Page 365 - IGC BOOK

Detectors

Detectors detects change in the state, this may be either through detection of smoke or heat.

Smoke Detectors

• Ionisation

• Optical
Ionisation detectors ionize the oxygen and the nitrogen in the air, which causes a small electric current
in the detector. When smoke enters the chambers, the current is reduced and the alarm sounds.

Optical detectors have a light source and a photoelectric cell in the detector. Smoke enter the chamber
either obscures the light between the cell and the source, known as a light obscuration type detector
or causes the light to reflect on the to the cell, known as light scatter type detector in both the cases
it causes the alarm to sound.

Heat Detectors

• Fixed Temperature

• Rate of rise
Heat detectors rely on the principle of the expansion of solids, liquids or gases to operate. Fixed
temperature detectors operate when a predetermined temperature has been reached whilst rate of
rise detectors allow workplace which generate a slow build-up of heat to do so without actuation.
However, should a sudden increase in temperature occur, as would be found in a fire situation, the
detector will activate.
Smoke Detection

Smoke is usually the first detectable sign of any carbonaceous fire;
therefore, smoke detectors are most frequently used

unless there are problems with steam or dust, etc., which may trigger
false alarms.

Ionizing detectors operate when smoke enters the detector and it
absorbs ions, reducing the flow of electrical current. These devices are
very sensitive to smoke and may be triggered by cigarette smoke and
dust, etc.

Light scatter detectors operate when smoke scatters a beam of light. Smoke Detector

Obscuration detectors operate when smoke enters the detector
preventing light from reaching a photoelectric cell.

ENSIGN | Unit IG2 – Element 10 –Fire 20