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Ask the Technician

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer





C Wrote:

I have a question... Is it important for smoke detectors to be polarity correct? Are all smoke detectors polarity sensitive?

Douglas Krantz Wrote:

Many smoke detectors are polarity sensitive; if the wires aren't landed so the positive wire is on the "+" terminal and the negative wire is on the "-" terminal, the smoke detector won't work. Some smoke detectors are not polarity sensitive; if the positive and negative wires aren't landed correctly, the smoke detector will still operate.

Of course, if a wire is connected to a third terminal on the detector for an annunciator or relay connection, or if the smoke detector is used directly on a Signaling Line Circuit (SLC), the smoke detector is still polarity sensitive.

Long term, if the service technicians have mixed and matched different detectors in the system, some of the detectors wired correctly and some wired reverse, future service on the system can be a nightmare.

Just as a matter of principle, I read the installation instructions that come with the detector, and I always connect the smoke detector as if all smoke detectors are polarity sensitive. That way I don't have to remember which make and model is polarity sensitive.







Douglas Krantz

Describing How It Works
writer@douglaskrantz.com
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612/986-4210

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Articles

How Does Class A Fire Alarm Wiring Work?-- Fire alarm systems save lives and protect property. Fire alarm systems also break down because... Read More

Just What Is a Signaling Line Circuit (SLC)? -- The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss ... Read More

How is a Buffer Relay Wired Into a Door Holder Circuit? -- Like a door stop, a door holder keeps a fire door open. When smoke is detected, the door holder releases, allowing the door to shut. The door holder looks simple and innocuous enough... Read More

How Does One Find a Soft Ground Fault? -- Normally, we think of resistance like that of a resistor. The amount of resistance is built-in; no matter what voltage is used to drive the electrical... Read More

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More



Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.