·
Original Message
From: Timothy Medema
To: basicstamps@yahoogroups.com
Sent: Wednesday, May 02, 2001 4:32 PM
Subject: Re: [noparse][[/noparse]basicstamps] N.O. vs N.C. switch
Mike,
········I'm no expert but I believe that STOP switches are normally closed for safety as the primary reason.· Should a failure of the STOP switch wiring happen (an open) the machine will stop running as the default.· If the switch were normally open and the same failure occur, the STOP switch would NOT work in an emergency or other situation and you would not know of the failure until the switch function was needed.· Hope this helps.· Maybe a few of the list professionals will provide more explanation.
Tim
At 10:24 AM 5/2/2001 -0700, you wrote:
Is there anyone out there with industrial experience who can explain why
STOP switches are normally closed? I can imagine that if I were making a
machine that "started" by using a relay, I could latch the relay with a
normally closed switch and "stop" it by opening the coil circuit. Other
than that, I can't come up with a good reason to have Start and Stop
switches to my stamp to be anything other than normally-open. I'm sure that
I have overlooked something, so I thought someone out there could shed some
light on this.
Thanks,
Mike
_________________________________
Mike Walsh
walsh@its.caltech.edu
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
Timothy Medema
CrystaLite, Inc.
3307 Cedar St.··················(425) 745-6000·· 800-666-6065
Everett, WA 98201···············Fax: (425) 257-0232
www.crystaliteinc.com···········timm@crystaliteinc.com
The information transmitted is intended only for the person or entity to whom it is addressed and may contain confidential and/or privileged material.· Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited.· If you received this in error, please contact the sender and delete the material from any computer.
Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
info right now!
thanks,
Mike
At 04:51 PM 5/2/2001 -0500, you wrote:
>Dang -- Yahoo spanked me again. It only took almost two hours for this
>message to be posted.
>
>
Original Message
>
> > So if a wire is shorted, the machine does not suddenly start up?
> >
> > Probably also the switch is in series with the relay coil, which would
> > interrupt the circuit no matter what the controller was doing.
> >
> > Might look into OSHA specs on this stuff for clarification.
_________________________________
Mike Walsh
walsh@i...
Original Message
From: "MIKE WALSH" <walsh@c...>
To: <basicstamps@yahoogroups.com>
Sent: Wednesday, May 02, 2001 8:13 PM
Subject: Re: [noparse][[/noparse]basicstamps] N.O. vs N.C. switch
> None the less, I appreciate your reply. I'm trying to get the OSHA and NEA
> info right now!
> thanks,
> Mike
>
> At 04:51 PM 5/2/2001 -0500, you wrote:
>
> >Dang -- Yahoo spanked me again. It only took almost two hours for this
> >message to be posted.
> >
> >
Original Message
> >
> > > So if a wire is shorted, the machine does not suddenly start up?
> > >
> > > Probably also the switch is in series with the relay coil, which would
> > > interrupt the circuit no matter what the controller was doing.
> > >
> > > Might look into OSHA specs on this stuff for clarification.
>
> _________________________________
> Mike Walsh
> walsh@i...
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
>
Usually, Stop switches are normally closed so that they can be
supervised. In applications where should the Stop switch fail,
someone could get hurt, you damned well better be sure that the
that switch along with all of its interconnected wiring are in good
shape and working.
Russ
Original Message
From: MIKE WALSH <walsh@c...>
To: <basicstamps@yahoogroups.com>
Sent: Wednesday, May 02, 2001 1:24 PM
Subject: [noparse][[/noparse]basicstamps] N.O. vs N.C. switch
| Group,
| Is there anyone out there with industrial experience who can explain why
| STOP switches are normally closed? I can imagine that if I were making a
| machine that "started" by using a relay, I could latch the relay with a
| normally closed switch and "stop" it by opening the coil circuit. Other
| than that, I can't come up with a good reason to have Start and Stop
| switches to my stamp to be anything other than normally-open. I'm sure that
| I have overlooked something, so I thought someone out there could shed some
| light on this.
|
| Thanks,
| Mike
|
|
|
| _________________________________
| Mike Walsh
| walsh@i...
|
|
|
|
| Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
|
|
|
>STOP switches are normally closed? I can imagine that if I were making a
>machine that "started" by using a relay, I could latch the relay with a
>normally closed switch and "stop" it by opening the coil circuit. Other
>than that, I can't come up with a good reason to have Start and Stop
>switches to my stamp to be anything other than normally-open. I'm sure that
>I have overlooked something, so I thought someone out there could shed some
>light on this.
A "STOP" with with NC contacts is "fail-safe". If the switch losses
connection for any reason, the effect is the same if you deployed the stop
switch. The are failure modes that this doesn't pick up, such as physical
damage to the mechanical portions of the switch, and there are applications
where this type of operation is not desired. However, for normal motor
controls, and whatnot, the "start" switch engages a relay which is
self-latching, and the "stop" switch interrupts the circuit, ceasing motor
action.
