False Alarms in Automatic Fire Alarm Systems
It is vital that a fire alarm system of a building is able to provide the earliest and reliable warning of fire. "Reliable warning" means it is trustworthy, certain, unquestionable and errorless.
Unfortunately, a great number of unwanted alarms is received by fire departments, also in automatic fire detection and fire alarm systems (FDAS), nuisance fire alarms occur frequently. A fire alarm signal activates a fire protection system, begins an evacuation process. A business or a production process is interrupted, stock spoiled, etc. Additionally, there are a range of other costs such as congestion, traffic accidents whilst fire brigades rush to attend false callouts.
Nuisance fire alarms cause a range of costs which must be paid by the emergency services, businesses, organizations and the general public. Another negative effect is that when a fire alarm system cries wolf, people lose confidence in it to the point where they will ignore a true emergency situation or, even more, will turn the system off.
Minimizing nuisance fire alarms is an important task both for developers of fire alarm systems and devices, designer engineers, fire alarm installation services and for building and business owners.
Causes of False Fire Alarms
A false alarm in an automatic fire alarm system is a fire alarm signal resulting from a cause(s) other than a fire. The most common examples include:
- a fire-like phenomenon or environmental influence (smoking outside designated areas; heat, smoke, steam or flame from cooking or production processes; dust, exhaust fumes or rapid air flows);
- accidental or malicious manual call point activation;
- lack of fire detector maintenance;
- fire alarm system devices testing or repairing without notifying a fire department;
- mechanical damage or electrical malfunction of devices caused by vibration, shock or corrosion;
- works in the monitored area performed by poorly trained personnel or without following precautions and safety measures (electric tools cause electromagnetic interference, fire detectors without protective covers);
- low level of fire alarm system devices protection from electromagnetic interference:
a) electromagnetic disturbance impact on the communication line between the control panel and fire detectors,
b) non-compliance of technical equipment with electromagnetic compatibility requirements for specific premises.
In this article, we will examine the causes of fire detector nuisance alarms and describe technical solutions to reduce their likelihood.
Nuisance or False Alarm Signals from Smoke Detectors
Fire detectors are the sense organs of a FDAS and these devices must meet high requirements. One of them is the minimum probability of false alarm.
Smoke detectors are the most widely used. Generally, they respond faster than heat or flame detectors, and they respond to smoke that is the most common sign of fire. That is why these devices give great part of overall false alarms.
Analyzing the operation of point photoelectric (optical) smoke detectors in real operating conditions the following causes of false alarms could be identified:
- construction features of the smoke chamber;
- dust or insects in the smoke chamber;
- electromagnetic interference induced on the detector's electronic circuit;
- incorrect smoke detector positioning;
- poor commissioning and maintenance.
Reduction of Optical Smoke Detector False Alarm Probability: Technical Solutions
Specialists of our company have developed a model of a point optical smoke detector. They gave special attention to reducing the probability of false alarm signals. Additionally, they worked on the detector enclosure design, improved electromagnetic compatibility of the device and signal processing algorithms.
To ensure the optimal operation of the smoke chamber, it is important to provide its sufficient ventilation and at the same time to minimize the dust ingress. A consultation with aircraft designers helped to calculate the opening dimensions.
With accumulations of dust and dirt inside the sensing chamber the smoke detector becomes more sensitive, which could eventually lead to nuisance alarms.
Due to individual calibration of each detector we achieve high reproducibility, maintain a constant level of response threshold value from specimen to specimen. Then, by means of a device programmer, one of three sensitivity levels calibrated by the degree of light attenuation in smoke can be set.
In our smoke detectors, an internal sensor signal processing algorithm is used that includes a sensitivity drift compensation in order to maintain a more constant level of response threshold value with time even when the smoke chamber is very dirty.
To improve devices' resistance to electromagnetic interference our tech team was focused primarily on increasing the signal-to-interference ratio. They saw two ways: to increase the sensor signal intensity and/or to reduce the interference impact.
To increase the sensor signal intensity, we could, for example, increase the infrared LED pulse power, but this could increase the detector's power consumption. In order to slow down the degradation of the infrared LED working at high currents, we reached a higher pulse power level due to a balanced increase of its pulse amplitude and duration.
For reducing electromagnetic interference impact, the photodiode external shielding and the input amplifier shielding with an additional layer of copper were made. Also, an optimal layout of the device's printed circuit board was designed with a minimized number of components and trace length.
In addition, the detector electronic scheme includes industrial frequency interference filtering and the photodiode signal integration. ADC signal is additionally filtered to exclude nuisance actuations.
The integral approach and an optimal combination of several methods allowed to achieve a significant reduction in false alarms produced by smoke detectors and to avoid increasing detectors' consumption.