IEC 60601-1 Clause 11.1.3 Winding Temperature Requirements

[Pads38]

Clause 11.1.3 (60601-1 ed 3) gives the details of temperature measurements to be taken and gives details of the change of resistance method for windings.

This includes a constant of 234.5 along with the resistance readings and room temperature compensation.

The question is where does the 234.5 come from and what does it represent?

(the temp coefficient for copper is 0.394% per degree C so this doesn't seem to be the whole answer).

 

[Peter Selvey]

I've just pulled out my thesis from 1998 on this one. It appears the formula uses the co-efficient at 0degC rather than 20degC.

If the 20C value was used, the formula would appear as:

dT = (R2-R1)/R1 * 251.9 - 20 -(t2 -t1)

Your instinct might say that since the co-efficient changes so much between 0 and 20C, a first order (linear) approximation is not suitable. Or to put it another way, the above formula might give different results.

In actual case, in the region of interest copper is quite linear, but the linear approximation does not pass through the point (0,0). If it did pass through (0,0) the slope and temperature co-efficient would be the same. Since the line does not go through (0,0), the "co-efficient" (change in resistance / degC) is not the slope, and changes a lot with temperature. But as long as the slope does not change, the linear approximation holds and we can estimate temperature accurately.

 

[Pads38]

Thanks Peter.

I suppose it doesn't really change how the test works, but it does help to resolve one more bit of 'head scratching'.