Wiring Basics 1

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DIY Alarm Systems / DIY Security Systems

Tennessee Alarm Store


Wiring Basics 1

Wire Sizes and Types

Since 1857, wire has been measured in the United States using the American Wire Gauge (AWG). This is sometimes called the Brown and Sharpe wire gauge. Wire can be either stranded or solid. While the stranded type is easier to bend, it has some draw backs. Because the strands are very fine, they can separate and go unnoticed when wrapping them around screw terminals or plugging them into grip or compression connectors (think of the manner in which wire sometimes plugs into stereo speakers). These stray wires can cause shorts. This is why I prefer solid wire for DIY alarm installations.

Twenty-two gauge wire measures .0253 inches in diameter and has a resistance of about 16 ohms for each 1000 feet. The larger the number, the smaller the wire. For example, a 30-guage wire is smaller than a 22-guage wire. In case you are wondering, normal house wiring uses 12 gauge wire for electrical receptacles. Now, this may sound like a bunch of electrical blah, blah, blah, but it is important for you to know that there are limits to the length a wire can be run before it loses its ability to efficiently and safely carry current.

Alarm systems use 22 gauge wire. This is a fine, relatively small wire, and is normally used for detectors. Depending on what you are wiring, you will need 22 gauge wire with either 2 or 4 conductors. (We will help you so you'll purchase the right thing.) When using this size wire, you can have up to 3000 linear feet of wire for each zone, but a typical zone will have less than 1,000 feet of wiring. Stay within these perameters and you will have a reliable installation.

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How Switches are Shown in Drawings

Magnetic SwitchI think we should briefly touch on the magnetic contacts that will protect your doors and windows. We will see how they are shown in electronic drawings and how to utilize them in different circuits.

Magnetic contacts can be purchased in two different configurations, normally open (N.O.) and normally closed (N.C.). Magnetic contacts (switches) have two parts: a permanent magnet and a hermetically sealed reed switch in a glass envelope. The normal position of the switch is how it behaves without the magnet close to the switch. Imagine the switch setting on a shelf and the magnet held in your hand. This would be the contact position at rest. So, in a magnetic switch with N.O. contacts, when the magnet is brought in close proximity to the switch, approximately one half inch, the contacts close. This is by far the most widely used configuration in modern DIY alarm systems. The other choice is the N.C. configuration, which is when the contacts are closed at rest, laying up on the shelf. When the magnet is brought close to the switch, the contacts open, breaking the circuit. Don't get normally closed switches confused with normally closed circuits. A normally closed circuit may consist of several normally open switches with their magnets close making them close. Drawings don't typically show the magnets, so when you see switches in a drawing you need to look for a foot note or something in the drawing that states, magnetic switches shown with N.C. or N.O. contacts. Don't get hung up on normally open or normally closed. I have seen manufacturers list their switches both ways. Not all switches require a magnet to operate and are mechanical in nature. In the case of smoke detectors the switch is normally open and they are closed electronically when tripped within the smoke detector itself. Other devices are motion detectors, glass break detectors, and photoelectric sensors.

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Series Circuits

Series CircuitA series circuit is one that is wired so there is only one path for current to flow in the zone or circuit. Think of it like traffic on a one-way street. Like a car entering a one-way street, when current enters a series circuit, there is only one path for it to enter, one direction for it to flow, and one direction for it to exit.

The total current must flow from the negative terminal around the circuit and through each device in order to return to the positive terminal. Any device that stops the flow of current at any one point must stop the total current for the entire circuit. Since DIY alarm systems are basically made up of a series of switches, opening any one of these switches will stop the current flow for the entire circuit. Opening additional switches after the first one will not have any effect on the system since current flow is already interrupted. In order to restore the current flow each and every switch must be closed to complete the circuit.

The easiest way to visualize this is to imagine a garden hose placed in a circle. If you kink the hose at any point the water flow stops. Probably the most striking examples of this is the older strings of Christmas lights. These were lights that were wired in series, and if one light burned out, the whole string of lights would not illuminate. You would have to figure out which bulb was burned out and replace that lamp to restore power to the string. A problem with this system is monitoring the wiring for shorts. A short is the touching of the wires together which can thwart the system and make it think the detectors are all closed. We will show you a way to supervise or monitor the wires when we get to the section on end of line resistors.

