Douglas Krantz - Technical Writer - Describing How It Works
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Why Would a Smoke Detector be Wired Incorrectly?

Smoke detectors are usually replaced when they fail to detect smoke during a test. There are usually a minimum of 4 unlabeled wires behind the smoke detector, sometimes the replacement isn't installed quite right. Sometimes the replacement detector, even though it looks right, is not what should have been used.

Smoke detectors can cause problems with other parts of the fire alarm systemm if they are misswired, and not be reliable if they aren't certivied to be compatable.

Greetings Douglas,

Do you know why a detector would not be wired correctly? Just wondered how that could happen as an electrician claims to have replaced 2 detectors because they were not wired correctly (even though he says he then correctly wired them), but why replace them then? I'm confused.

Thank you, M J

Reason for Looking at the Detector Wiring

There are several very common ways that a smoke detector can be wired incorrectly. I'm not sure how your electrician found the two detectors, but for me, I found detectors that didn't work during the annual testing. Testing is basically "Spray canned smoke into the detector and see what happens". When the horns and strobes don't sound the alarm, further investigation is needed.

Once in a while, investigation shows that the smoke detector was miss-wired. It could have been that the polarity of the smoke detector was backward, so it wouldn't work. With a minimum of 4 wires going into the back-box behind the smoke detector, there could have been other issues with wiring.

If the smoke detectors were miss-wired, there is a higher probability that the smoke detectors were "incompatible" with the exact make and model of the fire alarm panel. In one place, the smoke detectors were the wrong type (they looked identical to the right type), and even though they set of the alarms, the miss-wired/wrong type of smoke detector also shorted out the speaker circuits of the panel.

"An incompatible smoke detector" actually means "The smoke detector won't work with the make and model of the fire alarm control panel". An incompatible smoke detector might not work at all, or an incompatible smoked detector might work right now, but it's unreliable.

If the electrician replaced the smoke detector because it was not compatible, the smoke detector that was removed might have worked, but wasn't reliable.

Resident Smoke Alarm

I've run into a situation in a 24 story condominium building where the smoke alarms inside the residences would sound off inside the condominium, and also send a supervisory signal to the office to indicate that the smoke alarm in that condominium was in alarm.

The condominium's maintenance person had replaced a faulty smoke alarm with a new one. The old smoke alarm wasn't made anymore and the new one was the manufacturer's official replacement.

Included with the new smoke alarm was a plug with short wires to allow the smoke alarm to be plugged in or unplugged. The old plug looked identical to the new one, and plugged into the new smoke detector just like the old smoke alarm, so the old plug was reused. Then the new plug was thrown away with the old smoke alarm.

Later testing showed that even though the smoke alarm worked, it didn't send a signal to the office. Just to obtain a working plug and fix the wiring, the whole smoke alarm had to be replaced.

Many Possibilities

Miss-wired polarity, incompatible smoke detector, replacement of the smoke alarm while not replacing the pug are some of the possibilities for the need to replace a smoke detector.

The worst I've seen is where a fire alarm technician, who should have known better, had problems with a two-wire smoke detector. The technician took out the two-wire smoke detector, saw that there were four pairs of wires inside the back-box, and replaced the two-wire smoke detector with a four-wire smoke detector.

Yes, when tested, the four-wire smoke detector sent in an alarm to the fire panel. But at the same time, it now also shorted out the speaker circuit.

Because a field-replaceable fuse was now burned out in the speaker circuit of the fire panel, the technician told the owners of the 14 story hotel they needed a new fire alarm panel.

Even fire alarm technicians make mistakes in wiring.

Does It Work?

The fire alarm system is a life safety, property protection system. If something doesn't work properly, someone may be hurt or a fire could do a lot more damage.

If there's a question as to whether or not a smoke detector is going to work reliably, from a life safety, property protection point of view, replacement of smoke detectors that may be unreliable seems to me to be a better choice than relying on smoke detectors that are questionable.


An extremely important issue here is testing after rewiring and replacement. Canned smoke should be sprayed at the smoke detectors, one at a time, and the horns and strobes should have then sounded off.

This testing is the only way a person can know that the rewiring was done right.

Douglas Krantz
Mr. Krantz

Thank you so much for this reply. i can't tell you how helpful this is.

Thank you, M J
Life Safety
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