There is an increasing tendency for fire alarm systems to be configured for coincidence operation (double knock) where two detectors have to operate before the evacuation signal is actuated.

Should the fire detection system be categorised P1 or P2 there is no real problem as the system design is based on property protection, but should the requirement for the system be “Life Safety”(L system) one should think twice before applying this principle of operation.

Coincidence operation is a requirement for gas extinguishing systems to ensure the fire condition is confirmed before dumping expensive fire suppression gasses but should only be used for fire detection systems if persistent false alarms are occurring or the disturbance of people is a real problem.

“Double knock” operation is being requested by system designers on new installations with the fear of false alarms disturbing the staff or guests; this raises the question “have you designed a defective system that is going to give you false alarms?” “Double knock” is an excuse to treat the symptoms and not solve the cause; with today’s technology and the choice of so many fire detection types to choose from, elimination of false alarms becomes easier. Too often the automatic choice of fire detection principle is optical smoke detection where it is known these devices are sensitive to steam and dust. We have CO detectors, multi criteria detector, aspiration smoke detectors and dual LED optical smoke detectors all of which have their own unique principles of operation that will not trigger into alarm when granny burns the toast, or people smoke where they shouldn’t.

FireHotel rooms today are built with open plan bathrooms and provide kettles which are both subject to providing false alarms but contractors and designers persist in installing optical smoke detectors in these environments when other devices with differing principles of operation are more suited.

If false alarms are being experienced from a fire detection system, conduct an investigation to determine the source of the alarm and fix the problem rather than finding a remedy for the situation, “double knock”.

Consultants are calling for a double knock system at time of tender, is this because they are not confident in what they are doing or are they deliberately designing defective systems?

The actual purpose of the fire detection installation is often forgotten i.e. protection of property or protection of life by providing the earliest possible warning of fire to allow the safe evacuation of people from the effected building or providing warning for persons who can attend to the fire condition.

By utilising two detectors to trigger before raising a fire alarm can seriously reduce the time available for safe evacuation. A fire can generate dangerous fumes and toxic air within minutes; we do not always have minutes available to raise the attention of fire fighters!!

Several cases have been reported of new or refurbished hotel fire detection systems being designed with the “double knock” operation. There is only one smoke detector in the guests’ room but this is not allowed to raise an evacuation alarm until a second detector triggers a smoke alarm.

Hotel rooms are fire separated from each other with walls built to the concrete slab above and tightly fitting fire doors, rooms are built to reduce sound (and as a consequence smoke); for the smoke to be able to find a way out of the one bedroom and operate a second detector in the corridor or the room next door is going to be very long, the person, if asleep, will possibly not survive and the fire damage to the room will be great before the general alarm is raised. The explanation is that the first alarm will raise the attention of the security guard, or receptionist, at two o’clock in the morning who will investigate the first alarm. The weakest link in any fire detection system is the human being and numerous cases are reported of operators ignoring alarms or resetting them without investigation, leading to devastating circumstances.

If the areas are large open plan areas smoke can travel relatively freely and “double knock” or confirmation operation can be accepted, but in enclosed areas “double knock” can be dangerous or downright idiotic.

The client complains that the staff or guests must not be disturbed by fire sirens, if they never hear the sirens how do they know what type of sound it is when a real fire occurs? In many buildings staff has no idea what the fire evacuation signal sounds like because management have isolated the sounders. This is extremely dangerous in the event of a real fire alarm.

Double knockA preferred method of operation should a client not want immediate operation of evacuation procedures is to utilise “staff alarms”. Should a fire detector operate it sounds a “staff alarm” only in a 24 hour manned location. The operator can now investigate the alarm and reset the system should it be found to be a nuisance alarm or actuate the fire evacuation system in a real case of fire. The operator is given a fixed time period to respond to the alarm; the SANS standard specifies a maximum of 6 minutes, we recommend 3 to 4 minutes and if the alarm is not responded to, the evacuation sirens are automatically operated.

The operator must be made aware that the safety of people in the building now rests solely on his shoulders and all fire alarms must be treated seriously.

THINK TWICE BEFORE KNOCKING TWICE