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





L S Asked:

Hi Mr. Krantz,

I found your blog (www.douglaskrantz.com/Blogboosterpowersupply.html ) while looking for an answer to a question I have; for backup, does the signal power expander only use batteries as the power source or would a/c or a generator ever be used?

Douglas Krantz Answered:

As their primary source of power, fire Alarm Systems, including Fire Alarm Control Panels (FACP), Signal Power Expanders (SPX), and other auxiliary fire alarm power sources, use the AC utility power. This is the most consistent and reliable source.

However, the primary utility power, like any other source of power, sometimes fails. Batteries, as a secondary backup power source, are used only when the primary utility power fails. This is an automatic switchover from the primary source to the backup.

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A long time ago, some buildings had an emergency power entry so if the primary power source for the building failed, functions like emergency lights could stay running. At that time, there was no such thing as battery backup, so this emergency power was used to at least sound the trouble buzzer for the fire alarm system.

Nowadays, to back up the utility power for the building, many commercial buildings and larger residential complexes have an automatic standby (emergency) generator. The generator turns on when the utility power fails.

The generator doesn't start instantaneously; there's a short blackout period. During this time, if the fire alarm system is already in alarm, batteries are needed to keep it going.

As far as the generator goes, technically, the generator needs to be "Fire Rated", regularly tested, and monitored by the fire alarm panel as an official backup generator. A generator like this doesn't eliminate the battery requirement for any fire alarm panel; the generator just reduces the size of battery needed for backup.

The manufacturer of the fire alarm equipment has rated and tested the equipment with the batteries, so therefore, batteries are required.

Much of the time, this generator isn't used for the whole building, so for the generator to do any good, the whole fire alarm system (including the SPX) has to be connected to the generator.






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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 www.douglaskrantz.com/
ElecElectricalFlow.html
.