1/1000 x 1/1000 only holds for two independent events. If the gear production and preparation is linked personally, locally, informally, productively ... then t

1/1000 x 1/1000 only holds for two independent events. If the gear production and preparation is linked personally, locally, informally, productively ... then the probability of the backup device also failing is larger than 1/1000 if the main one failed.