Alarm Zone Circuit Types and States

One of the most difficult aspects of discussing alarm circuits with another person is the terminology used to describe the components used in an alarm circuit vs the programming of the alarm panel to achieve the desired behavior. Get it wrong and you can have the fire department dispatched when a smoke detector is removed rather than a simple trouble condition. 

Installers and programmers must work together to get it correct, every time. This article is intended to provide a baseline of understanding so that no confusion exists during the installation or service of Burglar and Fire alarm hardwired systems. 

Circuit Types and States

There are two TYPES of alarm circuits, Burglary, and Fire.  The terms "Unsupervised" and "Supervised" are the more correct terms, but for now we will use the terms Burglary and Fire since unsupervised circuits make up a vast majority of burglary zones and supervised circuits are always used in analog fire circuits.

Terms such as Open or Closed, Shorted and more can be used to describe the status of individual components in an alarm circuit but may not properly refer to the STATE of the circuit.

The STATE of an alarm circuit is a combination of the electrical properties (does current flow and if so, how much)  and the type of the circuit.


In a Burglary alarm circuit, there are only two states, Open and EOL (End of Line Resistor). Excluding a situation, whereas wires are shorted through wire damage or poor installation, the two states of a burglary circuit are illustrated below. 24 hours zones such as Panic Buttons are wired using Fire Alarm circuit design due to their inherent supervision properties. More on that later. 

Unsupervised alarm circuit in EOL state
Fig 1. Burglary alarm circuit in EOL state. 
unsupervised alarm circuit in open state
Fig 2. Burglary alarm circuit in OPEN state

In all burglary installations, a resistor is installed in series with the sensor. If you omit the resistor, the two states possible are Open and Closed (shorted), but only on Vista panels zones 2-8. All other Vista 20P hardwired zones require a resistor.
Bosch security panels ALWAYS require a resistor in the circuit, so for continuity SSNW installers  ALWAYS place a resistor in the circuit thereby making it best to use the term EOL rather than closed to describe the circuits above. You could also say its in an "EOL Closed"  state, either way, you are conveying the fact a resistor is installed.

A burglar alarm is armed and disarmed over and over, thereby each circuit is essentially verified as good before the system is armed. If the circuit is damaged or otherwise unable to function the system cannot be armed. The user is made aware and can take steps to correct.

Unsupervised circuit with cut wire
Fig 3. Unsupervised Burglary circuit with cut wire

An unsupervised zone cannot differentiate between a door/window is open or cut wire.  The user will not be aware of a problem unless they look at the keypad and notice the display indicating a door/window is open when they know its closed. Unless they notice something is wrong, the user will not be aware of an issue until they try to arm their alarm system.  Even worse is when the user performs a force bypass on a zone they expect to be closed after they leave, thereby including that zone in the protected area. If a wire is cut, that zone will never be added to the system.  It is not uncommon for residential security clients to not arm their alarm system for days at a time. Then when they try to arm their alarm, they cannot because there is a problem.    

Fire and Life Safety 

A fire/life safety circuit is 24/365.  It protects all the time and circuit damage or device removal must be detected immediately.  To accomplish the required supervision, a fire alarm circuit has three states; Open, EOL and Closed (Shorted ). 

In an unsupervised circuit, the panel is unable to differentiate between an open door and a cut wire. In a supervised circuit, regardless of its state, the panel can properly determine the difference between an open or closed contact (or switch) and cut wire. 

Supervised alarm circuit in EOL state
Fig 4. Fire Alarm circuit in an EOL state. This should create a Normal Condition at the panel.
Supervised Fire Alarm circuit in shorted state
Fig 5. Fire Alarm circuit in Closed state. This should create an Alarm or Supervisory Condition at the panel.
Fire Alarm circuit with cut wire in open state
Fig 6. Fire Alarm Circuit in Open state. This should create a Trouble Condition at the panel. 


A supervised alarm circuit MUST have a resistor in parallel to the alarm/supervisory device. Only a parallel resistor with an initiating device in an open state for the "normal" circuit state can be a supervised alarm circuit.  A resistor in series with the initiating device provides tamper detection properties, not supervision. In a supervised circuit, the system user is immediately notified of a wiring malfunction. In a tamper resistance circuit, the user will not be notified if a wire is cut, but if the wire is cut between the panel and device, the resistor will no longer be in the circuit, thus the user cannot arm the alarm system. They will then realize there is a problem.


In both unsupervised and supervised alarm systems the state of the circuit does not by itself create the CONDITION of the alarm panel. The condition of a panel is a combination of Circuit TYPE, circuit STATE, and PROGRAMMING. There are 5-panel conditions; Normal, Alarm, Supervisory, Trouble, and Fault. 



Circuit States

Circuit Type



Closed / Shorted

Unsupervised Unarmed




Unsupervised Armed







Alarm / Supervisory


A smoke alarm circuit (supervised) in a shorted state triggers an ALARM condition, but the same state of a water value tamper circuit results in a SUPERVISORY condition. Both a smoke alarm circuit and valve tamper circuit use a supervised circuit type and both circuits go into a closed/shorted state. It is only through programming that the panel condition is determined. 

An open burglary alarm circuit (unsupervised) in an unarmed alarm system results in a FAULT condition which is the same as opening a door or window, whereas an open heat detector circuit (supervised) results in a TROUBLE condition. Trouble conditions notify the local users as well as contacting the Central Station if not cleared in a timely manner.

As you can see in the above chart, circuits in an EOL state always result in a Normal panel or zone condition. If you think saying a circuit is in a "normal state" would seem to make more sense than saying it is in an EOL state, you would not be wrong. The problem is the term normal is often misunderstood in discussions.  Use the term normal carefully. 

Product Type