When you know what's happening, you can fix the problem. To figure out what's happening, you need to know what you're working with.
Loop - Circuit in the Building
With almost any building-wide Fire Detection and Alarm System (FDAS), a loop is a two-wire circuit that goes from the fire alarm control panel to devices in the building.
- IDC - The loop can be the wires going to the detectors and switches for a zone input to the panel (Initiating Device Circuit or IDC)
- NAC - The loop can be the wires going to the horns and strobes for output from the panel (Notification Appliance Circuit or NAC)
- SLC - The loop can be the wires of an electronic, data type of circuit to connect all the addressable detectors, input modules, and output modules to the panel (Signaling Line Circuit or SLC)
Ohms Law > Resistance - Voltage - Current
As an electronic technician, get familiar with Ohm's Law. Ohm's Law shows a relationship between resistance, voltage, and current.
Resistance, in any circuit, is not alone. Both voltage and current are interrelated with resistance, so if resistance is changed, either the voltage changes, or the current changes, or both the voltage and the current change. Because the panel is included with any measurement (the screen says that the resistance is too high) the panel is included in the relationship.
Ohm's Law shows us what is happening in an electrical circuit; voltage pushes and pulls the electron current. Resistance slows down the electron current.
- Voltage - "E" - Electromotive Force - Voltage is measured in Volts and is needed to push and pull electrons through an electronic circuit. Without voltage, there is no electrical current.
- Current - "I" - Quantity of Electrons or Intensity of Current - Current is measured in Amps and is the quantity of electrons going past a single point in a circuit.
- Resistance - "R"- Amount of Holding-Back the Current or Hindering the Free Flowing of Electrons - Resistance is measured in ohms and reduces the current.
Ohm's Law is
E equals I times R, or
E=IxR.
No matter what we do, the relationship shown in Ohm's Law stays the same.
When the resistance is kept the same:
- If the voltage is increased, the current automatically increases
- If the voltage is decreased, the current automatically decreases
When the voltage is kept the same:
- If the resistance is decreased, the current automatically increases
- If the resistance is increased, the current automatically decreases
High Resistance (Negative or Positive) in the Loop
When the resistance is high, according to Ohm's Law, the current is low. That means that somewhere in the building, something is stopping the current, or resisting the current, on one of the loops. This could be caused by:
- A wire has come loose from a connection
- A wire is broken
- A device (a detector, a switch, a horn, a strobe, etc.) has come loose or has been removed
Talk to Technical Support
Your job is to figure out the problem, and re-connect or fix whatever is stopping or slowing down the current in the building. Most of the legwork needed to find the problem will be done by you, but you can get help starting out.
Download the installation manual for the panel from the web. Read that to get started.
After looking at the manual, talk to Technical Support. The technical support team at the manufacture (Honeywell) can help you figure out exactly where to start looking for the problem. However, before contacting them, you need to have the following information ready.
- The exact model number of the fire alarm system panel
- Exactly which lights on the panel are lit
- Exactly which lights on the panel are not lit
- Exactly what the display on the panel is saying. All the numbers and letters give information, so everything shown on the display is important
Douglas Krantz
