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Fires and Explosions > Fires 8 > Flashcards

Fires 8 Flashcards

1
Q

What are the different ways that we can heat things up?

A
  • Light
  • Electricity
  • Friction
  • Contact with something hot
  • Chemical reaction
  • Nuclear fission
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2
Q

How do you achieve ignition?

A

The fuel (even if its just a small part of it) needs to have its temperature raised to its ignition temperature

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3
Q

How does radiation/light cause fires?

A
  • Fuel must be able to absorb more heat than it can dissipate
  • Radient heat from a fire can induce another fire (flashover)
  • Sunlight can be focused by lenses or mirror if positioned just right
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4
Q

How does electricity cause fires?

A
  • Sparks and arcs involves the movement of current through a gas, producing plasma
  • Very localised so only ignites gases or vapours
  • The build up of charge is big enough to ionise the gas between into a plasma
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5
Q

What is the difference between sparks and arcs?

A
  • Sparks are shorter and smaller
  • Arcs are longer and maintained
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6
Q

How does electricity cause fires in regards to overheating?

A
  • Electrical currents generate heat when they meet resistance
  • The amount of current that a wire can carry is proportional to its diameter, material and covering
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7
Q

What causes overheating in wires?

A
  • Excessive current (wrong fuse, short circuit)
  • Tightly coiled wires (which cant dissipate heat)
  • Poor or loose connection
  • Insulation breakdown (charred wood/plastic becomes a semi-conductor)
  • Aluminium fitting (old) - not as good as a conductor like copper
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8
Q

How does friction cause a flame in lighters?

A
  • Friction from striking the ‘flint’ generates hot shavings (mechanical sparks)
  • This process is hot enough to auto-ignite the cerium in the flint
  • These sparks are formed near a fuel which comes out when your thumb touches a button
  • This can also occur in poorly lubricated train bearings
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9
Q

What is spontaneous combustion?

A

When some substances have naturally occuring exothermic reactions

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10
Q

How is an equilibrium temperature reached?

A
  • Heat is lost from the surface of a material
  • But the interior temperature continues to rise
  • This accelerates the reaction but as this happens heat gets lost more rapidly
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11
Q

How does the size of an object affect a chemical reaction?

A
  • In a large quantity of material, the surface area is relatively small
  • Therefore there is more material to heat up and so it takes longer to reach the auto-ignition temperature
  • The equilibrium temp is higher
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12
Q

What naturally occuring chemical reactions can mimic arson attacks?

A
  • Sawdust, coal dust, hay, straw and landfill waste will get broken down by microbes and will exothermically generate heat
  • Unsaturated (drying) oils that are soaked up in rags and papers will oxidise in air exothermically and then become fuel
  • Some reactive species oxidise in air (K, Na metal, group 1)
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13
Q

Why do accidental fires need evidence of origin and faults?

A

Arsonists may disguise an intentional fire by setting it under an oven/BBQ to make them look accidental

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14
Q

What are examples of that can start gas flame fires?

A
  • Open flames
  • Cooking oils
  • Bunsen burners
  • Portable gas stoves
  • Grease
  • Towels
  • Paper
  • Clothing
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15
Q

Where will you find hot and burning fragments/sparks?

A
  • Rubbish
  • Bonfires
  • Lanterns
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16
Q

What are hot and burning fragments/sparks?

A

Bits of wood, cardboard, paper, which can travel whist flaming

17
Q

What affects the distanced travelled by sparks?

A
  • Material
  • Size
  • The wind
18
Q

What is the issue with sparks?

A
  • Can initiate secondary combustions because of burning fragments
  • So cannot limit area of search to where the fire was found because the wind may have taken a spark from another fire
19
Q

Why is it often unlikely that a cigarette caused a fire?

A

They have a low heat release rate and therefore need direct contact with a flammable material/fuel for a long time to start a fire

20
Q

General aspects at a fire scene

A
  • Anything flammable at the scene will be destroyed
  • Evidence might have been destroyed
  • Materials and surfaces that are far from the scene may have been affected by soot/debris or by the heat - need large cordons
  • Fire fighting techniques might cause physical damage and damage to evdience
  • Post fire clean up can destroy evidence
21
Q

What fire-fighting techniques can cause physical damage?

A
  • Doors and windows broken
  • Ceilings collapse under weight of water
  • Objects disturbed or removed
22
Q

What does it mean if materials and surfaces far away from the intial scene have been affected by the fire?

A
  • Affects ability to recover evidence
  • Cordons need to be set carefully and ideally as large as possible
  • DNA and fingerprints might be hard to find
23
Q

Why might insurance investigators and loss adjusters be involved in an investigation?

A
  • Fraud
  • Financial fraud to profit from building insurance
  • Financial fraud to profit from contents insurance
  • Intentional fires to destory business records of tax fraud
24
Q

What are some physical evidence that you should look for in a fire scene?

A
  • Burn patterns - the fire will burn a certain way because it follows the rules of chemistry and physics
  • Smoke records
  • Temperature indication - spalling (surface flaking) of plaster
  • Debris layer sequences - you can tell what fell first in a fire by looking at the order they’re lying on top of each other
  • Remains of ignition devices or suspicious containers
25
Q

What are some chemical evidence that you would look for at a fire scene?

A
  • Analysis of trace residues
  • Presence of accelerants
26
Q

What related circumstantial evidence would you look for at a fire scene?

A
  • Evidence of forced entry
  • Removal of goods before fire
  • Soot from the fire will show drag marks from items moved by the first responders - if something has been moved pre-fire there wont be any drag marks
  • Witness statements
27
Q

How do you know if someone has died before the fire started or after?

A

If there is smoke in the lunks and CO in the blood that is indicative of smoke inhalation
* The absense of smoke in the lungs or CO in the blood suggests that the victim died prior to the fire
* Contributory evidence of arson (not proof though)

28
Q

Before forensic investigations, what must be checked so that the site is demed safe?

A
  • Structural collapse of the building
  • Live electrical cables
  • Damaged sewer/drainage
  • Dust inhalation hazards
  • Unknown building contents
29
Q

What must the risk assessment be due ot the nature of fire scenes?

A

Dynamic
* Constantly assessing the scene for changes in the situation

30
Q

What is the V shaped smoke pattern?

A
  • Points to where the fire started
  • Due to rapid circulation of the fire
31
Q

What are directional indicators and what is an example of one?

A

The face of the material that is exposed to the fire will typically show the greatest extent of damage
* Glass and plastics melt towards the heat of the fire

32
Q

How to know if a fire was due to an electrical fault behind the socket or a fire outside?

A
  • Outlet itself is relatively undisturbed
  • Main damage is around the outside of the socket in a V shape
33
Q

How does fire cause walls to spall?

A
  • Plaster or concrete from walls will contain small amounts of water
  • This water will get heated up due to the fire and boil, this increases the pressure and makes the surface of the wall break off
  • Can also see if the damage happened before or after the fire due to soot patterns
34
Q

What should you look for in a fire scene?

A
  • Burn patterns that dont fit the profile of an apparent fuel
  • Trail evidence - streamers from liquid accelerants
  • Containers that could house accelerants
  • Ignition devices - timers, lighters, discarded matches
  • Evidence of forced entry
35
Q

What can be evidence of arson but not proof of arson?

A

If the fire has multiple seats
* Radiation induced flashover can create multiple seats of fire but the debris ordering can tell you if it was a secondary fire

Fires and Explosions (20 decks)

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