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Fire Alarm -- Description

Early methods of sounding a fire alarm involved breaking a glass pane
Here, the manual fire alarm station inherently latches on. When sounding the alarm, glass is broken. The fire alarm system cannot be reset until the glass is replaced.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Why do Fire Alarm Pull Stations Break?

By Douglas Krantz

Ever wonder why a manual pull station tends to break when someone attempts to push it back after setting off the alarms? I, too, have been wondering this. To figure this out, one of the keys is to look at the early fire alarm systems.

Manual Fire Alarm Activation

The first fire alarm systems were manual fire alarm systems. As opposed to automatic activation, like a smoke detector or waterflow switch, someone had manually trigger the alarms.

Anyone in the building could set off the alarms, but only someone in authority could turn off the alarms because stopping the sound of the alarms is like sounding the all clear. Once it's been activated the fire alarm system had to latch to prevent a false "all clear."

Non-Latching Fire Alarm Control Panels

Many of the original fire alarm control panels only provided power for the fire horns and supervised the system wiring. They did not latch, and they did not have a reset button.

To activate, the manual station would turn on the alarms, and to reset, the manual station would turn off the alarms.

By the 1960's and 1970's, these original non-latching control panels were mostly phased out of service, but a few were still in operation as late as the year 2014 (Couch was one manufacturer).

Latching Manual Stations

Instead of the fire alarm control panel latching, the manual stations themselves inherently latched. They were "break-glass" activated; when a hammer or glass breaker was used, a button would pop out, sounding the alarm.

Once the glass was broken, the alarms would keep sounding until someone replaced the glass. In essence, the manual fire alarm station kept sounding the alarm until an authorized person investigated.

Evolution to Manual Pull Stations

Over time, to avoid accidental false alarms when the manual station was bumped, it was changed. Now instead of a glass pane that could be bumped and broken, the manual stations had a lever that had to be pulled. This reduced false alarms, but without proper design the manual pull stations had the side effect of not latching; they could be reset by anyone who didn't like the noise of the alarm.

Latching versus Indicating

There's a difference between an indication that the manual station has been activated, and the manual station latching so it stays activated.

The lever could stick out when someone pulled it, a piece of glass could break showing that it had been pulled, but even when pushed back by someone trying to turn off the alarms, the manual pull station needed to stay on. It had to latch.

Latching the Manual Pull Station

Nowadays, all fire alarm control panels latch once they go into alarm, and they stay in alarm until manually reset.
Was this
Yes   No

But then again, there may still be a few of the old non-latching fire alarm panels around, so for the time being, pull stations have to latch.

Further Information on Manual Pull Stations

Of course, someone reading this may have more information on the reason so many manual pull stations break rather than being reset. I'd be interested in hearing about it.


Douglas Krantz

Describing How It Works

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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