be moved to another tile side to avoid short circuit problems which is not particularly user oriented fig3 shows an embodiment of a power unit 1 in accordance with the present invention the power unit 1 comprises a mains power input 2 through which the power unit 1 can be connected to a mains outlet and two output terminals 3 a 3 b by means of which the power unit 1 can be connected to for example a lighting tile the power unit 1 further comprises a power supply 5 which regulates the supply voltage v 1 of the power unit 1 to a value which is desired in the actual system where it is to be used this value can be adjustable through a voltage selector or it can be set to a fixed value to avoid that it inadvertently is set to an incorrect value the power unit 1 also comprises a power on detector 6 a power transfer switch 9 and a control unit 7 comprising a timer 8 the purpose of which is to initiate a power up procedure delay of random length immediately after the power unit 1 has been switched on the power on detector 6 detects the rising edge of the supply voltage v 1 output from the power supply 5 which initiates the timer 8 during this delay the control unit 7 disconnects the power transfer switch 9 and consequently no power is transferred to the output terminals 3 a 3 b the delay imposed may for example be multiples of 500 milliseconds in addition the power unit 1 comprises a volt meter 11 for measuring a voltage v 2 which will.