Intrusion Alarm Circuits Guide

Intrusion Alarm Circuits Guide 

Intrusion alarm circuits are a fundamental element of wired intrusion / burglar systems. Designing the intrusion alarm circuit greatly affects its performance. In particular, more efficient circuit designs introduce less resistance and cause fewer false alarms.

Alarm Circuits Overviewed

Intrusion alarm circuits use wires between an Intrusion alarm panel and various sensors. When the circuit / connection of those wires is broken (e.g., an alarmed window opens), the alarm is triggered if the system is armed / enabled.

How an Alarm Circuit Works

An intrusion alarm circuit consists of a pair of wires running from an intrusion alarm panel to a sensor, such as a magnetic contact. Electrical charge flows from the positive terminal, down one wire, and into the sensor. When something causes the sensor to close, it completes the circuit, allowing the charge to flow down the other wire and back to the negative terminal on the panel.

In the case of the contact, the circuit is complete because the magnet causes the reeds to touch, allowing current to flow from the reed on the positive side of the circuit to the reed on the negative side.

Using a pair of wires to connect both sides of the sensor to both terminals on the panel creates a large circle, which is where the word circuit comes from. The electrons flow freely all around this circuit, from the positive terminal on the panel, through the sensor, and back to the negative terminal. Opening the window will cause the reed to separate, which will break the circle and stop the flow of electrons. In other words, opening a window will create an open circuit. It is this open circuit that causes an alarm condition.

Loops vs Splices

The two most common ways to add multiple sensors to circuits is to use loops or splices.

Loops are preferred because it creates a short pathway, which means less resistance, fewer points of failure and faster to install. However, loops can only typically be used in new construction where the technician has the ability to run wires and loops through the window frames before the drywall is installed. Prewiring requires coordination with the general contractor or the carpenter. The alarm company needs to be able to schedule a technician to complete the wiring before the drywall crew is scheduled to begin installing drywall, and the carpenter should be told where to drill holes on the moulding or window frame.

Splices should be minimized because they add resistance and are more time consuming to install. However, adding alarms to exiting homes or businesses typically require this since it is not feasible to open the drywalls to run a looped circuit.

Two types of splicing exists: field splice and ITB (in-the-box) splice. The benefits of field splices are lower total circuit resistance and using less wire, but it requires a more skilled technician to hide the splice. By contrast, ITB splices are easier to troubleshoot, have fewer potential points of failure and they can be done by a less experienced installer, but they have higher total circuit resistance and require an installer to home run a wire to each individual window.

Loops Explained

The image below shows an example of a loop that allows two windows to share a circuit. Opening either the top sash or the bottom sash of either the right window or left window will cause the same zone to open. To accomplish this, a technician runs a single wire from one contact to another, allowing the current to travel around the windows in a circle. The technician leaves a loop at every window, which will bring the circuit in one side of the contact and out the other.

One wire of the pair runs to the window on the right, and the other runs to the window on the left. The technician has run loops of wires from one side of the contact to the other. One side of the contact on the top right window has a loop running to one side of the contact on the left (shown in black below). The loop between the top left contact and the bottom left contact (shown in RED) completes the circle, as long as all the windows are closed.

Field Splices

A field splice is one that is made at the device end of the wire, usually at the device itself. A skilled technician can make a field splice if the conditions are right. Splices must be accessible for future troubleshooting, so a field splice can only be made if there is someplace to hide them away from casual view. For example, when wiring windows, a technician can staple the wire to the underside of the frame or moulding, out of sight but easily found by an experienced / certified intrusion alarm troubleshooter. Intrusion alarm installers commonly wire all sirens and strobes in a location to a single circuit, and make field splices inside the siren box.

In this example, a pair of wires (green) is running to the alarm panel. Red wires are running to magnetic contacts on the bottom sash and the top sash of the right window, and blue wires are running to the top sash and the bottom sash of the left window. One red wire is spliced to one blue wire, leaving one red wire and one blue wire to be spliced to the green wires.

