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Fires and Explosions > Lecture 19 > Flashcards

Lecture 19 Flashcards

1
Q

how is ignition of a fuel achieved

what are ways things can be heated up

A

the temperature of the fuel needs to be raised to its ignition temperature

a source of heat is imperative but different amounts for different fuel types

light
electricity
friction
contact with something hot
chemical reactions
nuclear fission

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2
Q

radiant heat from a fire can induce another fire - what is this the principle of

A

flashover - the transition of a fire in a room to a room on fire

the heat can be transferred to surrounding materials where they then heat up, produce pyrolysis products and combust

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2
Q

what are electrical sparks and arcs

A

the movement of a current through a gas which can lead to the ignition of a fuel

only in gases and vapours as it is very localised

spark = short
arc = maintained

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2
Q

for radiation to effectively heat a fuel what must happen

A

the fuel must be able to absorb more heat than it can dissipate

if this isn’t the case the material won’t heat up

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2
Q

when does an electrical current generate heat

A

when they meet resistance

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3
Q

how does a lighter ignite a flame

A

using friction

striking the flint in the lighter generates mechanical sparks which ignite a petroleum distillate vapour or butane gas

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3
Q

what is the amount of current a wire can carry proportional to

A

diameter, material and covering

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4
Q

what can heat build up in a wire due to (5)

A

excessive current for the wire

tightly coiled wires - can’t dissipate the heat

poor/loose connection

insulation breakdown - can char or melt

aluminium fittings in old wiring systems are not as good of a conductor than copper and create oxide films = heat barrier

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5
Q

where is a friction related fire unlikely to occur

and likely

A

in the home

train tracks
industrial machinery that is poorly lubricated

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6
Q

where may spontaneous combustions be seen to lead to a fire

A

in sawdust, coal dust, hay, straw and landfill waste

unsaturated soils that are soaked into rags and paper

these can mimic arson attacks but were actually causes due to heat build up

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6
Q

what is spontaneous combustion

what results in an equilibrium temperature being reached here

when can this result in a fire starting in the bulk of the material

A

when a substance has a naturally occurring exothermic reactions

heat is lost from the surface of the material but the temp inside still rises, accelerating the reaction but when this happens heat generated us lost more rapidly also

in a large quantity of a material the surface area is quite small so heat may not be able to be dissipated as quickly as it needs to - this can take hours to happen though

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6
Q

when does the likelihood of a fire hazard in a drying oil increase (2)

A

with the number of C=C bonds and when the structures have conjugation

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7
Q

what can cause any substance to spontaneously combust

A

if it builds up heat and can’t dissipate it quick enough to the point where it reaches it’s ignition temperature

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7
Q

what are two things an accidental fire needs evidence of before it can be ruled in or out and arson can be considered

A

evidence of the origin of the fire and any faults with the appliance that may have lead to the fire on accident

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7
Q

is excessive gas normally the cause of a fire

when is this an exception

A

no

when there is an explosion

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7
Q

some species oxidise in air such as ….. what can this lead to

A

potassium and sodium

can lead to a spontaneous combustion

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8
Q

what can thermostat failure result in

A

fires initiating in broken furnaces or driers therefore they are running above their operating/threshold temps so heat up more

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9
Q

open gas flames are common fire sources - name two places these may be seen

A

Bunsen burners

gas stoves

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10
Q

give three sources of got and burning fragments that can lead to the ignition of another fire

A

rubbish
bonfires
lanterns

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10
Q

hot and burning fragments are another type of ignition source, what are these called

A

sparks - bots of wood, paper, card that travel whilst flaming

do not confuse with the other type of sparks we looked at

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11
Q

what does the distance of a hot and burning fragment (spark) depend on (3)

what is the common distance travelled of these

A

the material type, size, wind (convection currents from other fires too)

max of 6-12m is common

12
Q

what is meant by a secondary combustion

A

2 different sources of fire and 1 scene

13
Q

when can cigarettes act as an ignition source

A

when they are in contact with a material for a prolonged period of time

cigarettes have a low heat release rate

14
Q

what types of fibres are more vulnerable to smouldering combustion after contact with a cigarette

A

synthetic are less vulnerable than natural fibres

15
Q

what affects the ignition temperature of a cigarette (2)

A

the packing density of the tobacco
cigarette paper composition

16
Q

what was the outcome of the Michigan Millers’ Mutual vs Benfield case

why did this happen

A

the court held that fire investigation is a scientific discipline and not an experience based opinion

because investigators claimed they could identify if it was arson by looking at evidence - no samples, no tests and no proof of excluding other sources

knowledge and experience can be used to focus our investigation but scientific principles are the foundation of the conclusions experts present here

