I am working on a project that will have an exposed heat sink.

I know that UL has a standard limiting how hot an external surface can be. Unfortunately, I don't have access to the UL spec. right now. Can someone tell me what UL says for the maximum temperature that is safe to touch?

Thanks.

 

I am working on a project that will have an exposed heat sink.

I know that UL has a standard limiting how hot an external surface can be. Unfortunately, I don't have access to the UL spec. right now. Can someone tell me what UL says for the maximum temperature that is safe to touch?

Thanks.

I found this, which is indicative. Looks like about 70°C is the max, which makes sense. You can tolerate brief contact with 60°C water, but you can't keep your skin immersed in it.
http://msaworld.com/wp-content/uploads/2014/09/External-RangeTemperatures.pdf

 

I found this, which is indicative. Looks like about 70°C is the max, which makes sense. You can tolerate brief contact with 60°C water, but you can't keep your skin immersed in it.
http://msaworld.com/wp-content/uploads/2014/09/External-RangeTemperatures.pdf

Interesting document. I will have to look at it more closely later.

For reference, the Bob Pease rule is that if you can keep your finger on a heat sink for 5 seconds then it is about 85 degC which is about right.

Obviously, UL would never allow a surface to be that hot.

 

Interesting document. I will have to look at it more closely later.

For reference, the Bob Pease rule is that if you can keep your finger on a heat sink for 5 seconds then it is about 85 degC which is about right.

Obviously, UL would never allow a surface to be that hot.

The requirements set out in the product safety standard UL 62368-1 (Audio/video, information and communication technology equipment), allows accessible metal parts that need not be touched to operate the equipment to reach a maximum of 70C.

However, in relation to equipment heatsinks the temperature limit is 100C, with the proviso that the required instructional safeguard (warning) is marked on or close to the part.

The above temperature limits are based on an ambient of 25C, with no requirement to consider elevated ambient temperatures.

 

If you want to operate an exposed heatsink at a higher temperature, you could put a wire cage around it.

 

Hello,

Or cool the heatsink with forced air from a fan.
You can use a finger gard on the fan for safety.

Bertus

 

I think an exposed hot heat-sink is actually a feature. It helps make it more tamper proof and reduces the humidity that will build up on it.

 

A larger exposed heat sink would look even more impressive, but improving the circuit efficiency would both lower the temperature and avoid thermally stressing the parts as much. The heat from the heatsink certainly will not be any benefit to any of the other parts in the system.