Exhaust fans commonly found in the ceilings of bathrooms and laundry rooms cause many residential fires and fires in commercial and industrial buildings. Exhaust fan fires have caused more than $50 million in property damages.
Exhaust fan fires are frequently ignited by electrical arcing in the fan motor or by heating caused by the fan motor bearings “locking up.” The heat generated by the electrical activity or friction is sufficient to ignite plastic components of the fans such as the blades or grill. The burning plastic can and will fall down out of the fan to ignite combustible material in the room below. Burn patterns moving up and out from laundry baskets, plastic trash cans, and other similar items ignited by burning plastic dropping down from exhaust fans are frequently relied upon by investigators for fan and fan motor manufacturers to support an argument that the fan was damaged by a fire originating in the room below and being drawn to, and burning to, the fan operating in the ceiling of the room.
Investigators have identified one common cause of exhaust fan fires referred to as a crimp connection failure. Certain fan motors are manufactured with a crimp connection between a copper lead and an aluminum conductor. Copper and aluminum are dissimilar metals. Copper and aluminum expand and contract at different rates when heated and cooled. The differential rate of expansion and contraction will weaken the connection. The contact between the dissimilar metals also causes corrosion which results in oxidation. Aluminum oxides do not conduct electricity well and further weaken the connection. Weakening and failure of the connection results in resistance heating and electrical arcing sufficient to ignite fires.
There are three crimp connections in many exhaust fan motors. The first crimp connection connects the copper lead from the motor plug cord to the copper lead into the fan motor TCO. A crimp connection is merely a hollow connector that is used to connect one conductor to another. The first crimp connection is actually located within the plastic bobbin of the motor. The second crimp connection runs out from the fan motor TCO and connects to the aluminum windings. The second crimp connection actually rests on the aluminum windings. The third crimp connection connects the other end of the aluminum windings to the copper lead for the neutral that returns back to the plug cord. The third crimp connection is usually also located within the plastic bobbin for the motor.
The second crimp connection is a connection of a copper lead to an aluminum conductor. Heat generated by the connection failure can cause off-gassing of the varnish on the aluminum windings upon which the second crimp connection rests. Electrical arcing can occur as the second crimp connection fails and the aluminum windings are exposed by the off-gassing of the varnish. The electrical arcing will ignite the off-gassing varnish and combustible lint, dust, and plastic in the fan.
The melting of the copper lead at the second crimp connection is strong evidence that a failure at this connection ignited a fire. Copper melts at 1,980° Fahrenheit. ((c) NFPA 921, 2011 Edition, Table 126.96.36.199) Electrical arcing caused by the failure of the connection is typically the only possible source of heat sufficient to melt the copper. Fire attacking a fan motor from the exterior will typically not produce localized temperatures near 1,980° Fahrenheit, especially when combustible material such as plastic in the fan or wood framing near the fan with much lower ignition temperatures suffer little or no fire damage. A fire attacking an exhaust fan from the exterior will typically cause electrical arcing in another location on the fan electrical system and terminate current to the fan motor before the fire reaches the fan motor and causes arcing at the second crimp connection.
An exhaust fan must be on or energized for a crimp connection failure to occur and ignite a fire. Electrical arcing cannot occur unless electrical current is flowing to the crimp connection. However, a crimp connection can fail and cause a fan to stop operating or to operate intermittently when the fan switch is on and the fan is energized. Therefore, a fan may be on and energized even though it is not operating and the homeowner does not believe the fan is on. This situation often occurs when exhaust fans rendered non-operational by a crimp connection failure are controlled by two separate on/off switches in a laundry room or bathroom.
The connection of dissimilar metals in an electrical circuit is a recognized fire danger. Section 110.14 of the 2008 National Electric Code prohibits intermixing or splicing where physical contact occurs between dissimilar conductors (specifically identifying copper and aluminum as dissimilar conductors) unless a device is used that is approved for connecting the dissimilar metals. However, this code provision applies to building wiring and not to wiring within a product such as an exhaust fan.