A power supply system comprising: - a power processor having a power input with a nominal input voltage V Nin and an AC power output with a nominal output voltage V Nout and a nominal output current I Nout, said power processor being connected downstream to at least one downstream circuit breaker intended to be itself connected to a critical load and upstream to an upstream circuit breaker, said upstream circuit breaker being higher-rated than the downstream circuit breaker, and - a power supply safety or backup circuit connected in parallel to said power processor between said power input and said AC power output and comprising: - a bidirectional switch, and - control means for closing said bidirectional switch after detecting a short circuit condition downstream of the AC power output; characterised in that said backup circuit also comprises a current limiter connected in series to said bidirectional switch to limit the current that can flow through the backup circuit to a maximum current of αI Nout wherein α has a value comprised between 5 and 20, preferably between 7 and 13, and most preferably about 10, said maximum current corresponding to a tripping current able to trip the downstream circuit breaker that needs 10 ms to open without tripping the higher-rated upstream circuit breaker, the bidirectional switch being a triac and the current limiter being a resistor having an impedance value of V Nin α ⁢ I Nout and in that the control means is successively set: - for detecting the short circuit condition downstream of the AC power output and for closing the bidirectional switch, when the voltage at the AC power output falls below a threshold value of at most 80% of the nominal output voltage V Nout, in particular below 20% of V Nout and/or the current at the AC power output rises above a threshold value of at least 120% of the nominal output current I Nout, in particular above 150% of I Nout; and - for reopening said bidirectional switch when the output voltage reaches again 80% of V Nout, after the downstream circuit breaker has been tripped or for reopening said bidirectional switch after it has been closed for a predetermined time period, preferably 20ms. - a power processor having a power input with a nominal input voltage V Nin and an AC power output with a nominal output voltage V Nout and a nominal output current I Nout, said power processor being connected downstream to at least one downstream circuit breaker intended to be itself connected to a critical load and upstream to an upstream circuit breaker, said upstream circuit breaker being higher-rated than the downstream circuit breaker, and - a power supply safety or backup circuit connected in parallel to said power processor between said power input and said AC power output and comprising: - a bidirectional switch, and - control means for closing said bidirectional switch after detecting a short circuit condition downstream of the AC power output; - for detecting the short circuit condition downstream of the AC power output and for closing the bidirectional switch, when the voltage at the AC power output falls below a threshold value of at most 80% of the nominal output voltage V Nout, in particular below 20% of V Nout and/or the current at the AC power output rises above a threshold value of at least 120% of the nominal output current I Nout, in particular above 150% of I Nout; and - for reopening said bidirectional switch when the output voltage reaches again 80% of V Nout, after the downstream circuit breaker has been tripped or for reopening said bidirectional switch after it has been closed for a predetermined time period, preferably 20ms.