Alarm System Contacts

Normal is when there is no outside force acting on the switch, relay, contactor, whatever. In this case, it's when the magnet is away from the switch.

only if it is a Form C contact...

nate

@Pat:

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...

~~ Evan

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 cutters.

I have seen both types installed with the same magnet to operate different circuits...

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...

~~ Evan

Hi, If I were you I'd pick up reed switch with 3 wires. It can be used on either case N.O or N.C. The wire will be marked as C(common), NO(normally open), NC(normally closed)

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 its open without the magnet then its NO

Jimmie

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.

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: I agree.

In all the following the contacts are closed with the magnet next to the reed switch.

From an alarm perspective *NC* ("normal" is when the door is closed):

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 the above. end reply.

I agree with the above and it also agrees, I think, with what all the other replies have been. I thought this would lead to one of those 100 reply 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 seen use a circuit that gets interrupted to cause an alarm. Which makes sense, since then even if a wire breaks, it trips the alarm indicating 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.

Good answer!

Thanks for all the replies. Since I was planning to buy from Digikey rather than an alarm company, the clear answer seems to be N.O.

N.O. 'closed loop' to be precise.

You can use N.C. contacts in an alarm zone circuit, but the wiring would be in parallel. Most are wired in series, hence the NO 'closed loop'.

Smitty,

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.

Indeed.

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 expanders.

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. ^_^

TDD

Heh, yup. And you have to speak both when dealing with various vendors.

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 trouble.