Another thread discussing whether a switch in something was normally
open or normally closed reminded me of a similar question I have.
If I want to buy a reed switch/magnet pair to use as a door or window
contact in an alarm system and the system requires that the contact be
closed when the window or door is closed, do I buy a N.O. switch or
N.C. switch? I assumed N.C. until I started to over-think it. A
normally open switch is one that is open until it gets activated. Reed
switches are activated by the magnet getting close. So, a normally
open switch would be closed when the magnet is near and that happens
when the window is closed - which is normal - so normally, a normally
open switch is closed - aaaaaargh! I need a replacement switch a few
months ago and ended up paying extra for a double throw switch. It
wasn't labeled so I still don't know if I wanted N.O. or N.C. I just
used the contact that worked.
So, what is "normal" for a reed switch? When the magnet is near or
On May 25, 1:45 pm, firstname.lastname@example.org wrote:
N.O., or normally open, means the switch is open until a magnetic
force is applied to close the circuit...
(i.e. closing the window to align the magnet/switch pair)
N.C., or normally closed, means the switch is closed until a magnetic
force is applied to open the circuit...
As others have stated the requirements for one kind of switch or
the other will vary by: the alarm panel being used, the type of
switch being used by other devices wired in a series circuit and
the type of input being used on the given alarm panel...
Typically you want to use a normally open switch so that when the
magnet is close to it it closes. That way the circuit is complete
when the windo is closed. Anyone attempting to defeat the protection
by cutting the circuit anywhere trips it. Still can be defaeated by
shorting it somewhere else but that takes more than a pair of wire
I have seen both types installed with the same magnet to operate
The N.O. switch for the alarm system...
The N.C. switch used on an unrelated and unconnected indicator
panel where when a door or window is opened the indicator light
goes on so some guard/attendant can investigate... All lights
out meant "all openings closed"...
Some of the more elaborate installations of the indicator system
used a graphic annunciator panel with a plan of the building on it...
The security industry made up it's own vernacular for contacts. 99% are
N/O (shelf state), closed on magnetic reed trip (actuated).
But..... to avoid confusion, they started calling them "closed loop"
switches. It only added to confusion.
Have a look at GRI, GE, and Honeywell for top quality.
If you have a NO switch, you can easily convert it to a NC closed
switch..Mount the reed relay with a magnet nearby to close it. Then
use the movable magnet to cancel the field from the attached magnet
when the movable magnet is in the vicinity of the reed relay. All it
takes is one little extra magnet.
On 5/25/2011 12:45 PM, email@example.com wrote:
There was a thread on this about a year ago.
RBM's reply was:
As an electrician, I've been through this numerous times over the years.
It clearly depend on who you are talking to, and what trade they're in.
IMO, the "normal" position is the state the switch is in when nothing
affects it. Alarm people have the opposite take on it.
My reply was:
In all the following the contacts are closed with the magnet next to the
From an alarm perspective *NC* ("normal" is when the door is closed):
"Most switching devices are N.C. (normally closed) circuits, so when the
device is not in an alarm condition, the circuit is closed."
"Output signal: normally closed (switch contact is closed when the door
From a component manufacturer *NO*
C&K: "ELECTRICAL CIRCUIT: SPST NO (Contact Form A). Reed switch
opens when magnet is removed from proximity. Contacts are
held closed when magnet is within actuation range."
To know what the switch does you need to have a description like all of
I agree with the above and it also agrees, I think, with what all the
replies have been. I thought this would lead to one of those 100
threads, because it does get confusing.
But, if one wants a typical alarm reed switch for a door or window,
I think it's fairly easy to get the right one. All the alarms I've
a circuit that gets interrupted to cause an alarm. Which makes
sense, since then even if a wire breaks, it trips the alarm
something is wrong. If you just buy a switch clearly
intended for door/window use, very high probability you'll get one
that is closed when the magnet is present.
Alarm people 'do' know what they're talking about. You can blame the
manufacturers for the vernacular confusion, but 'alarm people" know.
Come on over to alt.security.alarms for further discussion if it bothers
you. I'm sure some of my colleges can define the reason better than I.
It is easy to understand if you just acknowledge that your security
system is normal/ready when the window loop is closed.
Fire alarms are also "ready" when the zone relays are energized. This
is to allow them to fail safe.
It's the same thing. Fire alarm devices like smoke detectors, heat
detectors, duct smoke, pull-stations, whatever.... They are 'open' in a
normal situation (no alarm) and close the contacts when the device is
actuated. The big difference is wiring. Conventional fire alarms MUST
be wired in parallel, with the EOLR (end of line resistor) in parallel
with the last physical device on that zone. That way, we can detect a
disconnected wire/device and get a 'trouble' condition (instead of
ALARM) on the panel.
The reason series wiring is used for burglary is the way the house is
wired. On a "good" pre-wire job (during construction), each window and
door has a cable "home-run" to the panel. Well, most homes have an
average of 15-20 windows, 3-5 exterior doors, a couple of motion
detectors, and perhaps a monitored fire zone. Your average burg. panel
has about 8 hardwired zones. It would not make sense to put each device
on it's own zone, it would be cost-prohibitive to add the zone
So, we identify the cables for a "set" of windows i.e. "living
room/kitchen" and wire those cables in series right at the panel. You
have to use the zones you have to work with as optimally as possible.
Because they will not be all the same type, for example: Zone 1 Fire,
Zone 2 is an entry/exit delay, Zone 3 for 'interior' devices', Zone 4 -
CO detector (or aux. device) Zones 5-8 for 'perimeter' devices and
extra interior devices.
Series wiring gives us the greatest flexibility to work 'creative'
solutions for security. Fire alarms are a whole different animal
because they MUST adhere to the NFPA 72 code, which does not permit
series wiring for a conventional class-B zone style.
I've heard a fire instructor tell us, "Fire alarms are science, security
systems are art."
People in different industries using the same types of technology often
have different terminology for the same process. I've worked across many
fields over the years so I'm well aware of it. In alarm tech, normal
state is considered the secured state. Which means the magnet
is against the magnetic switch closing the circuit which is usually
a supervised circuit with a set resistance in the loop. In the field of
electronics or control systems the normal state is considered to be no
outside force affecting a switch. As a broadcast engineer working with
remote signals, I had to work with engineers and techs with the phone
company. To get a higher signal level, the radio guy will say "increase
the gain", the phone tech won't grok that unless I told them to "lessen
the loss". Same technology, different dialect. ^_^
You may want to ask the doc. for a stimulant. If you fall asleep and
aspirate stuff from your mouth to the lungs you could be in a lot of
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