A negative pressure wound therapy system comprising - an electrical pump for generating negative pressure, - optionally a tachometer for determining a pump speed associated with the electrical pump, - a pressure sensor for determining negative pressure values, - a controller for controlling activity of the electrical pump, - an input means for adjusting settings on the negative pressure wound therapy system, said input means being operable by the user of the negative pressure wound therapy system, - a relief valve for venting the negative pressure wound therapy system, wherein the relief valve can be controlled by the controller, - a first fluid path (6, 14, 15) fluidly connectable to a wound site and to the electrical pump such that the wound site can be subjected to a negative pressure, wherein the pressure sensor is located in the first fluid path (6, 14, 15) between the wound site and the electrical pump, - a second fluid path (24, 25) fluidly connectable to the wound site and the relief valve, wherein the first fluid path (6, 14, 15) and the second fluid path (24, 25) are in fluid communication at the wound site, wherein the controller of the negative pressure wound therapy system is adapted to execute a method of determining a blockage condition in the negative pressure wound therapy system during a negative pressure wound therapy, comprising the steps of i. generating a negative pressure at the wound site by means of the electrical pump, ii. recording the negative pressure, iii. venting the fluid path (6, 14, 15, 24, 25) of the negative pressure wound therapy system by opening the relief valve, wherein during the ventilation the electrical pump is stopped, iv. determining and recording a negative pressure drop during the ventilation step, wherein the negative pressure drop is determined for a predetermined period of time, v. optionally closing the relief valve when the predetermined period of time has elapsed, wherein closing the relief valve finalizes the ventilation step, vi. generating a blockage signal in the controller of the negative pressure wound therapy system if the negative pressure drop observed during the ventilation step is less than a predetermined negative pressure drop. - an electrical pump for generating negative pressure, - optionally a tachometer for determining a pump speed associated with the electrical pump, - a pressure sensor for determining negative pressure values, - a controller for controlling activity of the electrical pump, - an input means for adjusting settings on the negative pressure wound therapy system, said input means being operable by the user of the negative pressure wound therapy system, - a relief valve for venting the negative pressure wound therapy system, wherein the relief valve can be controlled by the controller, - a first fluid path (6, 14, 15) fluidly connectable to a wound site and to the electrical pump such that the wound site can be subjected to a negative pressure, wherein the pressure sensor is located in the first fluid path (6, 14, 15) between the wound site and the electrical pump, - a second fluid path (24, 25) fluidly connectable to the wound site and the relief valve, wherein the first fluid path (6, 14, 15) and the second fluid path (24, 25) are in fluid communication at the wound site, i. generating a negative pressure at the wound site by means of the electrical pump, ii. recording the negative pressure, iii. venting the fluid path (6, 14, 15, 24, 25) of the negative pressure wound therapy system by opening the relief valve, wherein during the ventilation the electrical pump is stopped, iv. determining and recording a negative pressure drop during the ventilation step, wherein the negative pressure drop is determined for a predetermined period of time, v. optionally closing the relief valve when the predetermined period of time has elapsed, wherein closing the relief valve finalizes the ventilation step, vi. generating a blockage signal in the controller of the negative pressure wound therapy system if the negative pressure drop observed during the ventilation step is less than a predetermined negative pressure drop.