Intrusion alarm panels, zones and keypads

The intrusion panel for the Juniper Point storage lockers arrives in a gray steel cabinet with a circuit board, a transformer, a battery bracket and a bag of resistors. Hollis hands Maya the bag before anything else. Those resistors, he says, are what make an alarm system an alarm system rather than a set of switches: they let the panel tell the difference between a door that is closed, a door that is open, a wire that has been cut and a wire that has been shorted. This lesson covers how the panel watches its zones, how end-of-line resistors supervise the wiring, zone types and attributes, keypads and partitions, and the panel features that the installer sets up.

What the control panel does

The control panel is a small computer with inputs and outputs. Its inputs are zones, one for each detector or group of detectors, plus the keypads and any expansion modules on its data bus. Its outputs drive sirens and strobes, relays that signal other systems, and the communicator that reports to the monitoring station. The panel keeps the system's state, armed or disarmed, for each area, and applies the rules programmed for each zone: whether it is watched only when armed or all the time, whether it has an entry delay, and what it reports when it trips. It also watches itself: its mains power, its battery, its siren circuit, its tamper switch and its communication path.

Panels come in sizes from 8 zones for a house to several hundred for a campus. Most have a fixed number of zones on the main board and add more with zone expanders on a four-wire data bus, or with wireless receivers. The Juniper Point locker panel has 16 zones on the board and an eight-zone expander in a second cabinet at the far end of the parkade, joined by the bus cable. Larger systems, like the one at Riverside school, use addressable devices: each detector has its own address on a two-wire loop, so one cable can carry dozens of detectors and the panel still knows which one tripped.

Zones and end-of-line supervision

A simple zone is a loop of wire from a zone terminal, out to a detector's alarm contact and back to a common terminal. The detector's contact is normally closed: it is closed when everything is normal and opens on alarm. If that were the whole circuit, the panel could only see two states, closed and open, and a burglar who shorted the two wires together before opening the door would defeat it. Worse, a cut wire would look like an alarm only if the system was armed, and would go unnoticed the rest of the time.

The answer is the end-of-line resistor, or EOL. A resistor is fitted at the far end of the loop, at the last detector, so that in the normal state the panel sees a known resistance, for example 5.6 kΩ. When the contact opens, the panel sees an open circuit. When the wires are shorted, the panel sees close to zero ohms. The panel measures the zone's resistance and so can tell normal from alarm from trouble. That is supervision: the panel knows the wiring itself is intact.

The resistor must be at the end of the line, at the device, not in the panel. A resistor fitted at the panel terminals makes the zone read normal however the field wiring is damaged, because the panel sees only the resistor. It is a common shortcut on old installations, and it removes supervision completely. Monitoring stations and the installation standards require EOL resistors at the device, and Hollis checks for them on every service call.

Single and double end-of-line

With a single EOL and a normally closed contact in series, the panel sees the resistor in the normal state, an open circuit on alarm or a cut wire, and a short on a shorted wire. Because alarm and cut wire look the same, the panel treats an open as an alarm when armed and as a trouble when disarmed. With double end-of-line, or DEOL, a second resistor is wired across the contact, so the panel sees one resistor when the contact is closed, two in series when the contact opens, an open circuit only when the wire is cut, and a short when it is shorted. That gives four distinct states: normal, alarm, tamper and short. Many panels also use DEOL to bring a detector's tamper switch onto the same pair of wires.

# Expected zone readings for single and double end-of-line wiring
R1 = 5600.0      # ohms, end-of-line resistor
R2 = 5600.0      # ohms, second resistor across the contact (DEOL)
WIRE = 0.01614       # ohms per ft of one conductor, 22 AWG
run = 400      # ft from panel to the last device
loop = 2 * WIRE * run
def show(name, ohms):
    text = "open" if ohms is None else f"{ohms:8.0f} ohms"
    print(f"  {name:12s} {text}")
print(f"loop wire resistance: {loop:.1f} ohms")
print("single end-of-line:")
show("normal", R1 + loop); show("alarm", None); show("short", loop); show("cut wire", None)
print("double end-of-line:")
show("normal", R1 + loop); show("alarm", R1 + R2 + loop); show("short", loop); show("cut wire", None)

loop wire resistance: 12.9 ohms
single end-of-line:
  normal           5613 ohms
  alarm        open
  short              13 ohms
  cut wire     open
double end-of-line:
  normal           5613 ohms
  alarm           11213 ohms
  short              13 ohms
  cut wire     open

The panel accepts a band around each expected value, because the wire adds resistance and resistors have tolerance. Panel manuals give the limit, often a maximum loop resistance of a few hundred ohms excluding the EOL. On long runs to the far end of a parkade, the wire resistance is worth checking against that limit before the cable goes in, and the zone reading at the panel is worth recording at commissioning, so that a later reading can be compared.

