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A fire occurred in a machinery shed on a working farm containing two tractors with slashers attached, other tools and equipment and stored materials. All machinery items were damaged by heat and smoke.


One tractor and slasher had been used during the afternoon, when the weather was hot but the grass was not excessively long or dry. The farmer returned the tractor to the shed, switched off the ignition and left the keys in, and took his ute down the driveway to the main road to meet his wife, who was exercising the dog and gardening.


The couple spent about 30 minutes doing some general gardening at the end of the driveway before driving back up the hill. As they began the drive back, they could see thick black smoke coming from the shed. The insured called 000 and tried to enter the shed with a hose but the smoke was too thick. Firefighters arrived about five minutes later.


The rear wheels, power take off and slasher of the tractor were did not suffer burns. The front part of the tractor, from the operator seat forward, was burnt to varying degrees, with the front tyres consumed and the most severe damage around and just forward of the engine compartment. 


It was apparent that the fire originated in the general area of the tractor engine and/or forward battery compartment.


Close inspection of the alternator showed no indication of a pre-fire internal fault or electrical activity between the internal conductors and attached cables.


In front of the engine was a separate compartment containing the battery. This was badly damaged by heat externally and internally, with buckling of the plates inside the cells and destruction of the external plastic case. The heavy-gauge cables and terminals did not show any indicators such as arc or fusion damage.


From the positive terminal, a heavy-gauge multi-strand cable passed down to the base of the compartment, rearwards beneath and to the right of the engine to an isolator switch. 


The cable from the battery to the isolator showed electrical arc and fusion damage where it crossed a steel bracket welded to the chassis rail. The location and nature of the damage indicated that it probably resulted from abrasion of the cable insulation, caused by vibrating against the bracket while the engine was running and/or the machine being used, allowing the conductors to contact the bracket. 


There was a patch of localised intense heat damage to the surrounding steel assembly, but no burning had spread out from there due to a lack of surrounding combustibles to act as fuel. Instead, the burning had spread out from the battery, which was not in the immediate vicinity of the arced cable.


Once the positive cable insulation was sufficiently degraded, current could pass between the positive cable and the grounded (negative) bracket intermittently during moments of contact and eventually for longer periods. During the longer contacts, there would be high current flow through the battery due to the negligible circuit resistance.


It is likely that the damaged cable was lying against the bracket when the tractor was parked in the shed, but this would not have been visible from the operator position. In this position, there would be continuous discharge of the battery through the machine chassis. 


High current flow through a lead-acid automotive battery causes buckling of the internal plates, localised internal shorting, and high temperatures. If maintained, the liquid electrolyte boils off and the plastic casing can overheat to the point of ignition.


The root cause of the cable contact with the bracket could not be established. It is likely that a cable clip or tie had failed during use, allowing the cable to drop from its intended position onto the bracket.

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