Depending on their design and application, resistors must be protected against contact with live components or be installed accordingly. Resistors with direct cooling are manufactured in the protection class IP00 for installation or as guarded designs with IP20 to IP23. In all cases, sufficient cooling air feed must be ensured thus requiring sufficiently dimensioned inlet and outlet openings or intensive blower ventilation. Starting with IP30, the admissible openings are so small that ventilation is barely ensured (IP30: ingress of contaminants > 2.5 mm and IP40 > 1 mm must be prevented). High protection classes from IP40 can only be achieved by indirect cooling. Heat transfer mainly takes place by heat dissipation via a heat transport medium to the surface and from there to the surrounding cooling liquid. If at the same time a low surface temperature is required due to fire hazard or temperature-sensitive surfaces (paint coat), the load resistors used may take on a considerable size compared to air-cooled designs with low protection classes.