Laboratory stands for measuring the values ​​of power, torque and rotational speed, working mechanisms of machines, are often connected with belt transmissions. Transmissions with a V-belt are additionally characterized by the possibility of working as an overload clutch. However, if the belt is not tensioned properly, the measurement may not be performed correctly. The aim of the article is to show the range of belt tension force values ​​that enable proper measurement of the driving power of the positive pressure fan. Additionally, the analysis covered the influence of using one or two V-belts. The tested positive pressure fan worked at a maximum of about 4.15 ± 0.2 kW, the belt tension from 230 ± 2 N to 650 ± 2 N ensured operation with these parameters. Increasing the number of belts from one to two did not increase the fan operation range with greater power. The tests confirmed that the change of the belt tension force from the operating point of the belt not causing the belt slippage does not significantly affect the value of the driving power transmitted, if the system is not overloaded.