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Interview Questions on Smoke detection system

1.What are the types of ‘smoke detectors’ ?

Three types of smoke detectors are used. They are:

  1. Optical type,
  2. Radioactive type,
  3.  Ionization type.

2.Explain the operating principle of the different types of smoke detectors.

Optical type: when the smoke cuts the optical detection path, and unbalance in the detector current loop raises a smoke alarm. These type detectors are sensitive to larger particles of combustion 0.5 to 10 microns. It is suitable for applications where thermal turbulence can be expected.

Radio active type: when the smoke is detected by the radio active substance, the unbalance in the detector current loop raises a smoke alarm. This type of detectors are sensitive to air movement greater than 5m/sec. It is normally installed with the photoelectric (optical type) type smoke detectors.

3.Why is a smoke detector as important as a ‘heat switch’ and ‘UV detector’?

Smoke detectors are used in confined areas such as control rooms and electrical switch rooms.

4. How carbon monoxide detectors work?

Carbon monoxide detectors sound an alarm when they sense a certain amount of carbon monoxide in the air over time. Different types of alarms are triggered by different types of sensors.

  • Biomimetic sensor: A gel changes color when it absorbs carbon monoxide, and this color change triggers the alarm.
  • Metal oxide semiconductor: When the silica chip’s circuitry detects carbon monoxide, it lowers the electrical resistance, and this change triggers the alarm.
  • Electrochemical sensor: Electrodes immersed in a chemical solution sense changes in electrical currents when they come into contact with carbon dioxide, and this change triggers the alarm.

Once the alarm sounds, the carbon monoxide detector must be in a carbon monoxide-free environment to reset itself.

5.What happens when a smoke detector detects smoke?

When a smoke detector detects smoke, the detector loop current changes. The unbalance in the loop current is detected as smoke.

Once in every 10 secs the L.E.D on each smoke detector blinks as self loop check. On detection of smoke, this L.E.D remains continuous on (bright).

6.name the manufacturers of smoke detection systems.

Name of the smoke detector manufactures are Firecheta, Cloride Gent…etc.

7.why are periodic checks and maintenance required on smoke detection system?

Particularly on the optical type smoke detectors, the collection of dust in the sensor path may result in a false alarm. Also the periodic maintenance ensures the reliability of the detection system.

8.What are the common problems faced when maintaining a smoke detection system?

  • False alarm due to accumulation of dust, floor cleaning, water vapour
  • Loose connection results in the fluctuation of the detector loop current.
  • Improper base plug-in connection.

9. Commonly used  terms in Gas Detection :

  • LEL – Lower Explosive Limit. The minimum concentration of a combustible gas or vapor in air which will ignite if a source of ignition is present
  • UEL – Upper Explosive Limit. Most, but not all, combustible gases have an upper explosive limit which is the maximum concentration in air which will support combustion. Concentrations which are above the U.E.L. are too “rich” to burn.
  • PPM – Parts Per Million (toxic & VOC)
  • %VOL – Percent by volume (oxygen)
  • VOC – Volatile Organic Compounds (PID)
  • PID – Photo Ionization Detection (VOC)
  • TWA – Time Weighted Average (toxic gases)
  • STEL – Short Term Exposure Limit
  • IP – Ionization Potential & Ingress Protection
  • IS Rating – Intrinsic Safety (UL, CSA)
  • T90 – Time sensor needs to reach 90% full response

10.  Ionization smoke detector principle

The ionization smoke detector is widely used. Its capability to detect smoke originating from fire is best utilized for clean-burning fires that produce small particles during combustion.

The ionization smoke detector consists of an alpha particle producing a radioactive source, a smoke chamber, and charged detector plates.

  • The alpha source causes the air within the smoke chamber to become ionized and conductive
  • As smoke particles enter the smoke chamber, the smoke particles attach themselves to the ionized air molecules and the air in the chamber becomes less conductive
  • When the air conductivity within the chamber drops below a predetermined level, the alarm is triggered

Objective Questions of Building F & G Detectors

1 What is the best tool available to us that can prevent a fire from creating victims?

a) a smoke alarm
b) a fire hydrant
c) a telephone
d) an extinguisher

2 Is it mandatory to have a smoke alarm in a house or dwelling?

a) no b) yes, in most municipalities
c) yes, in all municipalities

3 How many smoke alarms is it suggested to have in a dwelling or a house?

a) one per room
b) one is enough
c) one per floor

4 At what location is it preferable to install smoke alarms?

a) in the residence’s main entrance
b) in the laundry room, the kitchen and the bathrooms
c) in the hallways, near the bedrooms
d) in a room exposed to fires (e.g.: kitchen, living room with fireplace, etc.)

5 In an apartment building, who is responsible for testing and maintaining smoke alarms, including replacing the battery?

a) all tenants
b) the tenant of each dwelling
c) the landlord

6 What should be done when the smoke alarm emits intermittent audible signals?

a) remove the battery
b) call the Fire Department
c) replace the battery
d) replace the smoke alarm

7 What should be done when the smoke alarm emits a continuous signal?

a) panic and scream
b) call the Fire Department right away
c) attack the fire
d) leave and gather outside the residence

8 When must a smoke alarm be replaced?

a) every 5 years
b) every 10 years
c) every 20 years
d) never

1 Answer : a) According to statistics, a smoke alarm that works reduces the risk of death by half in the event of a fire. It also considerably reduces material losses, since the firefighters are called to the scene of the fire sooner.

2 Answer : b) Most municipalities have a by-law requiring installation of smoke alarms. The smoke alarm allows the occupants of a dwelling or house to leave quickly before the smoke spreads. It saves lives, especially when a fire breaks out at night when everyone is asleep.

3 Answer : c) For greater safety, it is generally recommended to have one smoke alarm per floor, including the basement. It is also suggested that smoke alarms be installed in bedrooms where people sleep with the door closed.

4 Answer : c) In the hallways, near the bedrooms, ensuring that there are no obstacles that prevent smoke from circulating. They are installed on the ceiling or on the walls, at a distance of 10 to 30 cm (4 to 12 in.) from the wall or ceiling.

5 Answer : b) The tenant of each dwelling is responsible for testing and maintaining smoke alarms and replacing the battery as needed. However, the landlord must install at least one smoke alarm per dwelling and replace them when necessary.

6 Answer : c) The battery must be replaced when the smoke alarm emits intermittent audible signals. These signals indicate that the battery is too low. When moving the clock back or forward in the fall or spring, take the opportunity to test the battery and replace it as needed. Never install a rechargeable battery. To test the smoke alarm, press the test button located on the casing for a few seconds. You should hear the signal immediately. To check if the smoke alarm detects smoke properly, extinguish a candle near it.

7 Answer : d) You must leave immediately and call the fire department from a safe location. Ensure that all the occupants have evacuated the dwelling or house. It is suggested that you prepare an evacuation plan with your family or cotenants in case of fire and provide for a gathering point outside. You must never go back inside a burning building.

8 Answer : b) It is recommended that smoke alarms be replaced every 10 years. Go by the useful life generally indicated on the smoke alarm casing. If it is damaged, dusty or doesn’t emit a signal after the battery has been replaced, change it.

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azetou April 27, 2017 at 1:49 pm

Hi , I noticed in many of your articles that you use abbreviations that you do not define, Like PDO here, if you could incorporate an abbreviation definition section in your articles it would be great, Thank you.

manoj mathew July 6, 2018 at 9:25 pm

the first question 3 types of smoke detectors what is the difference between ionisation and radioactive you are mentioned three types as my knowledge both are same if not please clarify me


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