Lecture 19 Flashcards
how is ignition of a fuel achieved
what are ways things can be heated up
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
radiant heat from a fire can induce another fire - what is this the principle of
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
what are electrical sparks and arcs
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
for radiation to effectively heat a fuel what must happen
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
when does an electrical current generate heat
when they meet resistance
how does a lighter ignite a flame
using friction
striking the flint in the lighter generates mechanical sparks which ignite a petroleum distillate vapour or butane gas
what is the amount of current a wire can carry proportional to
diameter, material and covering
what can heat build up in a wire due to (5)
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
where is a friction related fire unlikely to occur
and likely
in the home
train tracks
industrial machinery that is poorly lubricated
where may spontaneous combustions be seen to lead to a fire
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
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
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
when does the likelihood of a fire hazard in a drying oil increase (2)
with the number of C=C bonds and when the structures have conjugation
what can cause any substance to spontaneously combust
if it builds up heat and can’t dissipate it quick enough to the point where it reaches it’s ignition temperature
what are two things an accidental fire needs evidence of before it can be ruled in or out and arson can be considered
evidence of the origin of the fire and any faults with the appliance that may have lead to the fire on accident
is excessive gas normally the cause of a fire
when is this an exception
no
when there is an explosion
some species oxidise in air such as ….. what can this lead to
potassium and sodium
can lead to a spontaneous combustion
what can thermostat failure result in
fires initiating in broken furnaces or driers therefore they are running above their operating/threshold temps so heat up more
open gas flames are common fire sources - name two places these may be seen
Bunsen burners
gas stoves
give three sources of got and burning fragments that can lead to the ignition of another fire
rubbish
bonfires
lanterns
hot and burning fragments are another type of ignition source, what are these called
sparks - bots of wood, paper, card that travel whilst flaming
do not confuse with the other type of sparks we looked at
what does the distance of a hot and burning fragment (spark) depend on (3)
what is the common distance travelled of these
the material type, size, wind (convection currents from other fires too)
max of 6-12m is common
what is meant by a secondary combustion
2 different sources of fire and 1 scene
when can cigarettes act as an ignition source
when they are in contact with a material for a prolonged period of time
cigarettes have a low heat release rate
what types of fibres are more vulnerable to smouldering combustion after contact with a cigarette
synthetic are less vulnerable than natural fibres
what affects the ignition temperature of a cigarette (2)
the packing density of the tobacco
cigarette paper composition
what was the outcome of the Michigan Millers’ Mutual vs Benfield case
why did this happen
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
why is a fire scene one of the most complicated scenes an investigator is likely to encounter (2)
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
what are the two main roles of the forensic fire investigator
what can this information then help us suggest
evidence gathering
detection and identification of material
these can help us suggest suspect, motive and cause of fire
what will anything flammable at a fire scene likely be
damaged or destroyed
how will materials and surfaces near a fire scene be affected
what does this mean needs to be set carefully
covered in soot or debris or affected by the heat given off
cordons
who is evidence at a fire scene likely to be destroyed by
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
what damage can fire-fighting techniques cause at a fire scene
physical damage e.g windows or doors broken, ceilings collapse under water weight, objects disturbed or removed
what are some of the first questions a fire scene investigators is looking to answer (5)
- where did the fire start
- are there multiple sources
- motive? arson? accident? negligence?
- consider fraud
- how did it spread
what is required post-fire clean up for investigators
scene reconstruction as post clean up can destroy evidence
what is the first priority of a fire scene investigator
make scene safe for first responders, investigators, bystanders and witnesses
causualties and bodies extracted
when is the starting of a fire for insurance claims more often seen
when there is a economic recession
what physical evidence should be considered when thinking of how a fire started (5)
- burn patterns
- smoke records (what was exposed to smoke and what not)
- temperature indications
- debris layer sequences
- the presence of ignition devices or suspicious containers
what relevant but circumstantial evidence should be considered when thinking of how a fire started (3)
witness statements
evidence of forced entry or removal of goods
soot will show drag marks of things moved by first responders
what chemical evidence should be considered when thinking of how a fire started (2)
analysis of trace residues
accelerants presence
- sample empty bottles as things may have evaporated
in a body what is indicative of smoke inhalation
what is this the most common cause of
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
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
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
what three things do we know about locating the seat of the fire (start)
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
what does no residue beneath and object at a fire scene suggest
the object was not disturbed during or after the fire
what are directional indicators at fire scenes
where are these most useful
when the face of a material is exposed to the fire is typically shows the greatest extent of damage
most useful in scene reconstruction
what characteristic feature is seen in the rapid circulation of a fire
what do these help us identify
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
how to glasses and plastics react to a fire
they melt towards the heat of a fire
what does a fire surrounding a plug socket but the socket itself is undisturbed suggest
there was something bulky plugged in protesting the socket
the fire started from within the wall rather than the outside
what it spalling and when it is seen at fire scenes
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
what are two types of complimentary evidence that house accelerants
jerry can
oil bottle
if a fire has multiple seats what is this evidence to suggest
what else could suggest multiple seats of a fire
arson but not proof of it
radiation induced flashover could suggest multiple seats of a fire
what it is imperative to take at a fire scene and what do these help with
photos, sketches and contemporaneous notes
helps with scene reconstruction
what can layers of debris help you determine
the order that things were destroyed
