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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 up to 40 kilobecquerels.
  • In accordance with the German Radiation Protection Act (StrlSchG) § 12 (1) no. 3, 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 according to § 45(1) no. 1 StrlSchG. The Federal Office for Radiation Protection (BfS) is the competent authority responsible for type approvals.

Ionisation smoke detectors (ISD) are smoke detectors using radioactive substances, in most cases americium-241 with activities up to 40 kilobecquerels.

Ionisation smoke detector Ionisation smoke detectorIonisation smoke detector Source: Apollo

Principle of operation

The radioactive material is usually electroplated on a backing layer (foil), so that it is fixed. 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, the ionisation current changes. This change is detected electronically and is used for setting the alarm. ISDs principally differ in number and design of the measurement and reference chambers and in the electronic systems used.

Licence or type approval

ICDs are equipment containing radioactive substances and, thus, require a licence according to § 12 (1) no. 3 StrlSchG. Such equipment may also be operated without a licence if it is type approved according to § 45 (1) no. 1 StrlSchG. The Federal Office for Radiation Protection (BfS) is the competent authority responsible for type approvals.

Requirements for granting a type approval

In order to gain a type approval ISDs have to meet various requirements according to § 16 of the German Radiation Protection Ordinance (StrlSchV).

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 sealed enclosure and covered in a way that it is non-touchable. Also in case of fire, an ISD must remain tight in order to prevent releases of the contained radioactive material.

Usually the manufacturer of an ISD already limits its useful life to between ten and 15 years. As an ISD is not subject to regulatory control, it has to be designed in such a way that no additional leakage tests for the radioactive source will be necessary during its useful life apart from the acceptance test by the manufacturer and the leakage test according to § 25 (4) StrlSchV, which might be necessary. The authority responsible for the type approval may define deviating regulations in the approval certificate.

Restrictions for the type 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 according to Annex 4 Table 1 column 2 StrlSchV. In the case of americium-241 the tenfold of the exemption level amounts to 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 owner of an ISD has to return it to the type approval holder after use according to § 25 (5) StrlSchV. If this is not possible, the equipment has to be returned to a State Collecting Facility or to a facility appointed by the competent authority.

If an ISD is type approved it may only be operated and stored without a licence. Persons or companies, who install, remove or maintain an ISD require a licence even if the equipment is type approved.

Release of radioactive material is not possible under normal operating conditions

The release of radioactive material is not possible under normal operating conditions. Because of the very low dose rate in the vicinity of an ISD (see above) and the usual working distance from a smoke detector, an individual is only exposed to a maximum dose of some ten microsieverts per year. Compared to the natural radiation exposure in Germany, which amounts to about 2 millisieverts per year, the health risk from an ISD is negligible under normal operating conditions.

The type testing is only carried out for one single prototype. Therefore all manufactured ISDs have to be identical to the prototype with respect to their characteristics relevant to radiation protection. For this reason, the manufacturer has to maintain a quality assurance system which has to be monitored by an expert appointed by the authority responsible for type approvals.

Responsibilites for granting a type approval

An application for a type approval for an ISD has to be submitted to the BfS. The BfS involves the Federal Institute for Materials Research and Testing (BAM) in the process of granting a type approval. The BAM performs a series of specialised tests particularly with regard to leak tightness, choice of materials and design of the enclosure of the radioactive material as well as quality assurance. These tests comprise also temperature resistance, mechanical strength (impact, vibration and drop test) and leak tightness of an ISD in case of fire.

Uses of ionisation smoke detectors

The application of ISDs is nowadays mostly limited to specific tasks. They are still used in highly explosive environments, on ships or similar areas. This is mainly because by now optical smoke detectors or combined detectors can be used as alternative detection systems in many areas of application.

In other countries (for example Great Britain, USA), ISDs are still widely used, including in private households.

State of 2023.06.02

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