ITB Splices

ITB splices, or in-the-box splices, are those made inside the alarm can. Separate wires are run to each individual contact, and they are then joined up at the panel using a splice.

ITB splices are much easier for inexperienced technicians, and much faster to wire up. Making ten splices, one after another, to add ten devices to five zones, is faster than making a splice at every entranceway and then figuring out how to tuck them away. They are also much easier to troubleshoot if properly labeled in the can, because a troubleshooter can quickly isolate the circuit branch causing the issue, and does not have to first find and identify the splice. However, running individual wires is more more labor intensive, and uses more wire.

Circuit Electrical Specifications

All devices in an alarm, including unpowered devices such as contacts, require a specific amount of charge flowing through the wires at a specific speed and pressure or it will not work.

Every circuit has a voltage, current, and resistance value:

• Voltage is measured in volts (V).
• Current is measured in amps (A).
• Resistance is measured in ohms (Ω).

Voltage is a measurement of how much electricity is available for use. Every device lists the amount of voltage it requires to operate.

• If given too few volts, the device will either not power up or will work erratically.
• If given too many volts, it will either shut down or overheat.

Current is the pressure at which the charge flows. All devices consume, or draw, electricity at a predetermined rate.

• Not enough amps may cause the device to work harder to draw voltage, which could cause it to either shut down, overheat, or work erratically.
• Too many amps is not harmful as the device cannot draw more current than it can use.

Resistance is anything that slows the current. Factors that increase resistance are

• Number of connected devices
• Wire length
• Splices
• Time
• Copper oxidization

Too much resistance on a circuit will lower the current, making it more difficult for the alarm to monitor the zone properly. Since resistance increases in all circuits as time goes on, improperly designed circuits have a high probability of causing false alarms years after being installed. Therefore, every effort should be made to keep the resistance as low as possible.

Parallel Versus Series

The two methods of joining multiple sections to a single circuit are parallel and series. Devices can either be wired in parallel or in series with each other. Circuits using parallel splices are called parallel circuits, and circuits using series splices are called series circuits.

In a series splice, one of the pair of wires is spliced to one the next pair of. This way, no matter how many pairs of wires are added, the end result is two wires that are simple to connect to two screw terminals. In between, lots of wires are spliced to each other. This makes troubleshooting simpler, because a technician can simply test a single branch of the circuit at a time. Series is typically best for connecting a small number of devices to a circuit. It is faster and easier to wire up. The downside is that series introduces a lot more resistance if too many devices are connected.

In a series circuit, the wires from each branch of the circuit will be spliced together until two wires not spliced to anything are left. These single conductors will be connected to the screw terminals, while the splices 'hang' in the air, not connected to anything but each other. No matter how many devices and wires are connected to the circuit, there will always be only one wire to connect to the positive and one to connect to the negative.

Parallel splices are typically best for connecting a large number of devices to a single circuit, or for circuits that draw lots of power. It greatly reduces the amount of resistance, allowing those large numbers of devices to be connected. However, it is harder / more complicated to implement, will not work if done incorrectly, and is more difficult to troubleshoot later on.

In a parallel circuit, all the matching sides of the wire pairs are simply spliced together, with the end result being two thick wire twists. This can be a challenge to connect to screw terminals. In order to troubleshoot the wire going to a branch of the circuit, a technician must first undo the entire splice.

Experienced alarm technicians develop a 'feel' for when to splice devices in parallel and when to splice in series. However, the key determinate is this: when a series splice will result in too much resistance, a parallel splice must be used. Before splicing anything, technicians can test a circuit in order to determine the amount of resistance present.

