Non-latching alarms.

Preamble

This document explains what a "non-latching alarm" is and how it can be produced and used with the Firepoint-2 fire alarm equipment.

Definition of non-latching alarm

Normally when a smoke detector or call-point is operated the fire alarm system must start sounding the alarms and continue to sound them until somebody intervenes to stop them sounding.  In other words the alarms must be manually silenced — the various regulations do not allow the alarms to silence automatically.   This type of alarm is therefore a latching alarm because once the fire is detected the system latches into the alarm state until human intervention cancels the alarm.

So, if alatching alarm is one that requires human intervention to end it, then a non-latching alarm is one that can silence and reset automatically without human intervention.

Non-latching alarms are very useful when you want to connect two or more alarm systems in different buildings to one another.  If a fire is detected in one building the alarms will sound in all buildings but they can also all be silenced from the building where the alarm originated.

What you need to have to implement a non-latching alarm.

Non-latching alarms are a standard feature of every Firepoint-2 fire alarm system so you do not need to modify or upgrade the control panel.  What you will probably need, however, is an “input unit”.  The input unit is functionally very similar to a manual call-point.  The difference between them is that a call-point is intended to be operated by a human while an input unit is intended to be operated by other equipment.  The other equipment could be a relay in another fire alarm system in an adjacent building, a door switch or anything that is capable of closing a pair of contacts.

The input unit is connected to the Firepoint-2 as another detecting device on the loops and will be interrogated just like a smoke-detector or call-point.  To generate a non-latching alarm from the input unit you simply need to use the DIL switches to set its address to "0".

With the Firepoint-2, any device at address "0" will always be treated as a non-latching device.

Because thre are two loop circuits on the Firepoint-2 it is possible to generate non-latching alarms from at least two sources because you can have an address "0" on both loops.

How it works

The input unit set to address "0" will be polled by the control panel in the usual fashion.  The input unit is activated by joining two terminals together (using a relay or switch).  When the control panel detects that the device is active the following will happen.

• The red fire lamps will illuminate with a scrolling pattern.  The non-latching alarm is not considered to be part of any "zone" in the usual sense so all the zone lamps are used.
• The alarms will start to sound but instead of being continuous they will sound intermittently on-off-on-off-on-off ....  This discontinuous action is one significant difference between a latching and a non-latching alarm.
• An entry will be made in the event log.
• The display will show the FIRE text for the activated device.
• The fire-relay does NOT change over.

When the input unit returns to its inactive state the Firepoint-2 will go through an automatic silence and reset sequence and return to its normal standby condition.  This will also be recorded in the event log.

The non-latching alarm can be silenced by a person before the input unit returns to the inactive state;  in this case the control panel will resume normal polling but the red fire lamps will continue to flash.  If the person resets the non-latching alarm before the input unit becomes inactive then the alarms will immediately resound.  A full reset will occur only when the input unit returns to its inactive state.

The discontinuous (on-off-on-off...) sounding of the alarms will be recognised as an "alert" signal but not as a proper fire alarm.  In other words, a person hearing the discontinous alarm will know that they should prepare for evacuation but that evacuation is not yet essential.  Safety, however, is not compromised because if a smoke detector or call-point connected to the panel is activated then the normal continuous alarm signal will begin.  A latching alarm will always take priority over a non-latching alarm.

If a latching alarm occurs during a non-latching alarm then the alarms must be silenced and reset manually.  Even if the input-unit returns to its inactive state the alarms will continue to sound because of the latching alarm.  A latching alarm will always over-ride a non-latching alarm.

Links

The Firepoint 2 fire alarm control panel.