by John Gardner
Following is a summary of some of the main points that were covered in my talk on electrical fires for the AFI workshop on 1st April 2004.
Electricity is a form of energy which is converted to other forms of energy when it is used to perform a task, and when this conversion occurs heat is always produced to a greater or lesser degree. There is also some heat produced in the wiring connected to the electrical device.
For example electrical energy is converted to:-
- light energy (electric lamp) plus heat
- mechanical energy (electric motor) plus heat
- heat or thermal energy (electric heater) plus infra red radiation
A convenient measurement of the heat produced by electric current flow is Watts.
If a fault develops, the current (and heat energy) can increase to the extent that either the appliance burns out, the connecting wiring overheats and melts the plastic insulation or the plug and socket may burn out.
An electrical fire can then occur where the overheated wiring, motor, plug etc are in contact with, or close to flammable or combustible material.
2.0 ELECTRICAL CAUSE OF FIRE
An electrical fire can be defined as a fire where an electric current or electrical fault is found to have been the ignition source. Some examples would be:-
a) Lightning strike -Where extremely high voltages and currents, for a few thousandths of a second, produce so much heat that surrounding materials catch fire and continue to burn.
b) Overloaded wiring - Where the electric current flowing in the wires exceeds the rating of the cables. The wiring heats up and melts the insulation and can set fire to flammable material nearby. Commonly found in flexible cords, or power board leads.
c) Loose wiring connections - The current flowing through the wiring encounters resistance at the connection and generates heat. This can start a fire in the wall at the back of a power point, in a wiring junction box in the ceiling, above light fittings or inside a switchboard.
d) Electrical "arcing" (or sparking) - Where wiring insulation has been damaged by an external occurrence, and which lets the copper conductors inside a cable just touch one another, or to just make contact with the metal case of an appliance.
A small current will then flow from the "live" conductor to the neutral or earth wire, or, to the earthed metal case. The current will initially be too small to blow a fuse or to trip a circuit breaker, but because the contact area is also very small (a few strands of wire) the heat produced at this point can reach sufficiently high temperatures to melt or vapourise metals such as copper, brass or sheet metal. The localised heating will ignite combustible materials in close proximity and start a fire.
Arc welding is an example of "controlled" arcing, an arcing fault is "uncontrolled".
NOTE 1:- CABLE INSULATION CAN BE DAMAGED IN A FIRE, AND, IF THE POWER IS ON, ARCING MAY ALSO OCCUR. THEREFORE SIGNS OF ARC DAMAGE ON WIRING DOES NOT NECESSARILY MEAN THAT THE FIRE WAS CAUSED BY AN ELECTRICAL FAULT.
NOTE 2:- FIRES CAUSED BY LOOSE WIRING CONNECTIONS, OR, INSULATION DAMAGE, COMMONLY OCCUR IN MOTOR VEHICLES WHERE THE POWER SOURCE IS A 12 VOLT BATTERY. THEREFORE ELECTRICAL FIRES CAN START IN LOW VOLTAGE OR HIGH VOLTAGE WIRING SYSTEMS.
NOTE 3:- BUILDING FIRES GENERALLY ARE NEVER HOT ENOUGH TO SIGNIFICANTLY DAMAGE METALS SUCH AS COPPER, BRASS OR STEEL. IF EROSION OR SIGNS OF LOCALISED MELTING OF BRASS TERMINALS OR CONNECTORS IS FOUND IN THE AREA OF FIRE ORIGIN, IT USUALLY INDICATES THAT THIS HAS BEEN A "HOT CONNECTION" OR HIGH RESISTANCE POINT AND COULD HAVE CAUSED THE FIRE.
e) Appliance Fire - Electrical faults inside appliances are a common cause of electrical fires.
The standby power switch and high tension circuits in television sets are mostly to blame. Damaged power leads and moisture in compressor relays or thermostats also cause fires in refrigerators.
NOTE:- APPLIANCES WITH PLASTIC CASES ALSO BURN VERY BADLY WHEN EXPOSED TO FIRES, AND OFTEN THERE IS NOTHING MUCH LEFT TO CONFIRM IF THE FIRE STARTED INSIDE OR OUTSIDE THEM. THEREFORE MAKE SURE THE APPLIANCE WAS PLUGGED IN AT THE TIME OF THE FIRE, AND, THAT IT WAS IN THE AREA OF FIRE ORIGIN BEFORE CONCLUDING THAT A FAULTY APPLIANCE CAUSED THE FIRE.
3.0 ELECTRICALLY RELATED FIRES
Fires can also be caused where there is no electrical fault, but there has been inappropriate use or faulty installation of electrical appliances or components. Some examples of these would be:-
- Insufficient ventilation around recessed lights in the ceiling, which can set fire to debris, timber or insulation material.
- A clothes dryer with blocked lint filter can catch fire or set fire to clothes inside the drum.
- Heater being placed too close to clothes or furnishings.
4.0 WHERE ARE ELECTRICAL FIRES LIKELY TO OCCUR
Electrical fires in a building can start in a number of locations, some of these are more prone than others because of the number of connections or risk of damage.
It should be noted:-
- That electrical fires will not generally occur in continuous runs of cable unless something has caused physical damage to the insulation.
- Water alone will not cause the PVC insulation on cables to break down.
- Electrical fires most commonly occur at "weak points" in the system, these are where the cables have been cut and joined, or where they have been cut and connected to a power point, light or similar. These points can enable moisture or dirt to build up across "live" terminals, or, the connections may loosen with time and localised heating will occur.
Following is an example of the major components of typical "fixed wiring" or permanent wiring in a house (which can also be extended to larger buildings) and some comments in regard to potential fire risk.