17
Q

why is a fire scene one of the most complicated scenes an investigator is likely to encounter (2)

A

because they are dominated by destruction - a lot of evidence types are likley to be degraded e.g DNA and fingerprints

everything looks the same when burnt really

they can be accidental or intentional

18
Q

what are the two main roles of the forensic fire investigator

what can this information then help us suggest

A

evidence gathering

detection and identification of material

these can help us suggest suspect, motive and cause of fire

19
Q

what will anything flammable at a fire scene likely be

A

damaged or destroyed

20
Q

how will materials and surfaces near a fire scene be affected

what does this mean needs to be set carefully

A

covered in soot or debris or affected by the heat given off

cordons

21
Q

who is evidence at a fire scene likely to be destroyed by

A

first responders e.g fire service or ambulance who use fire suppressants to stop the fire

first responders priority is safety and preservation of life not scene integrity

some suppressants look like accelerants

22
Q

what damage can fire-fighting techniques cause at a fire scene

A

physical damage e.g windows or doors broken, ceilings collapse under water weight, objects disturbed or removed

23
Q

what are some of the first questions a fire scene investigators is looking to answer (5)

A
  1. where did the fire start
  2. are there multiple sources
  3. motive? arson? accident? negligence?
  4. consider fraud
  5. how did it spread
23
Q

what is required post-fire clean up for investigators

A

scene reconstruction as post clean up can destroy evidence

23
Q

what is the first priority of a fire scene investigator

A

make scene safe for first responders, investigators, bystanders and witnesses

causualties and bodies extracted

24
Q

when is the starting of a fire for insurance claims more often seen

A

when there is a economic recession

24
Q

what physical evidence should be considered when thinking of how a fire started (5)

A
  1. burn patterns
  2. smoke records (what was exposed to smoke and what not)
  3. temperature indications
  4. debris layer sequences
  5. the presence of ignition devices or suspicious containers
25
Q

what relevant but circumstantial evidence should be considered when thinking of how a fire started (3)

A

witness statements

evidence of forced entry or removal of goods

soot will show drag marks of things moved by first responders

25
Q

what chemical evidence should be considered when thinking of how a fire started (2)

A

analysis of trace residues

accelerants presence

  • sample empty bottles as things may have evaporated
26
Q

in a body what is indicative of smoke inhalation

what is this the most common cause of

A

smoke in lungs and carbon monoxide in the blood

most common cause of fire related deaths

absence of these suggest victim dies before the fire so fire may have been started to destroy evidence and body = contributory evidence but not proof of arson

27
Q

what must be taken in consideration when assessing the safety of a fire scene (6)

what must the risk assessment of a fire scene be

A

primary hazards
structure collapse
live cables and electrics
damaged sewage/pipes
dust inhalation - asbestos
toxic chemicals within the building if any

the risk assessment must be dynamic and constantly assessed for changes in the situation

27
Q

what three things do we know about locating the seat of the fire (start)

A

the point of origin has burnt the longest

something had ignited the fuel package

fires burns up so the lowest point of burning tends to be the seat

28
Q

what does no residue beneath and object at a fire scene suggest

A

the object was not disturbed during or after the fire

28
Q

what are directional indicators at fire scenes

where are these most useful

A

when the face of a material is exposed to the fire is typically shows the greatest extent of damage

most useful in scene reconstruction

28
Q

what characteristic feature is seen in the rapid circulation of a fire

what do these help us identify

A

a V shaped smoke pattern on adjacent walls - a fire spreads up and out

these almost points down to the seat of the fire and suggest where to start our examinations

29
Q

how to glasses and plastics react to a fire

A

they melt towards the heat of a fire

30
Q

what does a fire surrounding a plug socket but the socket itself is undisturbed suggest

A

there was something bulky plugged in protesting the socket

the fire started from within the wall rather than the outside

31
Q

what it spalling and when it is seen at fire scenes

A

the breaking off of plaster or concrete from walls

small amounts of water within the material cause it to break off as boils when heated

this tells us the wall reached a certain temperature during the fire

bits that have fallen off after smoke exposure will leave spots on the wall

32
Q

what are two types of complimentary evidence that house accelerants

A

jerry can
oil bottle

33
Q

if a fire has multiple seats what is this evidence to suggest

what else could suggest multiple seats of a fire

A

arson but not proof of it

radiation induced flashover could suggest multiple seats of a fire

34
Q

what it is imperative to take at a fire scene and what do these help with

A

photos, sketches and contemporaneous notes

helps with scene reconstruction

35
Q

what can layers of debris help you determine

A

the order that things were destroyed

Fires and Explosions (20 decks)

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