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#75568 03/04/07 02:08 PM
Joined: Jul 2001
Posts: 311
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Are magnetic door switches and motion detectors usually "normally closed" devices in reference to a basic hardwired security system?

Joined: Dec 2005
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R
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They are normally closed wired.

Quite often a series resistor is used somewhere along the line, and sometimes very near it in the reed relay casing to avoid a tamper and bypass via an external wire ( zero omhs ) across it's terminals.
Values are around 3.3, 3.9, and 4.7 kOhms.

If say three reed switches are in series on a 4.7 kOhm circuit, best practise is if space allowes to put a 1.5 kOhm resistor in each reed switch casing if space allowes.

The loop will be sensed at 4.5 k Ohms by the alarm unit. You can can put a 200 R in the alarmbox to match exact 4.7 kOhm for the cct concerned.

edited for typo's



[This message has been edited by RODALCO (edited 03-04-2007).]


The product of rotation, excitation and flux produces electricty.
Joined: Jun 2004
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You can set them up either way.

Joined: May 2005
Posts: 54
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it really depends on the control panel (end station).

some of our systems rely on 2 resistors.

4.7k accross the redd (or relay contact on PIR`s)
and a 2.2k in seroes with one of the legs from the panel.

this gives 4.7k when closed (quite mode)
6.9k in alarm
O/C in tamper

this we have found is the best way, but it realy depends on the panel.

some panels use, 1.0k for both 4.7k or even 6.8k.

good luck, if you need any advice e mail me.

Britspark

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"They are normally closed wired.
Quite often a series resistor is used somewhere along the line, and sometimes very near it in the reed relay casing to avoid a tamper and bypass via an external wire ( zero omhs ) across it's terminals.
Values are around 3.3, 3.9, and 4.7 kOhms.

If say three reed switches are in series on a 4.7 kOhm circuit, best practise is if space allowes to put a 1.5 kOhm resistor in each reed switch casing if space allowes.

The loop will be sensed at 4.5 k Ohms by the alarm unit. You can can put a 200 R in the alarmbox to match exact 4.7 kOhm for the cct concerned."

RODALCO, has it right, anything less then full supervision, is not security, it's no more then a noise box, the so called basic security systems are usually sold by mass marketers with one objective, create large monthly revenue streams, not to provide security, if you are installing security seytems, be sure to check your liability coverages, and your contract, security installations, are more involved then just running wires and contacts, give some thought to the liability issues.

Joined: Aug 2001
Posts: 329
S
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Just as a point to keep in mind. Most systems are, as people have already said, wired normally closed. HOWEVER; they use a normally open switch to do it. It's all symantics but alarm people talk abut a "closed loop" system that actually uses NO switches that are held closed by the magnet or pressure when the system is armed. Most motion detector switches for a closed loop system are open until power is applied to the unit.
I only point this out so if you go to buy alarm switches you ask for "closed loop" or "open loop" switches. If you ask for NO or NC switches you may or may not get what you want; more than once I didn't.

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"closed loop" system that actually uses NO switches that are held closed by the magnet or pressure when the system is armed"

Good catch Steve!

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A security system panel seems opposite of a small conventional fire alarm system panel. Pull stations, smoke detectors, heat detectors (shunt trip devices) are installed on an open loop. Or maybe I'm getting my terminology confused.

Joined: Jan 2005
Posts: 5,316
Cat Servant
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Every alarm system is different. That's one reason you should not mix equipment from different manufacturers, or even product lines.

In general, a burglar alarm is designed so that any fault causes a contact to close in the panel. As mentioned, this is usually be having a NO contact held closed; open it, or cut the wire, and the alarm sounds.

Fire alarms are a bit different. they often operate in both modes; one for "alarm," and one for "let's make sure the line is working." Resistors, diodes, DC power are all part of the picture.

In short, there is no simple answer. Alarms are but control circuits ... so any combination is possible.


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