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Parallel Circuits

Parallel CircuitIn a parallel circuit there are two or more paths for current flow in the zone or circuit. If we look at the same negative and positive terminals that we used in the series circuit and wire in one switch across the terminals, we essentially have a series circuit. Now let's wire in a second switch across the terminals. Now there are two paths for current flow and you must open both switches to stop the current flow. If we keep adding more switches then they all must be open to stop the current, but only one switch needs to be closed to complete the circuit. Do you see the difference, in a series circuit all switches must be closed to complete the circuit and in a parallel circuit only one switch needs to close to complete current flow. The opposite is true in stopping current. In a series circuit only one switch needs to open to interrupt the current and in a parallel circuit all switches must open to stop the current. Again there is inherent problems with this system as far as monitoring the wiring. Since this wiring method is generally used with N.O. switches you will not know if there is a break in the wire. We will show you how to monitor you wiring in the end of line resistors section.

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Combination Series/Parallel Circuits

Series/Parallel CircuitThis last section on wiring covers the series/parallel circuit. These circuits are made up of a combination of series and parallel wiring. At some point the current branches out into several different legs, come back together and may branch out again. Eventually the current does come back together and return to the positive terminal. These circuits can become very complicated. They are not widely used in alarm systems but do have their place. Most DIY alarm systems come with 4 - 8 on board zones and can be programmed for use as a series circuit, a parallel circuit or with end of line resistors that allow for supervision or monitoring of external wiring. If needed a supplemental printed circuit board, known as zone expanders, can be added that increase the number of zones to 32 - 64 or more. We will touch on zone expanders in more depth in the section on zone expanders.

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End of Line Resistors (EOLR) for Supervising Wiring

Now let's look at a circuit that supervises your actual wiring for tampering or damage. A resistor is a semiconductor device that can have very little resistance or it can be made to have a very large resistance. Resistance is measured in units called Ohms and represented by the Greek letter Omega "Ω". Different manufacturers use different values of resistance for their equipment. End of Line resistors (E.O.L.R.) are resistors of a specific value that are used at the end of a zone or protective loop. By that I mean the device that is located furthest from the control panel and that is where the end of line resistor should be located. We will see how an end of line resistor works and what benefits they may provide.

Series Circuit with SEOL ResistorIn a zone without an end of line resistor, there are only two states for the zone terminals. These are either an open circuit or a closed circuit and these are the only two possibilities. If we add a resistor of a known value there can be three states. These are an open state, a closed state and a known value of resistance state. Looking at the diagram to our left, our current flows through a couple of N.O. switches, then through the end of line resistor, a couple of additional N.O. switches and back to the positive terminal. This is the normal state when all windows and doors are closed. Remember when the magnets are next to the switches, they are closed. The alarm is secure and ready to be armed.

If any switches are open, the alarm shows a fault and cannot be armed until the fault is corrected. Now let's suppose someone tampers with the wiring in an attempt to circumvent the DIY alarm system by shorting the conductors together. Again this would show on the panel as a wiring fault and you would not be able to arm the system. If the panel was armed at the time the wires were shorted, it would go into alarm and sound the sirens and make the notifications. This is called a supervised loop and I believe for the minor extra effort, it is well worth it.

Parallel Circuit with SEOL ResistorNow to give you all sides of this, there are installers who do not install the end of line resistors and their reasoning is that the chances of a short are miniscule and they don't like the extra effort needed to install them. On panels where they are required, they put them in the control box, wired directly to the zone terminal, essentially negating their effectiveness. If down the road you need to change out the alarm control panel and decide to go with a different manufacturer you may have to change all the end of line resistors because the new control panel uses a different value. Only you can make this decision, but let me give you a few more points to ponder. In a commercial location, where the wiring is quite often on the surface or partially out in the open, you should give serious consideration to using them. Also most smoke and heat detectors use N.O. contacts. On these circuits, end of line resistors monitor the integrity zone and are the only way of monitoring your zones for broken wires. In my opinion, end of line resistors are absolutely required in fire alarm zones. When we get to the section on actually wiring in the sensors I will give you my opinion on this subject.

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Continued on Wiring Basics 2 page.