Can I use the aluminium enclosure of my mains powered product as a heatsink?
I'm thinking about mounting TO-220 transistors pressed against the enclosure, to get away with passive cooling by using the large surface of my product's aluminium enclosure as a heat sink. I'd use transistors with insulated tabs.
However, I worry this isn't really safe. The IEC-60950-1 stipulates that the product must be safe even under a single fault.
I believe this requirement would be fulfilled on paper. If the insulation between the transistor and the housing fails, the product would be short circuited to ground which would immediately blow a fuse. I can arrange to have the resistance in the ground much smaller than the resistance through the rest of the product, so even the transient voltage of the housing should be safe.
However, I worry that it might be hard to get the aluminium box properly grounded. I'm thinking that a grounding wire would be attached to the housing using screws. But how reliable would such an connection be, electrically? Could aluminium oxide accumulate over time, increasing the resistance to unsafe levels?
Edit: I might need to clarify that of course the housing of the product is earthed. It is the reliability of this earthing bond that worries me.
Best Answer
There are some precautions to could be taken like coating the aluminum screw point with Alnox right after scrubbing it but before making the connection. This is up to US Gov standards http://www.usbr.gov/power/data/fist/fist3_3/vol3-3.pdf (see chapter 6).