The invention relates to an electrical power supply system for a programmable logic controller, comprising: - a first electrical power supply module (1) configured in a master mode for supplying at least one electrical supply voltage to the programmable logic controller, - a second electrical power supply module (2) configured in slave mode for supplying said electrical supply voltage to the programmable logic controller in the event of failure of the first electrical power supply module, - a control module arranged to temporarily control the switching of the first electrical power supply module (1) into slave mode and the switching of the second electrical power supply module (2) into master mode, - a test module arranged for testing the second electrical power supply module (2) when same is switched into master mode.