Resistance of Fire Detectors to Industrial Electromagnetic Interference
Fire detectors are the sensory organs of a fire detection and alarm system (FDAS). They collect and analyze initial information about the state of the environment. These devices must provide:
• early detection of fire;
• minimum (ideally - absence) of false alarms;
• good performance in any operating conditions.
It is important to ensure long-term reliability of a fire alarm system especially for facilities with difficult operating conditions, in particular, with electromagnetic interference impact.
Types of Electromagnetic Interference
Electromagnetic interference (EMI) can be defined as unwanted electromagnetic energy from an external source that disturbs the proper functioning of an electrical circuit. EMI may degrade the performance of the device or even stop it from functioning.
Electromagnetic interference can be classified into types based on several factors, for example:
In this paper we consider the impact of industrial EMI on fire alarm system devices, in particular, fire detectors.
Industrial Electromagnetic Interference
Industrial EMI can be defined as disturbances generated by electrical and electronic devices used in industrial facilities, science researches, medicine, households, etc.
Sources of industrial EMI are numerous: machines, motors, electrical instruments and appliances, welding equipment, power transmission lines, generators, transformers, electric trains and trams, etc.
All industrial facilities have a high EMI level. Especially complex, saturated, so to speak, is the electromagnetic environment of electric power facilities – power plants and substations.
EMI Resistance of Fire Detectors
Resistance (immunity) to electromagnetic interference is the ability of a device or equipment to endure disturbances without degradation of performance.
Since FDASs require superior reliability, they should be engineered to operate in the presence of some amount of EMI. But in fact, all electrical and electronic devices, both working and faulty, generate electromagnetic interference in a wide range of frequencies. In real conditions, it is almost impossible to predict the electromagnetic environment at the location of the system components. Thus, fire alarm system manufacturers have to take into account numerous factors to ensure resistance to industrial EMI of their products.
Electromagnetic interference in the same frequency band where are internal (data) signals is dangerous for fire detector operation. Such EMI goes through the device's input filters and can be processed as a valid signal.
Low-frequency range of EMI spectrum that doesn't reach the device operational frequency band usually impacts on circuit elements closest to the device input and even can cause their failure. High frequency range of EMI spectrum that is out of the operational frequency band is able to "get round" protectors and penetrate deep into the electronic circuit. Parasitic capacitance and inductance may be to blame.
Data exchange between the system internal components is often carried out without error detection and correction, that's why even a slight distortion of the information signal can cause a malfunction, a false alarm and even is able to block the entire system functioning.
Resistance of Ista Fire Detectors to Industrial EMI
Designing the first models of conventional smoke detectors, our specialists worked hard to increase their EMI immunity.
The success of implemented solutions was confirmed by certification tests that demonstrated compliance with DSTU 2465-94 "Electromagnetic compatibility of technical means. Immunity to power frequency magnetic field. Technical requirements and test methods" (test report number 26-20-10). In the test report, it had been recorded that the device under test passed the immunity test with the level 5 for continuous magnetic field and the level 4 for short duration field.
The standard mentioned above was replaced with DSTU EN 61000-4-8:2017 (EN 61000-4-8:210, IDT; IEC 61000-4-8:2009, IDT) "Electromagnetic Compatibility (EMC) – Part 4-8: Testing and measurement techniques – Power frequency magnetic field immunity test" with identical requirements. The standard defines test levels for continuous and short duration application of the magnetic field applicable to distribution networks at 50 Hz and 60 Hz and describes the selection of test levels:
"Class 4: Typical industrial environment ...
Fields of heavy industrial and power plants and the control room of H.V. sub-stations may be representative of this environment.
Class 5: Severe industrial environment ...
Switchyard areas of heavy industrial plants, M.V., H.V. and power stations may be
representative of this environment."
So, the test confirmed that our smoke detectors were able to operate in complex electromagnetic environment of industrial plants and electric power facilities.
Experience in improving EMI immunity of conventional smoke detectors was very useful for development of new models of addressable fire detectors and other devices. The use of the latest electronic components and advanced circuit solutions helped provide good technical characteristics of our modern devices, their high resistance to electromagnetic interference and minimize nuisance fire alarms of smoke detectors.
Fire detectors, developed and manufactured by our enterprise, installed at Zaporizhzhya NPP, Boryspil International Airport (in particular, in the airport's control tower where there are strong electromagnetic fields from radar and navigational equipment), in traction substations and in the sub-platform area of several Kyiv metro stations.
Reducing EMI Impact on Fire Alarm System Functioning
The task of improving a fire alarm system immunity to electromagnetic interference must be solved comprehensively. Besides ensuring EMI immunity of its components, it is necessary to have the entire system professionally designed and correctly installed.
Improper detector wiring, incorrect cabling, grounding and shielding can nullify the benefits of reliable devices.
It is necessary to follow rules and standards for low voltage cable system installation: alarm loop cables should be laid parallel to power distribution cables at a distance of at least 50 cm, and their intersecting should be at right angles. It is also important to provide electrical continuity of cable shield: the shield of each conductor must be connected to the signal-reference node.