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Thursday, March 12, 2015

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.

PR-1 Interface Relay for Fire AlarmI 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.