Measuring Resistance

Resistance is measured using a digital multimeter, or DMM. To measure resistance, turn the function dial of meter to the Ω (ohms) symbol, and touch the leads to the bare wires of the circuit. Technicians measure resistance in order to

• Test that all devices on a circuit are functioning normally
• Decide whether to keep devices on a zone or to split them up among multiple zones
• Decide whether a circuit should be wired in series or in parallel
• Record a baseline resistance at the time of installation
• Troubleshoot, as a zone with a resistance reading well over baseline can indicate a broken wire or defective sensor

How Much Resistance Is Too Much?

There is no clear answer to the precise amount of devices, and consequently the precise amount of resistance, that should be allowable on a single detection circuit. As a general rule of thumb, many installers try to keep the baseline resistance on a single circuit to ~40Ω. Remember that the resistance will inevitably creep up over time, depending on the number and nature of sensors connected to the circuit, the number and nature of splices on the wire, the exact composition of the wire, and even the environment, which could cause the wire to oxidize faster or slower. The following animation shows the current slowing the more devices are added to the circuit:

Different alarm panel manufacturers have different standards for what constitutes an alarm condition, but all those standards are based on the panel reading the resistance on a zone circuit. If a circuit has a reading significantly higher than 40Ω, consider either using a parallel splice or splitting the zone, removing some devices from one zone and wiring them to a new zone.

Some installers prefer ITB splices because it allows them to decide to split the zone later on. If devices have been wired together using wire loops or field splices, this is not possible.

How Many Devices Is Too Many?

In theory, an installer can connect all the doors on a single zone, all the windows on another zone, all the motion detectors on a third zone, and so forth. However, this is quite an inefficient way of using an alarm. The more devices are on a single zone, the harder it will be for the user to figure out which device is causing the zone to be open.

If all the windows in a room are on a single zone, the user will have to check all of them before being able to arm the system. If a user wants to keep a single window open while arming the rest of the system, they would have to bypass all the windows in the room, which is a higher security risk than simply bypassing a single window. If central station needs to dispatch police or fire to the user's site, they will only be able to give a vague description of the location of the problem, not a specific location.

Most panels can only handle a limited number of zones out of the box. Using additional zones requires purchasing and installing zone expanders, which add to the overall cost of the installation. Using multiple zone expanders may require adding a second can, which likewise adds to the cost. The question of when to combine devices on a single zone and when to separate devices into separate zones has no easy answer, but becomes clearer with experience.

Wiring Sirens

Wiring sirens is slightly different than wiring other alarm devices.

• Sirens are rated in watts, not volts like all other alarm devices
• Watts are a measure of how much a power a device outputs
• Volts a measure of how much power a device uses
• Most sirens are rated to 30 watts

Most panels only have a single siren output. However, many applications call for the volume to be lower on some sirens than on others. For example, a siren mounted indoors would cause hearing damage if sounding at a full 110dB, but 110dB is necessary for notification outdoors. In order to have different volumes from a single output, installers can choose to wire sirens in either parallel (for louder volume) or in series (for lower volume). They can even choose to wire some sirens in parallel and other sirens in series. The sirens in series have more resistance than the sirens in parallel, so that there is more resistance, and therefore fewer watts, forcing the siren to sound at a lower volume.

Intrusion alarm siren wiring is a perfect illustration of Ohm's Law.

Ohm's Law

Understanding Ohm's Law makes alarm troubleshooting much easier. Ohm's Law states that there is a direct relationship between volts, amps, and ohms, specifically that volts equals amps times ohms. Amps is ohms divided by volts, and ohms is volts divided by amps. Adding resistance (ohms) makes the current (amps) go down, and adding current (amps) makes the resistance (ohms) go down.

The simplest way of lowering or raising the power available to the siren is by raising or lowering the resistance. This raises or lowers the amount of available power, which in turn affects the operation of the siren.

Understanding Ohm's Law can help a technician diagnose and repair power supplies, sensors, circuits, and sensors. Changing a splice from series to parallel, replacing a power supply for one that outputs the same voltage at a higher amperage, or adding a resistor are all repair options that an installer has once they understand how Ohm's Law works.

Source: IPVM.com & circuitstoday.com