Continuous running duty (CRD) S1
Short-time duty (STD) S2
Intermittent periodic duty (IPD) S3
In the case of intermittent periodic duty the cyclic duration factor (c.d.f.) in percent is an important figure, since the excess heat is allowed to dissipate by convective cooling during the stoppages. Therefore the intermittent periodic duty consists of the on-time and the breaks for cooling. This is also called a duty cycle. The observation period for power tools is always ten minutes. 40% c.d.f. means that the power tool can be operated for four minutes with the rated power input, which is followed by a cooling period of six minutes.
Rated load 4 minutes+ cooling pause 6 minutes= work cycle 10 minutes
The cyclic duration factor is usually 20%, 40% or 60%. Circular saws and belt sanders are used in intermittent periodic duty due to their particular operating characteristics. For electric welders other units of time apply.