Zone types and attributes

Each zone is programmed with a type that says how the panel responds. The names vary by manufacturer, but the ideas are the same. A delay zone, used for the door people normally come in by, starts an entry delay when it opens while armed, giving the user time to reach the keypad. An instant zone, used for other doors and windows, alarms immediately. An interior or follower zone, used for motion detectors inside, is ignored when the system is armed in stay mode and follows the entry delay when the delay door has opened first, so that a person walking from the door to the keypad does not set off an instant alarm. A 24-hour zone is watched whether the system is armed or not; panic buttons, holdup buttons and the tamper switches on cabinets are 24-hour zones. Some panels also have fire, gas and water zones, but a fire alarm system is a separate, code-governed system, and in commercial buildings intrusion panels do not replace it.

Zones also carry attributes. Bypass permitted lets a user exclude a zone, for example a window left open for ventilation, when arming. Chime makes the keypad beep when the zone opens while disarmed. Swinger shutdown stops a faulty zone from sending dozens of alarms in one night. Cross-zoning requires two zones to trip within a set time before an alarm is sent, which reduces false alarms from a single detector. Each attribute changes how the system behaves in a real event, so the installer sets them from the approved zone list, not by guessing.

Areas, partitions and users

Many buildings need parts of the system to arm separately. The panel divides zones into areas, also called partitions, each armed and disarmed on its own. At Juniper Point the board wants the storage lockers armed from midnight to six, the management office armed whenever staff leave, and the gym watched only for its emergency button. That is three partitions on one panel, with the parkade cameras and the gym button shared. Users get codes that work on some partitions and not others: the night cleaner's code disarms only the office, and only between 10 p.m. and 2 a.m.

Each user has a code, and the panel records who armed and disarmed and when. The master code, the installer code and the user codes are different levels of authority. The installer code allows programming and must be changed from the factory default at installation and handed over with the system, never left at a value printed in the manual. The owner's administrator manages users; the installer does not keep a list of user codes.

Keypads

The keypad is where people meet the system, so its location matters as much as any detector's. It goes inside the protected area, close to the entry door, within reach of a user who has just come in, and where an intruder outside cannot watch codes being entered. It has its own tamper switch, and it shows the state of every partition it controls. Keypads with a reader built in let users arm and disarm with their fob. On a bus, each keypad has an address set by switches or programming; two keypads with the same address make the bus misbehave in ways that are hard to trace, so addresses are recorded on the drawing as they are set.

The keypad also shows trouble conditions: AC lost, low battery, a communication failure, a zone trouble. A system that shows a trouble every day is a system that people learn to ignore, so part of commissioning is clearing every trouble before handover and explaining each one to the owner's staff.

Bells, sirens and outputs

The siren output is supervised like a zone: the panel watches for a cut or shorted siren wire, usually through a resistor at the siren or by sensing its current. Sirens are mounted out of easy reach, and outdoor sounders have their own tamper and often their own battery. The panel's programmable outputs drive relays that do other things on an event: turn on lights, signal the access control system, or start a camera recording. Many buildings now use silent alarms for some areas, with no local sound, because the goal is a response, not noise. The owner decides which, and the bylaws in some cities limit how long an outdoor siren may sound.

Programming from the zone list

The designer's zone list is the installer's programming document. It names every zone, its location, its device, its type and attributes, its partition and how it reports. Maya enters the list into the panel, labels each zone with a plain description that will appear on the keypad and at the monitoring station, and then tests each zone, recording the resistance she measured and the event the panel produced. The completed list, with measured values and test results, goes into the handover package.

# Check a zone list for gaps before programming
zones = [  # number, label, type, partition
    (1, "Locker room door", "delay", 1), (2, "Locker room motion", "follower", 1),
    (3, "Locker aisle B motion", "follower", 1), (4, "Locker room door 2", "instant", 1),
    (5, "Office door", "delay", 2), (6, "Office motion", "follower", 2),
    (7, "Gym panic", "24-hour", 3), (8, "Panel tamper", "24-hour", 1),
    (9, "Expander tamper", "24-hour", 1), (10, "Locker aisle C motion", "follower", 1),
]
types = {"delay", "instant", "follower", "24-hour"}
seen = set()
for n, label, t, part in zones:
    if n in seen: print(f"zone {n}: duplicate number")
    if t not in types: print(f"zone {n}: unknown type {t}")
    seen.add(n)
for part in (1, 2, 3):
    pz = [z for z in zones if z[3] == part]
    has_delay = any(z[2] == "delay" for z in pz)
    kinds = sorted({z[2] for z in pz})
    print(f"partition {part}: {len(pz)} zones, types {kinds}, entry delay {'yes' if has_delay else 'none'}")
missing = [k for k in range(1, max(seen) + 1) if k not in seen]
print("unused zone numbers:", missing if missing else "none")

partition 1: 7 zones, types ['24-hour', 'delay', 'follower', 'instant'], entry delay yes
partition 2: 2 zones, types ['delay', 'follower'], entry delay yes
partition 3: 1 zones, types ['24-hour'], entry delay none
unused zone numbers: none

Partition 3 has no delay zone, which is correct: the gym button is 24-hour and the gym is never armed. A partition that is meant to be armed by users but has no delay zone, though, means every user who walks in will cause an instant alarm. Checking the list before entering it saves a night of false alarms.