Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer
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Fire Alarm Testing
Can a Magnet Really be Used to Test a Smoke Detector?

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When a smoke detector is tested, should canned smoke be used or a magnet?
Yes, many manufacturers install magnet testing switches into their smoke detectors. The question is, which should be used for function testing of a smoke detector... Smoke or Magnet?
By Douglas Krantz

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 reed switch).

Both ways determine if the electronics within the smoke detector head will activate in alarm, but only the application of smoke determines if the smoke chamber inside the smoke detector will detect smoke.

Magnet Testing is Good For Testing the Fire Alarm System

Determining if the electronics of a smoke detector go into alarm and what the fire alarm system does with the alarm can be done with a magnet test.

Activating with a magnet determines whether the rest of the fire alarm system will cause: The problem is that the magnet test can't determine whether or not the smoke detector actually detects smoke.

Testing the smoke detectors using canned smoke has at one time been known to leave a residue. A magnet doesn't leave that kind of residue. However, the discussions on the pros and cons of using canned smoke will be discussed at another time.

Royal Chew Out over Magnet Testing

Most Fire Marshals view smoke in the chamber to be the only test of a smoke detector.

One time though, a fire marshal chewed me out because the smoke detectors installed by our company were not capable of being tested using magnets. He said that magnets were better for testing because those doing the testing (like me) could pump in as much smoke as we wanted for function testing, and by implication we would not get an accurate test. The opinion of that particular Fire Marshal was the magnet test is a calibrated test to determine if the smoke detector would function correctly.

Testing the Magnet Test

A few months later, with that chewing out still ringing in my head, I was at an older apartment building I had never been before. Previously, magnets had been used to test the smoke detectors. I decided this was a good place to see just how well a magnet would test them.

First, I tested a smoke detector with a magnet; the smoke detector went into alarm. Then I tested using smoke from the test can; the smoke detector did not activate.

Wanting to make sure I knew what was happening, I sprayed a tenth of a can of smoke on that one detector; the smoke detector still did not activate. The smoke detector passed with a magnet but failed to detect smoke.

In the same way, the other 30 smoke detectors in that building failed. Because of that inspection, that apartment building bought a new set of smoke detectors. So much for magnet testing.

Magnet Test vs Smoke Test

The problem with the magnet testing of smoke detectors is the actual smoke chamber is not tested. The magnet only closes a magnetic reed switch in the electronics. This switch mimics the signal that indicates that the smoke chamber detects smoke. However, the magnet does not test whether or not the smoke chamber can detect any smoke.
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The smoke detector looks for particles in the air. If there are particles in the air, then the chamber tells the electronics of the smoke, and the smoke detector latches into alarm. Once the smoke detector is in alarm, the fire alarm panel does its thing.

For testing what the fire alarm panels do when a smoke detector activates, magnets are great. However, the purpose of testing the smoke detectors is to find out if the smoke detector will detect smoke. To see this, the only test for smoke detection is the application of smoke.

Magnets just don't test to see if a smoke detector detects smoke.



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