Current low power WLC devices/systems use closed-loop foreign object detection (FOD) methods (compare RX power to TX), but these are not accurate enough in high power transmission, for example, around 5 W or above. There exist some foreign object detection (FOD) method(s) that are accurate enough, but they may only be applied when there is no power transfer (TX) ongoing. This is the case, for example, if the Q-factor of a receiver of a transmitter coil is used to detect foreign objects (FO's). Q-factor can be measured, for example, by measuring the length of the self-ringing of an RC circuit after applying an energy pulse to it. When power transfer is not allowed during a FOD method, the power transfer needs to be paused once in a while to perform a FOD cycle in order to detect a foreign object(s) (FO(s)) which may have arrived to the magnetic field i.e. onto a charging area or in proximity of the charging area. Dangerousness of foreign object(s) on the charging area or in proximity of the charging area depend on the transferred power (TX); higher power is more dangerous than the lower; when higher power has been transmitted, foreign object(s) can reach high temperature faster than in the case of lower power, which high temperature may cause above mentioned damages and/or fire accidents. Therefore, for this reason, it may be more appropriate to perform loop foreign object detection (FOD) methods more frequently when transferred power (TX) level is high than when it is low. Performing of foreign object detection (FOD) methods more frequently i.e. increasing the foreign object detection frequency comprises shortening a time interval between adjacent foreign object detections. In other words, FOD detection interval could be proportional to the power transmitted; it is possible to safely use a longer FOD interval i.e. lower FOD detection frequency when TX power is low and a shorter FOD interval i.e. higher FOD detection frequency when TX power is high. This improves the power transfer efficiency since the power transfer does not need to be interrupted so often, when there is no need for so frequently done FOD detections.