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Type approval of ionisation smoke detectors (ISD)
- Ionisation smoke detectors (ISD) are smoke detectors using radioactive substances, in most cases americium-241 with activities between 15 and 40 kilobecquerels.
- In accordance with the German Radiation Protection Ordinance (StrlSchV), a licence is required for the handling of equipment containing radioactive substances.
- It is also possible to operate such equipment without a licence if it is type approved. Since 2001, the Federal Office for Radiation Protection (BfS) has been the competent authority responsible for type approval.
Principle of operation
The radioactive material is usually applied to a backing layer. This radioactive foil is firmly mounted within the ISD casing with holders. The radiation emitted from the foil ionises the air in the ISD. An electric voltage applied within the ISD produces an ionisation current. If fire aerosols enter the ISD through its vents, the size of the ionisation current changes. This change is detected electronically and is used for setting off the alarm. ISDs principally differ in number and design of the measurement and reference chambers necessary for measuring the ionisation current and in the electronic evaluation system used.
In accordance with the German Radiation Protection Ordinance (StrlSchV), a licence is required for the handling of equipment containing radioactive substances. It is also possible to operate such equipment without a licence if it is type approved. Since 2001, the Federal Office for Radiation Protection (BfS) has been the competent authority responsible for type approval.
Requirements for granting type approval
ISDs have to meet various requirements before being granted type approval.
The ambient dose rate must not exceed 1 microsievert per hour at a distance of 0.1 metre from any touchable surface of the device. The radioactive material has to be contained in a tightly sealed enclosure and covered in a way that it is safe to touch. Also in the event of a fire, the sealing of an ISD must remain sufficiently tight to prevent unlawful releases even then.
Usually the producers already limit the useful life of an ISD to between ten and 15 years. As an ISD is not subject to regulatory control, it has to be designed in a way that no additional leakage test for the radioactive material incorporated into it will be necessary during its useful life after the acceptance test conducted by the manufacturer. The BfS may set out divergent regulations in the approval certificate.
Restrictions on granting approval
Moreover, a type approval may only be granted if the activity of the radioactive material incorporated in the equipment does not exceed the tenfold of the exemption levels. In the case of americium-241, that is a value of 100 kilobecquerels.
Non-destructive dismantling or opening the casing of the ISD should only be possible with special tools to prevent accidental removal of the radioactive foil.
The owners of the ISDs have to return these to the type approval holder after termination of use. If this is not possible or would involve disproportionate effort, the equipment has to be returned to the Federal Land Collecting Facility or to a facility appointed by the competent authority. Whoever installs, removes or maintains ISDs, requires a licence even if type approval has been granted.
Release of radioactive material not possible under normal operating conditions
The release of radioactive material is not possible under normal operating conditions. On account of the very low dose rate and the working distance from the smoke detector, an individual is only exposed to a maximum dose of some ten microsieverts per year. Compared to natural radiation exposure, which is about 2.4 millisieverts per year, the health risk from ISDs is negligible when they are used as intended.
As the equipment is tested for its suitability using only one single prototype during type testing, all ISD samples manufactured later have to be identical to the prototype with respect to the characteristics relevant to radiation protection. For this reason, the manufacturer has to operate a quality assurance system which is monitored by an expert who is appointed by the BfS.
Stakeholders in granting type approval
The BfS involves the Federal Institute for Materials Research and Testing (BAM) in the process of granting type approval for an ISD. The BAM performs a series of specialised tests particularly in regard to the assessment of leak tightness, choice of materials, design of the capsule enclosing the radioactive material as well as quality assurance. In this way, the ISDs are tested for their temperature resistance, mechanical strength (impact, vibration and drop test) and leak tightness in the event of a fire.
Uses of ionisation smoke detectors
The application of ISDs is becoming increasingly limited to specific tasks. They are, for example, used in highly explosive environments, on ships and the like. This is mainly because the requirements of the German Radiation Protection Ordinance (StrlSchV) and the requirement of separate disposal together result in additional costs and at the same time, the advancement of optical smoke detectors and combined detectors has led to the development of alternative detection systems in many areas of application.
In other countries (Great Britain, USA), where ISDs may be employed without a licence and do not have to be disposed of separately, they are still widely used, including private households.
State of 2018.04.05