Class 2 transformers and plastic enclosed printed circuit boards: A potentially perilous combination

From a fire development perspective, Class 2 transformers are generally regarded by the design engineering and standards (listing agencies) community to provide a safe method in which to power a wide variety of consumer and commercial electronic/electrical devices due their “intrinsic” current limiting features as stated in their specifications. However, Class 2 transformers can deliver current levels much higher than the 1.67 amps that is frequently accepted as the maximum current which can be provided by the transformer. These high current levels produce rapid I²R heating that can ignite flame-rated circuit board laminate, as well as the flame-rated plastic that houses the printed circuit board. Environmentally driven directives are creating a more challenging design environment via the introduction of materials and components that have been proven to be more sensitive to failure, more prone to ignition, and present increased safety hazards. Consequently, it is important that the design engineer considers the actual performance characteristics of the Class 2 transformer (not merely the abridged specifications). The design engineer must also consider the performance of the circuit board laminate and the plastic housing of the device in fault modes that can develop excessive I²R heating.