Regards,
Rob
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com
timm@crystaliteinc.com writes:
·······I'm no expert but I believe that STOP switches are normally closed
for safety as the primary reason. ·Should a failure of the STOP switch
wiring happen (an open) the machine will stop running as the default. ·If
the switch were normally open and the same failure occur, the STOP switch
would NOT work in an emergency or other situation and you would not know of
the failure until the switch function was needed. ·Hope this helps. ·Maybe
a few of the list professionals will provide more explanation.
Tim
At 10:24 AM 5/2/2001 -0700, you wrote:
Is there anyone out there with industrial experience who can explain why
STOP switches are normally closed? I can imagine that if I were making a
machine that "started" by using a relay, I could latch the relay with a
normally closed switch and "stop" it by opening the coil circuit. Other
than that, I can't come up with a good reason to have Start and Stop
switches to my stamp to be anything other than normally-open. I'm sure
that
I have overlooked something, so I thought someone out there could shed
some
light on this.
Thanks,
Mike
Mike:
······Tim is correct, the reason for making the limit switches and emergency
stop switches on industrial machines is indeed for safety reasons. ·If for
ANY reason a wiring harness should be damaged or one of the wires inside a
harness should separate inside it's insulation due to flexing, etc. then the
machine would shut down by default. ·If you used a normally open switch to
latch a relay then you would need another switch to unlatch that relay so why
not just use one switch. ·You can still use the switch(s) in series to
control a single emergency relay or you can wire them in parallel with
separate relays that have NC contacts in series. ·Depends on how much money
you want to spend and how complex you want the circuit. ·Of course any
engineer worth his salt will not make the circuit(s) any more complex than
absolutely necessary. ·Mainly you keep it simple (the KISS principle) because
every level of complexity you add is just another thing that can cause
problems.
I hope this helps some in answering your question
Regards,
Randy Abernathy
Woodworking Machinery Specialist
4626 Old Stilesboro Road NW
Acworth, GA 30101
Phone / Fax: 770-974-5295
E-mail: cnc002@aol.com[/font]
I'm not very good with ascii art, but maybe you can figure this out. In
the schematic below, one side of the normally closed stop button is
connected to the supply voltage and the other side is connected to the
start button and a normally open contact on the relay (x). The other side
of the start button is connected to the other side of the normally open
relay contact and then to one side of the relay coil. To complete the
circuit the coil is connected to common.
When the start button is momentarily closed, the coil is energized and
closes the normally open contact labled (x1). Because the stop button is
normally closed, this contact keeps the relay coil energized until the
stop button is momentarily pressed (opened) at which time the relay coil
is deenergized opening contact x1.
stop start coil (x)
_
supply volt
*_*---.--* *--.---mmm-- common
| (x1) |
|--||---|
Hope this helps.
Charles Huffman
On Wed, 02 May 2001 10:24:36 -0700 MIKE WALSH <walsh@c...> writes:
> Group,
> Is there anyone out there with industrial experience who can explain
> why
> STOP switches are normally closed? I can imagine that if I were
> making a
> machine that "started" by using a relay, I could latch the relay
> with a
> normally closed switch and "stop" it by opening the coil circuit.
> Other
> than that, I can't come up with a good reason to have Start and Stop
>
> switches to my stamp to be anything other than normally-open. I'm
> sure that
> I have overlooked something, so I thought someone out there could
> shed some
> light on this.
>
> Thanks,
> Mike
>
>
>
> _________________________________
> Mike Walsh
> walsh@i...
>
>
>
>
> Your use of Yahoo! Groups is subject to
> http://docs.yahoo.com/info/terms/
>
>
> connection for any reason, the effect is the same if you deployed the stop
> switch. The are failure modes that this doesn't pick up, such as physical
> damage to the mechanical portions of the switch, and there are applications
> where this type of operation is not desired. However, for normal motor
> controls, and whatnot, the "start" switch engages a relay which is
> self-latching, and the "stop" switch interrupts the circuit, ceasing motor
> action.
also, as an added bonus: if the power goes out and then comes back on
again you won't get machines unexpectedly starting up!
"well, the power's out, so let's do some maintenance" ... one could
easily overlook the fact that a regular power switch is still on. but a
latching switch avoids that problem.
--
_______________________________________________
Jason Lavoie
jlavoie@e...
jlavoie@o...
ICQ#:10604243
Electrical Engineering III, Carleton University
terminals or any other connection is cut or damaged, it will stop the
equipment. This forces repairs to be made.
If the cable were to break on a NO circuit you would not find the failure
until you needed the Estop. This could be a dangerous situation.
I have been using Stamps in industrial machinery for years and this is the
way I wire them. I also use NC for any safety or possible machine damaging
sensing. Travel limit switches are done this way. You do not want to find a
failure after the machine commits suicide.
Alan Bradford
Plasma Technologies
NH[/font]