PC Power supplies trip circuit breaker

PC power supplies (PSUs) have been a common source of hardware failures in our experience. But in 2018 we experienced a new variant of this problem: Power supplies that failed and tripped the circuit breaker, resulting in downtimes for all machines on the same circuit.

The following PSUs were involved:

Bequiet BQT S6-SYS-UA 550W (twice)
We have two identical machines (with a Xeon E3-1220) from 2012. In 2018 first one of these failed, with the end result being a dead PSU and a tripped circuit breaker. Half a year later, the same happened with the other machine.
Bequiet BQT-E6 400W
On a machine from ~2005 (with an Athlon 64 X2 4600+), we originally had a different PSU, which failed at some unknown time without further ado. We replaced it with the BQT-E6 400W, which we had in storage. In 2018, the machine failed; we found it with the PSU off. When we attempted to turn the machine on, it tripped the circuit breaker. The PSU was subsequently found to be dead. From the looks of the PSU, it had been in use only for a short time (but this model was first on sale in 2008, so we probably bought it long ago).
You may wonder whether the exclusive mention of Bequiet PSUs above is due to us having only Bequiet PSUs. The anser is "no". We have a number of different PSU makes.
Anton Ertl