On Sun, 02 May 2010 09:56:37 -0400, wrote:

On Sun, 2 May 2010 06:34:49 -0700 (PDT), wrote:

On May 1, 10:53 am, wrote:
On Sat, 1 May 2010 06:31:46 -0700 (PDT), wrote:

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.

I agree.

Not quite right. An alarm SYSTEM being normally closed requires
normally open SWITCHES. This is a series string system, where when one
- any one - switch opens, the "normally closed" loop is open and the
alarm sounds. The SWITCHES are normally open, and closed when the
magnet activates the switch.

A Normally OPEN system is a parallel string circuit - where the
normally CLOSED switches are activated when the doores are closed and
the magnet is close - and ANY ONE switch being CLOSED activates the
system.

The techs (if they know what they are doing and talking about) KNOW
this.

I think Bud and RBM who are both electricians have this one right.
There is no consistency on this. At the very least, I can show you
an alarm switch I quickly found that clearly calls switches that are
CLOSED when the magnet is next to them NORMALLY CLOSED SWITCHES. You
can pull up the PDF "manual" as well and it is consistent with that
terminology. It didn't take long to find this, it was the first place
I looked after googling alarm switch.

I think Buds statement that you have to carefully read the full
description to figure it out is spot on. And even then, in some
descriptions you may not be able to tell because they really don't
say. In this one, it is very clear:

http://www.smarthome.com/7113-10/Sur...-Contact-Switc...

"Surface Mount Magnetic Contact Switch, Normally Closed (10-Pack

Essential Info
The most basic sensors of any alarm system are the door and window
Surface Mount Magnetic Contact Switches. There are 10 pairs of
switches included in this kit allowing you to monitor multiple areas
around your home. This type of contact switch is for normally closed
circuits; this means that the when the magnets are lined up, the
switch is closed."

Note that they call the circuit normally closed and also call the
switch for that circuit normally closed.

No, they do not call the switch a "normally closed" switch. They say
when the magnets are lined up they are normally closed.
The definition of a "normally open" switch meets that spec.

You DO have to read carefully what they are saying - and you have to
understand what is really going on.



Yeah YOU sure do need to read carefully what they are saying. And you
obviously didn't. Because if your read the description in the link I
provided, they CLEARLY call that switch normally closed. It's right
in the freaking one line TITLE:

"Surface Mount Magnetic Contact Switch, Normally Closed (10-Pack)"


Here's another link to a switch supplier that discusses the issue and
says this:

http://www.iheartswitch.com/blog/cho...h-your-project

"For unknown reasons, there are some companies classifying their
magnetic switches against the grain. Their NC switches turn things OFF
when the two halves are apart, while NO does the opposite. This is why
even some salespersons are confused as well. Therefore, before
purchasing any magnetic switches, read the product descriptions
carefully."
From the site referenced :

Surface Mounted Normally Open Magnetic Reed Switch
Philmore # 30-17050

Use this switch if you'd like to turn something OFF when the two
halves of your switch are apart. Commonly used with security systems.


Specifications


Action: Closed Loop / Normally Open (when magnet is beyond the
actuations range)
Maximum Gap (actuation range): 1-1/4" Max
Mounting Hole Centers: 2-3/32"
Terminal Type: Screw
Type: SPST
Current Rating: 0.5A @ 100VDC


There is NOTHING contrary in this description. With the magnet beyond
actuation range, the NO switch is turned off - exactly as it should
be.

Radio Shack's is normally closed when contacts are apart (poor
description - but basically when magnet is beyond actuation range)

Digi-Key gives no description other than "normally closed"


The OP can make up his own mind. Listen to two electricians and a
switch company that says confusion does exist and the only way to know
is to carefully read the description, if it's even contained in the
description. Or you, who says the terms are only used one way.
Clearly in the case I just showed you, if you went by the one line
title of the switch, you'd be getting the opposite of what you
expect. And in most cases, that one line is about all they say about
the switch, ie there is no further explanation.





It all gets back to what your definition of "normal" is. If it means
the condition with no outside signal or magnet on it, which is
reasonable and how we approach relays then I agree with your
interpretation. But I also see how what RBM said is true. That in
the alarm industry, which BTW is an obvious big user of these devices,
they call a switch "normal" if the window is shut and the magnet is
next to it. It becomes not-normal when the window is moved away and
it trips the alarm.


A doorbell button is a clear and easy to understand example of a
normally open switch.