Tcase is a maximum value indicated by Intel for the case temperature of the processor, which has to be measured by manufacturers by placing a sensor in the geometric center of the CPU - a cooling solution must always contain this value. This guarantees that the actual processor core functions within permitted temperature parameters.

If the maximum core temperature of approximately 110°C is exceeded, the CPU initiates the THERMTRIP signal, on the basis of which the motherboard has to switch off. This maximum value is ascertained by the processor internally and has nothing to do with the integrated temperature sensor initially, which is read by the BIOS. The temperature difference between both measuring points is approximately 10°C.

During the tests, we observed the effect that several boards shut down when operating with a high power load, even though the thermal load on the processor was way below the permitted values. Therefore, we subjected all boards to an unusual test. We prepared the processor for this by isolating the pin responsible for THERMTRIP. During the test, we deactivated the fan of the processor cooler during operation, so that the CPU temperature increased within the shortest time. As THERMTRIP can only be activated by the processor, the boards must be responsible for the premature shutdown - which we could not confirm, as the uncooled system crashed within two to three minutes.

It is still unclear why this happens when temperatures are too low; by the time we had finished the test we were no nearer to explaining the reasons for this effect with respect to this article.

The Intel boxed cooler represents an additional aggravating factor in that the fan speeds are designed in such a way that the permitted, high case temperature of the P4 processor can also be achieved during operation. The reference cooler only exceeds the sound-critical limit of 3000 revs/min upwards of a temperature of 85°C.