Smoke Detectors

Smoke Detectors
Smoke detectors/alarms are important safety devices, because of their obvious potential to save lives and property. There are two types of smoke detector commonly available in many countries.

There are two types of smoke alarm:

·      Ionization Type

·      Optical Type

Ionization Type

Referring to the adjacent diagram Electrode A is treated with a Very level of Radiation material and a potential is applied between Electrodes A and B. On account of Radiation the air between the Electrodes A and B is ionized and ionization current flows between the Electrodes. In the presence of smoke between the electrodes the ionization current varies (due to the smoke particles) and thus the smoke is sensed. The Output from the electrodes is fed to an Electronics amplifier and a schmmit Trigger which will give an ON / OFF output for the Presence / Absence of smoke.

The smoke Detector is tested periodically (once in 6 months or as the case may be depending on the policy of the organization) by an Aerosol spray from a smoke canister. (NOT BY CREATING A SMOKE)

The Optical Type

The Optical type is based on the principle that when smoke passes through a source of Light (LED) and a photodiode / photocell the current through the Photocell / Photodiode varies. The arrangement is a source of light (says LED) is mounted in front of a Photodiode or a Photocell or may be a photo-resistor, under “No Smoke” condition the light falls on the photocell and a quiescent current flows in the circuit. When smoke passes through (between) the Light and the photocell the current in the circuit varies sensing the smoke. The signal is further processed (amplified and fed to a schimmt trigger) and a switch action is generated for the presence or absence of smoke.

The smoke Detector is tested periodically (once in 6 months or as the case may be depending on the policy of the organization) by an Aerosol spray from a smoke canister.

(NOT BY CREATING A SMOKE)

Usually one smoke detector will be located for each 3 Square meters of area. There is a NFPA (National Fire Protection Agency / Association, USA) regulation for the number of detectors for a given area.

Normally a number of smoke detectors will be associated with one zone. Due to atmospheric humidity many times smoke detectors may give rise to false alarms. As a result whenever there is a trip connected with smoke detectors it will never be based on one smoke detector sensing the smoke. Normally a given area will be divided into two zones and when one smoke detector from each zone in the same area simultaneously sense smoke then only the control action will be initiated. Consequently while testing the smoke detectors ensure that the smoke detector under test is (alarm acknowledged) reset before testing another smoke detector in the same area (look at the cause & Effect Sheet).

Each F & G system has a “Zone Module” to which the field devices are connected. Each Zone module gives an output depending on the alarm generated by the field devices. Each zone module has the following indications:

1.   Power On Indication

2.   smoke / Fire Alarm indication

3.   Card / Device Inhibit indication

Each zone module has the following controls:

1.   Alarm Accept / Acknowledge

2.   Alarm Reset

3.   Output Action Inhibit

Normally there will be a facilities module in the Panel (may be in each row) to which all the

Alarms and control actions are wired in the row.

The power to the field devices is usually provided from the zone module. So, whenever a field device is to be replaced, if we pull out the zone module the power to the field will be last. However before pulling out the module check the cause & effect sheet.

HSSD / VESDA

High Sensitivity Smoke Detection system / Very Early Smoke Detection Apparatus works similar to Optical smoke detector except that a high sensitive detection system (LASER) is used for detection. Further an Aspirator / pump is installed in the detection system that sucks the air sample and the same is passed through the detection system. This arrangement combined with the high sensitive detection system gives a high sensitive detection / Very Early Detection of smoke.

The principle difference between a normal smoke detector and High Sensitivity Smoke Detection system / Very Early Smoke Detection Apparatus is that in the latter the smoke is detected very early even when the smoke is in incipient stage (beginning to from) so that corrective action can be taken at a very early stage of smoke.

There is no fire without smoke. So the objective is to tackle the situation before it can get out control! The smoke is detected very early before it is too late.

There is a special test for testing the Very Early Smoke Detection Apparatus. A wire provided by the manufacturer is connected to a transformer (provided by the manufacturer) and the transformer is powered. The wire is located where the air will be sucked by the pump, this will be sensed by the VESDA and alarm is generated in the device.