On Apr 30, 9:02 pm, wrote:
On Fri, 30 Apr 2010 10:41:41 -0500, bud--
wrote:



RBM wrote:
"Bitzer" wrote in message
...
I'm replacing a broken surface mount magnetic switch mounted on my garage
door that controls a signal light in the house that comes on when the door
is open.

Thus, I need a switch that is OPEN (meaning no current flows) when the
magnet is near the switch and CLOSED (meaning current flows) when the
magnet is removed from the switch.

So do I need a normally open switch or a normally closed one? Different
manufacturers/vendors seem to use the terms differently. Is it "normal"
when the magnet is near the switch-- or away from it?

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...10-Pack/p.aspx

"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.


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.