No Battery Trouble on FACU or FACP

Remove Fire Alarm Battery Trouble

Before you read on please be advised that this tactic is only to be used in training FACU / FACP applications.  It is not the intention of Fire Alarms Online to provide this information to eliminate the need for battery backup on a FACU Fire Alarm Control unit, MNS Mass Notification System, RPS Remote Power Supply, etc.

FACP is Normal without Batteries WOW!

There is a way to connect an FACU Fire Alarm Control Unit without backup batteries and have it remain in the normal condition.  Now like I mentioned above, this is typically only used if you have a training and or demonstration FACU that you travel with.  You can also use this trick if you have a redundant FACU in your shop used for training (only if the system is not an approved life safety system for the site).

The backup battery charger on an FACP, FACU, MNS, RPS, etc. is always looking for 24VDC.  This is how it supervises the backup batteries.  In other words, this is how the fire alarm control unit knows that backup batteries are present.  The same terminals on the charger, also put out 24VDC in order to constantly charge the backup batteries so that they are always ready in the event of primary power loss.

FACU Backup Battery Requirements

Most jurisdictions require 24 hours of standby and 5 minutes of alarm for a horn/strobe system and 24 hours of standby and 15 minutes of alarm for a voice system.  See here for battery backup calculations.

With the use of one simple diode, you can accomplish this feat.  Simply place the Anode (+ Solid Black) side of the diode into the non-resetable positive 24VDC power output on the FACU / FACP.  Now place the Cathode (- Stripe) side of the diode into the positive terminal of the backup battery charger.

How this works:  Think of a diode as a one way gate.  Electrical current can flow through it in only one direction.  So this trick is actually quite simple.  The diode is providing a positive current path from the +24VDC output on the panel FACU / FACP and into the + terminal of the battery charger thus tricking it into believing there are backup batteries in place.  The reason for the diode is very important.  You need to remember that the battery charger puts out 24VDC as well.  With that said, we need to block that power from coming back into the +24VDC output on the FACP / FACU.  Here is a picture to help explain this trick on how to keep an FACP / FACU in the normal condition without backup batteries.


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Magnetic Door Holders and Battery Backup with FACU

Dropping Door Holder Power with Loss of AC Voltage

When designing a fire alarm system you may come across the need for magnetic door holders.  There are no codes nation or state wide that require doors to be held open.  With that said, magnetic door holders are used as a convenience to hold doors open during normal operation.  If a customer wants to hold open a door along a non fire rated wall then they can prop it open.  Now if the customer wants doors along a fire rated wall to be held open then they must use fire rated doors and magnetic door holders tied into the building fire alarm system.  Make sure you understand the correct smoke detector spacing for the activation of smoke doors.

I wanted to write this post to assist installers and system designers with a practice that will help eliminate the need for larger fire alarm system back up batteries to achieve 24 hours of standby and 5 or 15 minutes of alarm.  If the building loses 120 VAC power, the fire alarm system must switch over to backup batteries calculated to handle all of the devices tied to the system.  Door holders draw a lot of current and quickly add to the size of batteries required to properly backup a fire alarm system.

Since magnetic door holders perform their intended function with the loss of power, it is common to install a PR-1 interface relay to drop them out in the invent of AC power loss.  Simply power up the PR-1 interface relay with the AC power at the FACP / FACU or RPS (remote power supply) and break the door holder power circuit switch leg through the N.C. (normally closed) contacts on the PR-1 relay.  If the AC power to the FACU or RPS is lost, the PR-1 relay will change state and drop power to the field magnetic door holders.

Keep in mind that magnetic door holders are self supervising so there is no need to install any end of the line power supervision modules.  Just in case that did not make sense, the door holders perform their intended function (close) upon the loss of power.  You would only need to monitor the power at the end of line if the devices on that circuit require power to operate.