4.1 Service lines - The overhead wiring from the street to the "Point of Attachment" on the front of the house.
Unlikely, - can occur when tree branches fall through them but the fire would generally be outside.
4.2 Point of Attachment - junction box or terminals on the front of the house which connect the "Service Line" to the "Consumers Mains".
Unlikely in newer houses with separated insulated connectors, more likely on older houses with metal junction boxes. Water entry and aging insulation can cause insulation breakdown, arcing can occur inside the conduit and burn through to timber roof material.
NB ELECTRICAL HAZARD-
As an important aside and word of warning:- An even greater risk from electrical breakdown at the metal junction box is not from fire but from electrocution. The metal conduit is always connected to the house earth system, which is also always connected to the water pipes. When an electrical fault occurs in the junction box the conduit becomes "live" and so does the household plumbing system. To make matters worse the power in this case comes directly from the street so it cannot be disconnected by turning off the main switch on the switchboard.
4.3 Consumers Mains - these are the cables that run from the point of attachment down to the switchboard, in many cases via the roof space and then down inside the wall cavity.
Unlikely on newer houses (1970's say) because they are reasonably heavy duty cables with thick insulation, higher risk with old wiring.
Some possible causes of damage:-
- rats or other vermin in the roof chewing the insulation.
- deliberate act, attempts to bypass electricity meters by cutting into the mains (it happens).
- heat from a nearby chimney or combustion heater flu.
- insulation deterioration with age and heat (under tin roof) - older wiring.
4.4 Main switchboard - contains meters, main service fuses, and fuses or circuit breakers for light and power circuits.
Low on newer houses with circuit breakers and possibly earth leakage "safety switch", high on old houses with rewirable fuses, and old wiring behind the panels
Possible causes (some only):-
- Loose connection to fuses, not so likely with circuit breakers because they have a better connection arrangement.
- Faults usually start with the smaller light and power circuit wiring (BUT NOT ALWAYS).
- Overloaded circuit and overheated wiring causes the PVC insulation to melt and burn into other wiring insulation as wiring is usually tightly bunched behind the hinged panel.
- Old style porcelain fuse holders lose spring tension and start to overheat, eventually heat is conducted back through wires behind the panel.
4.5 Sub circuit wiring & connections - (PVC insulated & sheathed cables) - Comprises the circuits for lights, power points, water heaters stoves, air conditioners etc. and is usually rated at 15 to 20 amps, or 32 amps for stoves.
a) Wiring Above the Ceiling - Fire Risk
Unlikely without mechanical damage, such as rats, possums etc., and is also prone to damage by other heat sources such as fire place chimney or combustion heater flu.
NOTE: ALWAYS LOOK FOR OTHER POSSIBLE IGNITION SOURCES ABOVE THE CEILING BEFORE CONCLUDING THAT AN ELECTRICAL OR WIRING FAULT OCCURRED.
IF MULTIPLE ARC DAMAGE IS FOUND ALONG A LENGTH OF WIRING AFTER A FIRE IT SUGGESTS THAT THIS WAS A RESULT OF THE FIRE AND NOT THE CAUSE.
b) Connections to Light Fittings - Fire Risk
Highly likely "weak point" as a result of:-
- loose connection into the top of the lamp holder, or heat from the lamp below causing deterioration of the wiring insulation. Often occurs over a long period of time.
- accumulation of dirt or moisture on top of exposed terminals, eventual "tracking" between terminals - eventual ignition. Particular likelihood near bathroom or laundry exhaust fans that vent into the ceiling, or roof leaks.
- mice, possums, rats - nests often built above surface mounted lights which warm the underside of the ceiling, also can also cause wiring insulation damage.
NOTE:- LOOK FOR ARC DAMAGE TO BRASS CONNECTORS OR SCREWS AT THE BACK OF THE LIGHT AFTER THE FIRE.
c) Connections to Junction Boxes in the ceiling - Fire Risk
Highly likely, AS FOR LIGHT FITTINGS, as a result of:-
- Loose connection on "BP" connectors causing high resistance connection which generates localised heat, burns through other insulation and eventual ignition
- Accumulation of dirt or moisture inside junction box "tracking" between connectors - eventual ignition. Particular likelihood near bathroom or laundry exhaust fans that vent into the ceiling, or, roof leaks.
- Rats also like to eat the plastic covers.
NOTE:- LOOK FOR ARC DAMAGE TO REMAINS OF BRASS CONNECTORS OR SCREWS WHICH WILL PROBABLY INDICATE THE CAUSE OF THE FIRE. (IF IT IS FOUND IN THE AREA OF FIRE ORIGIN).
d) Ceiling Exhaust Fans (Unducted) - Fire risk
Likely but not common, as a result of the motor overheating, which can be caused by accumulation of dirt from roof space mixed with or powder and moisture residue from the shower below.
e) Sub Circuit wiring and connections in walls
Unlikely without mechanical damage
- If arc damage is found in a number of locations along the cables it usually suggests that damage was caused by a fire, and not the cause.
f) Power Outlets -
Not likely because the connections are less exposed than those in the ceiling, but could occur due to:-
- moisture running down wall and then behind the power outlets in bathrooms or kitchens.
- Ants and termite nests which have a high moisture content, built up on the back of the outlet can cause "tracking" (carbonisation which becomes conductive) across the terminals, eventual heat build up and ignition of flammable material
5. ELECTRICAL FIRES INSIDE THE HOUSE.
Note:- In TV fires it is difficult to diagnose the exact cause as there is usually too much internal damage
5.2 Power cords on all appliances
5.3 Extension cords